Your decision to do this full time is completely validated. Your ability to convey your expertise and thought processes in a diligent and understandable way, should not be under-estimated! Well done...
at least an electron or bbc can 'recover' from tape load errors by trying a block again, a spectrum would stop with R tape loading error and you'd have to rewind the whole tape and start again, electrons and bbcs split loading into small blocks..
@@andygozzo72 Yeap, and there's 2 errors, Data? and Block?. Normal rule of thumb was if you got a block error, you would rewind a little, adjust the volume up, and play. If you got a data error, you'd do the same but adjust the volume down!
@@mikespangler98 yep, but thats all you had for many systems, unless they make a disk interface,, and you could afford it.....many cost more than the computer itself
This, combined with a dodgy power supply plug in later years made using my electron quite stressful in later years! An age to get a game loaded and then a small knock and the system would reset!
The first of those op-amp circuits is simply an amplifier to increase the signal strength from the cassette player as much as possible. The second stage is a low-pass filter to get rid of any frequencies above the data frequencies, and the final op-amp stage is a clamping circuit to limit the amplitude of the signal.
I've never actually learned what opto couplers, op amps, and how different transistors, mosfets are used or can be used for... and it's because I can easily look up what type of behavior I should see from good parts and have not had to learn the actual operation of them... which I need to do... so when I don't have access to my pocket computer I can actually diagnose properly...I have learned everything I know by jumping in head first and figuring it out on the go... thank you for your time and effort! I Hope You Are Having A Great Day Or Night!
Further to BFLmouse's reply, I expect the diode was added in a later revision because that opamp is on +/- 5V rails and it is not unusual to get large turn on transients on an opamp. With a 10v from positive to negative you might occasionally get a greater then 5V transient the other side of the cap and certainly a lot bigger than the 1.2V it seems to have been designed for. This will be passed onto the ULA via the cap at turn on. This with repeated exposure it probably damaged the input. The diode would clip that to 0.7 ish volts, however it looks like they didn't put any resistance in line so that diode is pulling the output of the opamps current each transient. That transient will happen with every positive change in the signal through the diode. That is probably quite a lot of current on each pulse hence eventually the diode fails. Two diodes would have prevented the turn on transients without overloading the diodes as it would be about the same voltage as the transistor diodes are clipping the feedback loop too. Or they could have put a small resistor in series to limit the current through the one diode. The current circuit relies on the output resistance of the opamp and the ESR of the cap plus the trace resistance and inductance to limit the current.
Nice find with that diode! Here’s my quick circuit analysis, ‘cuz I like analog and audio electronics: Second opamp looks like a Sallen-Key two-pole low-pass filter, cutoff at 4.1 kHz. Third opamp is a symmetrical “soft” clipper, to make the signal very square, over a wide range of input amplitudes (as you stated!). They’re being tricky in the clipper, wiring transistors as diodes. I’m guessing it was easier and cheaper to throw on two more BC239 transistors than to source small-signal diodes just for this part of the circuit.
That weird transistor arrangement is very common in the input protection section of many multi-meters. I think it was actually patented by Fluke decades ago, I'm sure Dave Jones has a video explaining it. It is some kind of Zener clamp.
Using a diode-connected transistor gives a more ideal diode characteristic. Not sure why exactly that would matter in our limiter circuit though. It's more important in other applications like current mirrors. The transistor solution would be more expensive even today.
@@PileOfEmptyTapes The transistors prevent saturation of the OpAmp output. OpAmps used as comparators are prone to that effect. Recovery time from saturated state is not neglegible and will impact the slew rate and thus the frequency response.
You have to look back in awe and appreciate what 6-11 year olds in the UK were being exposed to in the 80s via BBC Basic and hexadecimal checksums. It really is a sight to behold. And to reinforce this, there was a "game" on the BBC Micro called Granny's Garden. Most student's passed it off as a game, but actually this "game" was a kinda like "The Last Starfighter" in that students who SOLVED the dragon section, were actually solving LOGIC GATES. This to me is incredible looking back at what the BBC and the Government were doing.
If the attempted repair was done in the 80s, access to an oscilloscope would have been limited. Access to schematics might have been available, but as you point out, the diode isn't in the schematics. Even access to a multimeter with diode testing would have been somewhat pricey. Not to mention, the years of experience you have with troubleshooting 8-bit computers would have been confined to electrical engineers. So parts cannon was the only method we had unless we knew someone in university.
Can't you remove the diode and use a battery or power supply to flow current through it in both directions to run a light bulb and verify functionality? Potentially using larger or smaller bulbs to somewhat load the part? Thank You For Your Time And Effort! I Hope You Are Having A Great Day Or Night!
The diode (D8) appears in the circuit diagram published in the Acorn Electron Advanced User Guide which corresponds to the issue 6 board. The diagram in the Acorn Electron Service Manual corresponds to earlier boards. For what it's worth, I did a KiCad version of the circuit that also includes this diode.
Oscilloscopes weren't hard to find in the 1980s, but the ones available to the hobbyist had limited bandwidth. Heathkit sold an oscilloscope kit, and I bought a used one. It was damaged in a flood in the 1990s, so I don't have it any more, but it definitely worked, at least up to being able to look at TV signals and lock onto the NTSC pedestal. New, modern oscilloscopes at the time were very expensive, but not required for debugging this problem. I was a poor college student, but was able to afford that oscilloscope.
Yeah, it's really mesmerizing to think that we have really access to super advanced instruments today -- digital storage oscilloscopes, logic analyzers and super cheap FPGAs. Stuff that cost a fortune back in the day when these computers were developed.
I admire your repair abilities! Congrats on narrowing down the issue to 1 simple component and fixing this machine! That little outtake at the end was fun! :D
Couple tips. First, azimuth and cleaning of the cassette deck. Bad azimuth causes higher frequency tones to blur together. Easy to set if you listen to some music, adjust the screw until it sounds most crisp (most high-frequency details audible). Next, crank that volume! You'll get more dropouts if the volume is too low at the "barely detectable" edge of the signal. Hey, fun watch none the less!
Old timer tip: check for the little pad thing under the tape that pushes the tape against the reader head. Sometimes it would sag a little, causing the tape to not make good contact with the head. If you remove it and fold it so that the pad is a little higher, everything works wonderfully again. Regards from Argentina!
In case you or someone else is interested in what the 3 Opamps are doing .. the fisr one is a Sallen-Key-Highpass with an edge frequency of 1.2kHz, the second one a Sallen-Key-Lowpass with an edge frequency of 4.5kHz. So this limits the audio signal to the 2 frequencies, which according to your scope are (very, VERY roughly) 1500Hz and 3000Hz. The gain here is very low, most likely to compensate for the overlap of the 2 filters. The last one is a limiting amplifier, making a square of defined amplitude out if it.
Hey man, you're a great representative of old school "Keep Portland Weird". Not a computer enthusiast or hobbyist but I love puzzles and the way you go through these is fascinating to watch! Good for you buddy! Fun to watch! Keep it up!
Good job! As for shotgun troubleshooting... if you've got a fat supply of every part, it's a really fast way to get stuff running. In fact, that's the DoD system of aircraft maintenance: replace every box that might be wrong to get the plane running, sending all of the suspect boxes bask to supply. Then supply sends those suspect boxes to repair centers to check out, where most run their test programs fine and only a few need to be actually fixed. Since boxes are filled with cards, you can recursively apply the shotgun approach ;)
It can be a problem if you get the parts cannon out and end up installing a new part incorrectly, bridged a solder joint or something like that. Now you have a new problem and likely still have the original as well. Swapping boards or modules is easier and much less prone to cause a new problem but shotgunning at the component level is a great way to make a door stop.
The BBC loads tape programs in blocks. The counter is the block count. If any block gets miss read you can wind back just that block and try again rather than re-start from the start. The DIN connector also supports motor control so the computer can stop the tape between programs.
Yes, the BBC tape format was far superior to any other. Most of them would have a single header block with the name and loading address, and then a constant stream. BBC had a small header on every block with a CRC. This is all the counting from 01...05, etc. BUT you could rewind the tape a few blocks if the CRC failed. Also it allowed C120 tapes to be packed with games on both sides. Total genius.
The TRS-80 Model I and III tape format was similar. It was record based with the name having a particular record ID etc. For a 'system' (ie machine code) program each program record was up to 256 bytes and had its own checksum as well as a discrete start address for that block so it was possible to directly write to the screen RAM during a normal program load. Someone could write a tape that included a few bytes directed to the screen to show a fairly time accurate counter of how long to go until the program load was completed. The record ID for the last in the stream was different again. The only lacking for the standard TRS-80 loader was no ability to stop, rewind and try again. If it failed at the last record, rewind to the beginning and try again. Being able to do a retry of a failed record like the BBC does is a nice idea.
In addition, each block is 256 bytes, so it was simple for the Acorn engineers to transfer the blocks to make each one a sector on a disc system if installed. Basically the same reading and writing format in terms of the OS, but on different media.
I just wanted to thank you for all of your videos. You inspired me to attempt a repair (power supply re-cap) of my BBC Micro. I haven't held a soldering iron in over 30 years, but I bit the bullet, bought a new soldering iron and the capacitors needed and gave it a try. And now I have a working BBC Micro! I probably wouldn't have even tried had it not been for your enthusiasm for this stuff. Thank you.
It has got me wondering if my dad still has his A300 (+ARM3 +4MB upgrade) in the loft. I think he still has his Sinclair Z88. I got rid of my RiscPC (StrongARM710+486+VRAM) a long time ago. Could have been a classic. In fact he may still have the Electron with Plus 1 (cartridge). I have memories of playing the Electron Plus1 Starship Command in a BBC Master.. Smoother and the high score game play is super fast :D
I used to have an Electron back in the 80s. The ULA used to overheat, so I always had the lid unscrewed to allow me to lift the lid to dab a bit of water on the ULA heatsink. Not really the recommended method of keeping electronics cool, but it worked! You using a lupe to read the diode's number is something I have to do now too. I remember the days when I used to be able to read components with my naked eyes in dim light!
Watching you put the cassette down on top of the cassette player speaker reminded me of the computer reseller where I worked in the early 1990s and the office person religiously backed up her computer to DC2120 tapes. She stacked the tapes on top of a clock radio on her desk once they were written and "verified". The only time that the backups were needed, many of them were too corrupted to restore sufficient data and about a months work of data entry had to be recreated and manually entered.
That was a plot point in _Halt and Catch Fire,_ the clean-room BIOS programmer had left a stack of disks on top of a big speaker and they had to redo all the code they couldn’t reconstruct.
Hey Adrian! Just want to thank you, all the knowlege you share through your vídeos are helping me to ressurrect an eletronic organ (a johannus prestige 10)from 1997!!!! It’s my church’s organ gifted from another church( it came all the way from germany to here, Brazil in 2000) it runs a Motorola 68008 as its processor and 128k of ram! I will post another update when the new parts for the board arrive!! And again, thank you so much my friend!! I am following your channel now for almost 5 years and your content are aways amazing to see!!!
This was so fun to watch. I only ever witnessed cassette storage at a friends house, and by the time I had my own 8 bit machine floppies were available. You're a hero for bringing these machines back to life. Thank you!
Such nice work with this troubleshooting. I always love watching the detective-like logical step by step narrowing down of the culprits until you get to the conclusion and seeing that "aha moment" is great. Keep up the great work it's like watching a detective show sometimes!
Came for the repair, remained for the singing at the end. :) Good job. I know nothing about these British micros, but this was a really enjoyable, methodical repair.
Adrian I'm watching your channel since the beginning. I love your troubleshooting skills, you start at the beginning and follow the way until the fault shows up. This diode is the best example. Thanks for your videos, don't stop making such content. Love it. Have a nice time and take care. Best wishes from Germany, Robert
Damn those game cassette tapes brings back some memories. I remember there used to be so many of them it was like a plague as they were really easy to copy, and everyone had them. Now you'd be lucky to find any surviving original copies. As tapes are not known for their durability, especially after 30+ Years.
Congratulations Adrian for this success and the teaching in showing your repair with great humor. I have this type of cassette recorder and use it to load programs onto my vintage computers.
I was a teenager running a small shop for one of the UK distributors for Acorn back when the Electron was released. It was very disappointing having been a BBC user for a while to see how much of a poor relation this was. The slow speed was an issue obviously but the thing that broke a lot of games was the lack of Mode 7 graphics. A lot of games etc. used this for in-game text because of its low memory requirement. A lot of BBC Micro software *will* run quite happily though, albeit slowly, but there was enough incompatibility between the 2 machines to make it frustrating and hit sales figures. I think if they'd made it as a BBC with ports removed but still running at full speed it's impact on the market would have been much greater over here. As it was, it was a mere shadow of the BBC Micro.
@@wembleyford That port of Commando was just lazy, easy money on the part of Elite, very much in the vein of the games industry at the time, with the whole concocted phenomenon of teenagers buying sports cars, Imagine Software, Ocean Software, and so on. All facilitated by the tame/corrupt reviewers of the day exhorting the readership to "go and buy this game immediately" (or equivalent) regardless of whether it was any good. See page 12 of Electron User, June 1986 for the evidence in this case. In fact, regardless of the original programmer's efforts, the publisher was so lazy that the title screen of the game shows the name of the game on two successive character lines. This is the hallmark of mode 7 double-height text shown on the Electron's mode 6 screen. Which means that someone got the BBC Micro version and did the bare minimum to "port" it to the Electron, which may have been nothing at all! Adrian practically demonstrated this by showing the Beeb version which is effectively the same. I suppose it shows how compatible the Electron was with the Beeb, though.
Came for the video, stayed for the outtake. In all seriousness, great video demonstrating the importance of taking your time when troubleshooting; throwing parts at the symptoms without knowing the problem is always more expensive and not a definitive way to solve one.
I love the debugging of the audio input. I always wondered how that was done. Analog signal to digital - just massively overdrive it, and cut off the peaks! So darn simple!
That was FUN. Thanks for bringing back a task I did so many years ago. But most of my repairs where without a schematic. Had to trace out the area to be worked on. Find a tape drive and cut into the motor feed and use the relay.
Adrian, yep Searching then the header which tells you the title, [if not the one you want fast fwd, and look for another header. The Acorn TapeFilingSystem is great. The numbers you see are block numbers in hex, if it miss reads a block, rewind a bit, clean head et-al, and try again, yep it will pick up from where it couldn't read the block. Side note when I had an Electron, a friend gave me Aviator for the BBC B, (I thought why not, give it a try) it worked, but very slowly. The Elc is the machine I learned 6502 on which led me on to a career in being a software dev! Well done!
Page is the top of memory, where your basic program starts. you could change page up by say 128 bytes then you have 128 bytes for user defined graphics vdu 23,ch,b0,b1..b7
This took me back to the 70's when I purchased a kit to build the UK101 single board computer. I rushed it home and by about 8 pm I had it up and running, using the TV which I had prised away from my wife and children - with some effort. But it would not store programs. I had all the components installed correctly, but it was not until someone suggested that I check the capacitor used in the 90/150 c/s (Hz) generator that generated the tones recorded onto tape that my error came to light. Instead of using a capacitor with something like a +/- 5% tolerance (may have been a 0.1 uF) I had used one of the decoupling type capacitors which had massive + tolerances. I swapped to the correct type and away it went! I had a lot of fun with that machine, but loading and storing programs was so time consuming!
33:40 I had a C64 that loaded nothing from tape. I then noticed that the tape read contact in the cassette port was shorted to ground. I traced it as far as I could, noticed that the copper leads to one CIA but also branches elsewhere. I experimentally cut the branch trace, leaving the connection between tape port and CIA intact, and VOILA, loading from tape worked. Everything was fine as far as I could tell, until I acquired a 1541. Trying to access the disk crashed the computer immediately, so I thought it might be related to the tape issue. I took out the schematics, and found out that yes, the serial port shares the tape read line, plus, every signal line in the serial port section goes through a diode snubber network. These are a set of 8 diodes, wired in series pairs in reverse between +5V and ground, the signals passing through the center junctions. Once again experimentally, I snipped the related diode on the ground side, and now the serial port works too. I have left it as is, maybe fix properly later. EDIT: obviously I bridged the previously cut trace, otherwise it would not work.
Oh the memories ! My high school had a room full of the BBC's, and my neighbour had an Electron that I used to go next door and use nearly every day :) My first purchase was the Amstrad CPC-464 with tape drive - often games took 20 - 40 minutes to load, and then crashed !!!
You don't want or need the asterisk there. The Electron and BBC tape formats are identical. The problem you're having here is that you're looking at the Issue 4 schematic and you have an Issue 6 board- there's an Issue 6 schematic on the stardot forum.
Nice work, really glad your ULA wasn't fried those things are getting rare... I think I read somewhere that the BBC Master tape input uses the same, or very similar conditioning circuitry so you might've been able to follow the signal through the Master and compare it with the Elk. However not needed as you got your man in the end 😊
It is very similar and you can load BBC games on an Electron. Sometimes they don't work, sometimes they work more slowly, as there's obvious differences between them. But the tape structure is the same.
I had an Acorn Electron when I was 12 back in the 80's. Unfortunately Acorn missed the Xmas sales and had to throw away thousands that didn't sell. I enjoyed using the Machine and learned to program BASIC on it. The games probably ran slow in BASIC, they did better when I typed in machine code from the Acorn User Magazines. I often think about getting one from Ebay for Nostalga, but they are a bit pricy. I'd love to know what happened to all the excess stock. The worst part of the machine was the soldering on the power connector. Often, the computer reset, deleting all the work I'd done, unless I had saved it to tape. Thanks for the Video, great working fixing the computer, very interesting. I might have to give it a try myself. I haven't played around with electronics for years, but it would be worth the challenge.
Adrian, I know you're always evaluating your content, and I want to make sure that when you said, "This is probably like watching paint dry..." you were, happily, dead wrong! This speaks ill of those who DO watch paint dry, and doesn't account for the fascination I felt as I watched the first-ever realtime loading of software from a cassette tape. To me? That's riveting, coherent content--that I love. Big thumbs up, sir. Jonathan in Seattle
When I first saw this I thought I knew exactly what the issue was but it was hardware related for you. On the Ti994A we had we used the the generation before that Panasonic tape player (RQ-309DS a touch heavier due to more steel in it). It had a tone control and my brother and I found that you could not have the tone set all the way to low or high but the Ti preferred it in the middle for loading software.
One thing about those old cassette players is some had line-level inputs and outputs, while others had speaker-level outputs (like most transistor radios or boomboxes). The only way to tell (if it’s not printed/embossed on the case) is to look at the schematic, or open it up and see if the “out” jack wires go to the speaker or not. I think most 8-bit micros want line-level input, but I could be wrong. 😮
The price difference between the master and electron seems to be represented well in how well the game runs on each system. We are used to this sort of thing with PC gaming but I was a little surprised there would be that much of a difference here on 8 bit Micros.
those 2 transistors connected like that make them work as diodes, with good hard 0.6v ish drop, and in the feedback loop like that, male that op amp a 'squarer' and amplitude limiter
BTW if you want to capture colour it's fairly easy to build a RGB scart lead for the BBC family of machines. Just a few resistors in the back of the scart plug to bring the levels down.
Just seen your video, we use to have a Electron when growing up. We also had Acorn Electron Advanced User Guide, it had more upto date schematic, which does show the D8/D9 diodes on the cassette in/out from the ULA.
What made the BBC tape loading rather slow was because the data is split into blocks with a second or so between them, which you probably heard. The advantage of this is that if an error occurs you can rewind the tape back a little, resume playback, and the computer will retry loading from the block that failed. Most other computers (Spectrum etc) would completely error out if there was a playback glitch, and require you to re-start the tape from the beginning.
The Acorn Electron was my first computer and it started my 25 year career in IT. My favourite games were Snapper and Arcadians. I hope you do a follow-up video where you get it hooked up with colour output and try out some more games! Thanks very much.
4:23 I had a little 'ohh NO' moment, when you pressed the play button and the record button got engaged too. 😱 weird concept of having to press the same button for play and record.
One game people should ALWAYS check out on the Electron is Tynesoft's Kastle (with K). That one really, really showcases what the little computer was capable of.
It has some large sprites that are nicely masked, and so on, but there are better platform games. Citadel is probably the one people should start with.
FWIW those two transistors in the op amp circuit were being used as diodes (thus the shorted pins), probably for pulse shaping. Presumably the two diodes near the ULA were added after that schematic for protecting the input to the ULA.
A very common problem with cassette data is the alignment of the heads on the cassette player. Even though you can hear sound when you play the tape, the computer won't recognize it. The very first thing to try is - using the little screw at the base of the play head, try to realign the tape head. It should go from a muffled sound to a bright sound when you do so, and amazingly it generally fixes the problem. That and cleaning the heads and tape path with isopropyl alcohol.
So satisfying! A component that shouldn't have been there was faulty - I can see why the previous owner changed the ULA, with the schematic you (and presumably everyone) have, it did look like the ULA was faulty or there was a shorted trace. An advantage the ZX Spectrum had over the other machines at that time was that it loaded at 1500bps and didn't pause between data chunks (C64 was 300...). That saved a good chunk of time. Turbo loaders helped a lot too. Impressive how the audio analogue circuitry deals with wildly different volume levels though.
There is a later version of the schematic in the Acorn Electron Advanced User Guide that includes the diode. That schematic pertains to the issue 6 board that Adrian has here.
The two transistors across IC3 form a clamping circuit. When the output voltage is lower than their base to emitter breakdown voltage (typically 0,6-0,7V) they will remain open, so the gain is determined by R9, which is 270K -- basically really, really high. But once the output goes past the breakdown voltage, the transistor will act as a short from collector to emitter, making the feedback resistance zero and limiting gain just enough for the voltage to remain at +/-0,7v. Otherwise the input would be 10vpp from the opamp power supply rails which would damage the chip. A diac or a set of two diodes in anti-parallel would have worked the same way. D8 and D9 might have been added at a later board revision with a newer ULA that doesn't need a negative supply voltage, so the input signal has to be DC offset to be processed properly.
I remember loading BBC game tapes on my Electron back in the eighties. About 50% of the time they even ran. The main problem was when they needed mode 7 or some other full Beeb function. So no Granny’s Garden for me. Which is a roundabout way of me saying that I’m surprised the Master wouldn’t load the Elk tape or vice versa.
ZX:/ (left, right, up, down) is a common key combination to a lot of Acorn/BBC Micro games, with RETURN to fire/jump/etc. I still use these as the direction keys when I'm playing PC games, instead of WASD, which I can't get on with. However, I always use AZ,. and space (up, down, left, right) when playing Chuckie Egg, for reasons lost in the midst of time. Some better games to showcase the Electron would be the aforementioned Chuckie Egg, plus the Repton series.
Loading games from cassette tape... the struggle was real. We were so spoiled here in the U.S. Floppy disk drives weren't too uncommon for many "well to do" families. Even then, my Commodore 64 friends had to wait longer, if they didn't have the Fast Load cartridge. Ahh... good times.
Excellent methodical trouble shooting. Every time you cut from the schematic I expected to see a shot of surface mount stuff under a microscope, not the glorious gigantic old school components. Fun stuff, congrats!
This really takes me back - I was shouting *tape at the screen early on. 😁 My Dad had a BBC Micro when I was a kid which I 'permanently borrowed' 😀. The machine had a great feature which allowed you to type 6502 assembly language straight in from within even a basic program (I think it was within brackets but could be wrong - it was a long time ago!). It got me programming assembler when I was 12 so I could write my own games and lead to a long and successful career as a developer.
This brings back memories…. My first computer was the electron, I remember for loading games the volume level was very important. somewhere between 3/4 and full depending on game then the dreaded syntax error….. happy days
Great video. Some Electron tapes will load on a BBC B or Master. I loaded up the electron version of Chuckie egg on BBC B. Also to load a tape you can also use the shortened command of CH.""
The Acorn Electron, BBC Micro and Master are all compatible with their cassette recordings but may not be able to run the code due to machine differences. BASIC Programs using modes 0-6 will run. Also programmes are recorded in blocks so you can do *CAT and play the tape at any point and it will tell you what’s there. Also if you have an error you can rewind the tape to before that block and try again from that point. The tape is stopped and started between each block so there’s a gap in the carrier tone. It’s why the loading feels slow even though it’s not. The data to block over head means there’s more data than just the programme. Fast loaders ditched the block loading at the cost of reliability.
In fact, the BBC Micro supports a 300 baud transfer mode as well, which is not supported by the Electron, as far as I am aware. Modern experiments with the cassette interface have achieved some interesting results, however, so I am not ruling anything out.
Love these old computers. I had a Vic 20, CBM 64, Spectrum , Atari 400, Amiga 500, Amiga 1200 - I wish I still had all of them rather than Binning them 😞...well done you
15:23 I prefer watching the grass grow ;) I remember trying to figure out how to play some games on c64. Sometimes it required changing the joystick port or pressing a specific key, and sometimes I just needed to press almost every single key on the keyboard. I'm glad that those days are long gone.
Given my 8-bit experiences when young were solely loading form tape, yeah, the frustration of it taking so long was often problematic, cos you'd set the computer off loading, go off to do something else, forget it was loading, wonder why you hear noises from some other part of the house, then suddenly remember about that game you were loading over an hour ago... :P
That was a really interesting one, Adrian! In your situation, where the inherited machine arrived with partially de-soldered ULA, suspicion immediately falls to that device. Very glad to see that in the final analysis, it was a readily replaceable discrete component that had failed. - P.S.: Welcome to loading from cassette tape hell! You "missed out" on this growing up with an Apple //c with integral FDD. My impression is that the majority of folk in the USA who had 8bit machines tended to have FDD drives. -> In the rest of the world, most of us at least started out with cassette as the data storage medium on 8bit machines and the FDDs usually cost as much or more than the computer itself did!
2 points. Point 1: The input of an Opamp is always near zero unless the Opamp clamps. Example: on input you have a signal of say 200mV via 10K to the minus input of the Opamp. This will make the output shoot down (a theoretical Opamp has infinite amplification, a real Opamp several hundred thousant times). But if there is a 100K feedback then then the output reaches -2 V (Opamps usually have a negative power as well) then the input is in balance at 0V. The ratio between the input resistor and the feedback resistor determine the amplification ratio, that's how all these work, in this case the amplification is 10 time (100k/10k). Unless the output is clamping to the plus or minus rail, the minus input will always be around zero (exactly 0 for a theoretical Opamp). What they did here is also have 2 transistors in the feedback, at low volume the amplification is determined by the input and feedback resistors, but as soon as 0.6 V is dropped over the feedback resistor the transistor start conducting and the amplification ratio plummets to a very low ratio. One transistor for +0.6V and the other for -0.6V. Not all Opamp circuits have a negative voltage rail but then there minus input just has a bias voltage. The DC voltage on the minus input is not 0 volt in that case but still there is almost no AC signal because the current from the input and the current from the feedback are in balance. I hope this helps understanding these circuits better and why there is almost no signal on the minus input. If there would have been a large signal then that would have indicated a problem... Second point: The 2 diodes on the ULA cassette input are most likely 2 anti-parrallel diodes. They just clamp the signal to +/- 0.6 V for the input of the ULA, protecting it from a too high voltage on the input, destorying it. Very likely the added them afterward to protect the ULA better. You already saw what happened when the transistors were removed from the feedback. Without the diodes that may have been the dead blow for the ULA. In your case not because it has the diodes and one was even a short. But without it, it could easily be the end the ULA which is internally a low voltage sensitive component.
Your decision to do this full time is completely validated. Your ability to convey your expertise and thought processes in a diligent and understandable way, should not be under-estimated! Well done...
That “data? Rewind tape” message brings back so many bad memories from 40 years ago! You could easily spend an hour trying to load Elite.
at least an electron or bbc can 'recover' from tape load errors by trying a block again, a spectrum would stop with R tape loading error and you'd have to rewind the whole tape and start again, electrons and bbcs split loading into small blocks..
@@andygozzo72 Yeap, and there's 2 errors, Data? and Block?. Normal rule of thumb was if you got a block error, you would rewind a little, adjust the volume up, and play. If you got a data error, you'd do the same but adjust the volume down!
Cassettes were so slow. 😒
@@mikespangler98 yep, but thats all you had for many systems, unless they make a disk interface,, and you could afford it.....many cost more than the computer itself
This, combined with a dodgy power supply plug in later years made using my electron quite stressful in later years! An age to get a game loaded and then a small knock and the system would reset!
I was watching this on my TV. I took the time to log in to my account so I could comment: the outtake was absolutely worth waiting for.
The first of those op-amp circuits is simply an amplifier to increase the signal strength from the cassette player as much as possible. The second stage is a low-pass filter to get rid of any frequencies above the data frequencies, and the final op-amp stage is a clamping circuit to limit the amplitude of the signal.
I've never actually learned what opto couplers, op amps, and how different transistors, mosfets are used or can be used for... and it's because I can easily look up what type of behavior I should see from good parts and have not had to learn the actual operation of them... which I need to do... so when I don't have access to my pocket computer I can actually diagnose properly...I have learned everything I know by jumping in head first and figuring it out on the go... thank you for your time and effort! I Hope You Are Having A Great Day Or Night!
Further to BFLmouse's reply, I expect the diode was added in a later revision because that opamp is on +/- 5V rails and it is not unusual to get large turn on transients on an opamp. With a 10v from positive to negative you might occasionally get a greater then 5V transient the other side of the cap and certainly a lot bigger than the 1.2V it seems to have been designed for. This will be passed onto the ULA via the cap at turn on. This with repeated exposure it probably damaged the input. The diode would clip that to 0.7 ish volts, however it looks like they didn't put any resistance in line so that diode is pulling the output of the opamps current each transient. That transient will happen with every positive change in the signal through the diode. That is probably quite a lot of current on each pulse hence eventually the diode fails. Two diodes would have prevented the turn on transients without overloading the diodes as it would be about the same voltage as the transistor diodes are clipping the feedback loop too. Or they could have put a small resistor in series to limit the current through the one diode. The current circuit relies on the output resistance of the opamp and the ESR of the cap plus the trace resistance and inductance to limit the current.
@@gfiandy Looks to me like a clamping diode, to set the DC bias of the signal coming out of the cap. Not necessarily to catch over-/under-shoot.
Nice find with that diode!
Here’s my quick circuit analysis, ‘cuz I like analog and audio electronics: Second opamp looks like a Sallen-Key two-pole low-pass filter, cutoff at 4.1 kHz. Third opamp is a symmetrical “soft” clipper, to make the signal very square, over a wide range of input amplitudes (as you stated!). They’re being tricky in the clipper, wiring transistors as diodes. I’m guessing it was easier and cheaper to throw on two more BC239 transistors than to source small-signal diodes just for this part of the circuit.
Awesome -- yeah it's a bit magical to me, but I at least get the general gist of what it's supposed to be doing just from previous experience.
That weird transistor arrangement is very common in the input protection section of many multi-meters. I think it was actually patented by Fluke decades ago, I'm sure Dave Jones has a video explaining it. It is some kind of Zener clamp.
Yet they added a 1N4148 in a later revision instead of using another BC239.
Using a diode-connected transistor gives a more ideal diode characteristic. Not sure why exactly that would matter in our limiter circuit though. It's more important in other applications like current mirrors. The transistor solution would be more expensive even today.
@@PileOfEmptyTapes The transistors prevent saturation of the OpAmp output. OpAmps used as comparators are prone to that effect. Recovery time from saturated state is not neglegible and will impact the slew rate and thus the frequency response.
You have to look back in awe and appreciate what 6-11 year olds in the UK were being exposed to in the 80s via BBC Basic and hexadecimal checksums. It really is a sight to behold. And to reinforce this, there was a "game" on the BBC Micro called Granny's Garden. Most student's passed it off as a game, but actually this "game" was a kinda like "The Last Starfighter" in that students who SOLVED the dragon section, were actually solving LOGIC GATES. This to me is incredible looking back at what the BBC and the Government were doing.
Students. Why can’t anyone use apostrophes correctly anymore?
Great comment so true!!
@darkstatehk: Yes, that dragon puzzle was quite the sting in the tail!
@@bradallen8909 At least the OP is using punctuation and it is broadly being used correctly!
Granny's Garden, haha! "Do you want to help the King and Queen?" "You DO want to help the King and Queen!" 😂
If the attempted repair was done in the 80s, access to an oscilloscope would have been limited. Access to schematics might have been available, but as you point out, the diode isn't in the schematics. Even access to a multimeter with diode testing would have been somewhat pricey. Not to mention, the years of experience you have with troubleshooting 8-bit computers would have been confined to electrical engineers. So parts cannon was the only method we had unless we knew someone in university.
Can't you remove the diode and use a battery or power supply to flow current through it in both directions to run a light bulb and verify functionality? Potentially using larger or smaller bulbs to somewhat load the part? Thank You For Your Time And Effort! I Hope You Are Having A Great Day Or Night!
The diode (D8) appears in the circuit diagram published in the Acorn Electron Advanced User Guide which corresponds to the issue 6 board. The diagram in the Acorn Electron Service Manual corresponds to earlier boards. For what it's worth, I did a KiCad version of the circuit that also includes this diode.
Oscilloscopes weren't hard to find in the 1980s, but the ones available to the hobbyist had limited bandwidth. Heathkit sold an oscilloscope kit, and I bought a used one. It was damaged in a flood in the 1990s, so I don't have it any more, but it definitely worked, at least up to being able to look at TV signals and lock onto the NTSC pedestal. New, modern oscilloscopes at the time were very expensive, but not required for debugging this problem. I was a poor college student, but was able to afford that oscilloscope.
Yeah, it's really mesmerizing to think that we have really access to super advanced instruments today -- digital storage oscilloscopes, logic analyzers and super cheap FPGAs. Stuff that cost a fortune back in the day when these computers were developed.
I admire your repair abilities! Congrats on narrowing down the issue to 1 simple component and fixing this machine! That little outtake at the end was fun! :D
Couple tips. First, azimuth and cleaning of the cassette deck. Bad azimuth causes higher frequency tones to blur together. Easy to set if you listen to some music, adjust the screw until it sounds most crisp (most high-frequency details audible). Next, crank that volume! You'll get more dropouts if the volume is too low at the "barely detectable" edge of the signal. Hey, fun watch none the less!
Old timer tip: check for the little pad thing under the tape that pushes the tape against the reader head. Sometimes it would sag a little, causing the tape to not make good contact with the head. If you remove it and fold it so that the pad is a little higher, everything works wonderfully again. Regards from Argentina!
In case you or someone else is interested in what the 3 Opamps are doing .. the fisr one is a Sallen-Key-Highpass with an edge frequency of 1.2kHz, the second one a Sallen-Key-Lowpass with an edge frequency of 4.5kHz. So this limits the audio signal to the 2 frequencies, which according to your scope are (very, VERY roughly) 1500Hz and 3000Hz. The gain here is very low, most likely to compensate for the overlap of the 2 filters.
The last one is a limiting amplifier, making a square of defined amplitude out if it.
Yes, I had an Acorn Electron many years ago. Spending ages and ages waiting for games to not load from tapes. Happy memories. 🙂
I remember everything being good until the power went off and you heard the dreaded reset beep.
Hey man, you're a great representative of old school "Keep Portland Weird".
Not a computer enthusiast or hobbyist but I love puzzles and the way you go through these is fascinating to watch! Good for you buddy! Fun to watch! Keep it up!
Good job! As for shotgun troubleshooting... if you've got a fat supply of every part, it's a really fast way to get stuff running. In fact, that's the DoD system of aircraft maintenance: replace every box that might be wrong to get the plane running, sending all of the suspect boxes bask to supply. Then supply sends those suspect boxes to repair centers to check out, where most run their test programs fine and only a few need to be actually fixed. Since boxes are filled with cards, you can recursively apply the shotgun approach ;)
It can be a problem if you get the parts cannon out and end up installing a new part incorrectly, bridged a solder joint or something like that. Now you have a new problem and likely still have the original as well. Swapping boards or modules is easier and much less prone to cause a new problem but shotgunning at the component level is a great way to make a door stop.
The BBC loads tape programs in blocks. The counter is the block count. If any block gets miss read you can wind back just that block and try again rather than re-start from the start. The DIN connector also supports motor control so the computer can stop the tape between programs.
Yes, the BBC tape format was far superior to any other. Most of them would have a single header block with the name and loading address, and then a constant stream.
BBC had a small header on every block with a CRC. This is all the counting from 01...05, etc. BUT you could rewind the tape a few blocks if the CRC failed.
Also it allowed C120 tapes to be packed with games on both sides. Total genius.
“Dara Rewind Tape” I seem to remember. *edit I’ve just got to that point lol, I wasn’t far off.
The TRS-80 Model I and III tape format was similar. It was record based with the name having a particular record ID etc. For a 'system' (ie machine code) program each program record was up to 256 bytes and had its own checksum as well as a discrete start address for that block so it was possible to directly write to the screen RAM during a normal program load. Someone could write a tape that included a few bytes directed to the screen to show a fairly time accurate counter of how long to go until the program load was completed. The record ID for the last in the stream was different again.
The only lacking for the standard TRS-80 loader was no ability to stop, rewind and try again. If it failed at the last record, rewind to the beginning and try again. Being able to do a retry of a failed record like the BBC does is a nice idea.
In addition, each block is 256 bytes, so it was simple for the Acorn engineers to transfer the blocks to make each one a sector on a disc system if installed. Basically the same reading and writing format in terms of the OS, but on different media.
I just wanted to thank you for all of your videos. You inspired me to attempt a repair (power supply re-cap) of my BBC Micro. I haven't held a soldering iron in over 30 years, but I bit the bullet, bought a new soldering iron and the capacitors needed and gave it a try. And now I have a working BBC Micro! I probably wouldn't have even tried had it not been for your enthusiasm for this stuff. Thank you.
It has got me wondering if my dad still has his A300 (+ARM3 +4MB upgrade) in the loft. I think he still has his Sinclair Z88.
I got rid of my RiscPC (StrongARM710+486+VRAM) a long time ago. Could have been a classic.
In fact he may still have the Electron with Plus 1 (cartridge). I have memories of playing the Electron Plus1 Starship Command in a BBC Master.. Smoother and the high score game play is super fast :D
I used to have an Electron back in the 80s. The ULA used to overheat, so I always had the lid unscrewed to allow me to lift the lid to dab a bit of water on the ULA heatsink. Not really the recommended method of keeping electronics cool, but it worked!
You using a lupe to read the diode's number is something I have to do now too. I remember the days when I used to be able to read components with my naked eyes in dim light!
That Panasonic tape recorder brings me memories... My dad had one just like it. Exactly the same model.
Watching you put the cassette down on top of the cassette player speaker reminded me of the computer reseller where I worked in the early 1990s and the office person religiously backed up her computer to DC2120 tapes. She stacked the tapes on top of a clock radio on her desk once they were written and "verified". The only time that the backups were needed, many of them were too corrupted to restore sufficient data and about a months work of data entry had to be recreated and manually entered.
That was a plot point in _Halt and Catch Fire,_ the clean-room BIOS programmer had left a stack of disks on top of a big speaker and they had to redo all the code they couldn’t reconstruct.
Love the way you track down and diagnose problems. Excellent forensic work. Thank you.
Remember my Acorn Electron cassette lead having three jacks; input, output and control to stop start the tape.
At 13:00 he mentions the extra pins in the din connector used for switching the tape motor.
I remember many cassette players had a 2.5mm jack for pause. Doesn't look like that panasonic one has one though.
Yes, output for saving programs to tape - did lots of that, still have the tapes but sadly not the Electron :-(
I was given a data recorder for my Electron back in the 80's. I had to adjust the azimuth of the head to get a better signal from the tape.
Hey Adrian! Just want to thank you, all the knowlege you share through your vídeos are helping me to ressurrect an eletronic organ (a johannus prestige 10)from 1997!!!! It’s my church’s organ gifted from another church( it came all the way from germany to here, Brazil in 2000) it runs a Motorola 68008 as its processor and 128k of ram! I will post another update when the new parts for the board arrive!! And again, thank you so much my friend!! I am following your channel now for almost 5 years and your content are aways amazing to see!!!
This was so fun to watch. I only ever witnessed cassette storage at a friends house, and by the time I had my own 8 bit machine floppies were available. You're a hero for bringing these machines back to life. Thank you!
Such nice work with this troubleshooting. I always love watching the detective-like logical step by step narrowing down of the culprits until you get to the conclusion and seeing that "aha moment" is great. Keep up the great work it's like watching a detective show sometimes!
Came for the repair, remained for the singing at the end. :) Good job. I know nothing about these British micros, but this was a really enjoyable, methodical repair.
Adrian I'm watching your channel since the beginning. I love your troubleshooting skills, you start at the beginning and follow the way until the fault shows up. This diode is the best example. Thanks for your videos, don't stop making such content. Love it. Have a nice time and take care. Best wishes from Germany, Robert
Damn those game cassette tapes brings back some memories. I remember there used to be so many of them it was like a plague as they were really easy to copy, and everyone had them. Now you'd be lucky to find any surviving original copies. As tapes are not known for their durability, especially after 30+ Years.
Congratulations Adrian for this success and the teaching in showing your repair with great humor. I have this type of cassette recorder and use it to load programs onto my vintage computers.
I was a teenager running a small shop for one of the UK distributors for Acorn back when the Electron was released. It was very disappointing having been a BBC user for a while to see how much of a poor relation this was. The slow speed was an issue obviously but the thing that broke a lot of games was the lack of Mode 7 graphics. A lot of games etc. used this for in-game text because of its low memory requirement. A lot of BBC Micro software *will* run quite happily though, albeit slowly, but there was enough incompatibility between the 2 machines to make it frustrating and hit sales figures. I think if they'd made it as a BBC with ports removed but still running at full speed it's impact on the market would have been much greater over here. As it was, it was a mere shadow of the BBC Micro.
I love the Acorn line - but you've had to felt embarrassed for anyone who's been brought a copy of Commando if they'd ever seen the C64 version.
@@wembleyford That port of Commando was just lazy, easy money on the part of Elite, very much in the vein of the games industry at the time, with the whole concocted phenomenon of teenagers buying sports cars, Imagine Software, Ocean Software, and so on. All facilitated by the tame/corrupt reviewers of the day exhorting the readership to "go and buy this game immediately" (or equivalent) regardless of whether it was any good. See page 12 of Electron User, June 1986 for the evidence in this case.
In fact, regardless of the original programmer's efforts, the publisher was so lazy that the title screen of the game shows the name of the game on two successive character lines. This is the hallmark of mode 7 double-height text shown on the Electron's mode 6 screen. Which means that someone got the BBC Micro version and did the bare minimum to "port" it to the Electron, which may have been nothing at all! Adrian practically demonstrated this by showing the Beeb version which is effectively the same. I suppose it shows how compatible the Electron was with the Beeb, though.
To make it worse, if I’m remembering correctly the Electron shipped a year late due to problems manufacturing that massive ULA.
taught myself to juggle waiting for tape loads on the BBC :)
Came for the video, stayed for the outtake. In all seriousness, great video demonstrating the importance of taking your time when troubleshooting; throwing parts at the symptoms without knowing the problem is always more expensive and not a definitive way to solve one.
I love the debugging of the audio input. I always wondered how that was done. Analog signal to digital - just massively overdrive it, and cut off the peaks! So darn simple!
First comment!
Yay, my old BBC Master 128 is seeing some more use! 👌
All the best from Cambridge UK 🇬🇧
2nd, from a former resident of Cambridge, UK 🇬🇧
Third - most of my family are in Cambridgeshire
That was FUN. Thanks for bringing back a task I did so many years ago. But most of my repairs where without a schematic. Had to trace out the area to be worked on. Find a tape drive and cut into the motor feed and use the relay.
Adrian, yep Searching then the header which tells you the title, [if not the one you want fast fwd, and look for another header. The Acorn TapeFilingSystem is great. The numbers you see are block numbers in hex, if it miss reads a block, rewind a bit, clean head et-al, and try again, yep it will pick up from where it couldn't read the block. Side note when I had an Electron, a friend gave me Aviator for the BBC B, (I thought why not, give it a try) it worked, but very slowly. The Elc is the machine I learned 6502 on which led me on to a career in being a software dev! Well done!
Page is the top of memory, where your basic program starts. you could change page up by say 128 bytes then you have 128 bytes for user defined graphics vdu 23,ch,b0,b1..b7
Really enjoyed this one! I love when it comes down to a simple fix, and your ability to work through the board and find it is always inspring!
...well done sir! ...and thank goodness you didnt see Commando in colour..... its horrendous!
Ah the good old hexadecimal counter of tape loading on the Acorn machines.
This took me back to the 70's when I purchased a kit to build the UK101 single board computer. I rushed it home and by about 8 pm I had it up and running, using the TV which I had prised away from my wife and children - with some effort. But it would not store programs. I had all the components installed correctly, but it was not until someone suggested that I check the capacitor used in the 90/150 c/s (Hz) generator that generated the tones recorded onto tape that my error came to light. Instead of using a capacitor with something like a +/- 5% tolerance (may have been a 0.1 uF) I had used one of the decoupling type capacitors which had massive + tolerances. I swapped to the correct type and away it went! I had a lot of fun with that machine, but loading and storing programs was so time consuming!
I really enjoyed the electron videos, as this machine is part of my fondest childhood memories!
Thanks!
Nice basic diagnostic search for the failing element. (a shorted/defective diode) I enjoy watching your videos. 👍
33:40 I had a C64 that loaded nothing from tape. I then noticed that the tape read contact in the cassette port was shorted to ground. I traced it as far as I could, noticed that the copper leads to one CIA but also branches elsewhere. I experimentally cut the branch trace, leaving the connection between tape port and CIA intact, and VOILA, loading from tape worked. Everything was fine as far as I could tell, until I acquired a 1541. Trying to access the disk crashed the computer immediately, so I thought it might be related to the tape issue. I took out the schematics, and found out that yes, the serial port shares the tape read line, plus, every signal line in the serial port section goes through a diode snubber network. These are a set of 8 diodes, wired in series pairs in reverse between +5V and ground, the signals passing through the center junctions. Once again experimentally, I snipped the related diode on the ground side, and now the serial port works too. I have left it as is, maybe fix properly later.
EDIT: obviously I bridged the previously cut trace, otherwise it would not work.
That Panasonic tape deck brings back memories from my childhood. I was rocking Licensed to Ill on it back in 86
Oh the memories ! My high school had a room full of the BBC's, and my neighbour had an Electron that I used to go next door and use nearly every day :) My first purchase was the Amstrad CPC-464 with tape drive - often games took 20 - 40 minutes to load, and then crashed !!!
You don't want or need the asterisk there. The Electron and BBC tape formats are identical.
The problem you're having here is that you're looking at the Issue 4 schematic and you have an Issue 6 board- there's an Issue 6 schematic on the stardot forum.
My first computer. 1984/5
Nice work, really glad your ULA wasn't fried those things are getting rare... I think I read somewhere that the BBC Master tape input uses the same, or very similar conditioning circuitry so you might've been able to follow the signal through the Master and compare it with the Elk. However not needed as you got your man in the end 😊
It is very similar and you can load BBC games on an Electron. Sometimes they don't work, sometimes they work more slowly, as there's obvious differences between them. But the tape structure is the same.
I had an Acorn Electron when I was 12 back in the 80's. Unfortunately Acorn missed the Xmas sales and had to throw away thousands that didn't sell. I enjoyed using the Machine and learned to program BASIC on it. The games probably ran slow in BASIC, they did better when I typed in machine code from the Acorn User Magazines. I often think about getting one from Ebay for Nostalga, but they are a bit pricy. I'd love to know what happened to all the excess stock. The worst part of the machine was the soldering on the power connector. Often, the computer reset, deleting all the work I'd done, unless I had saved it to tape. Thanks for the Video, great working fixing the computer, very interesting. I might have to give it a try myself. I haven't played around with electronics for years, but it would be worth the challenge.
Well in defense of the old ways. Who had all the fancy diagnostics equipment in their shed in the 80's or 90's?
The transistors are there to clip inside the feedback loop. So with low amplitude on the feedback loop, they will make no difference, as you saw.
Great video. You can't beat the anticipation and joy (when it works) from tape loading 😅
Adrian, I know you're always evaluating your content, and I want to make sure that when you said, "This is probably like watching paint dry..." you were, happily, dead wrong! This speaks ill of those who DO watch paint dry, and doesn't account for the fascination I felt as I watched the first-ever realtime loading of software from a cassette tape. To me? That's riveting, coherent content--that I love. Big thumbs up, sir. Jonathan in Seattle
When I first saw this I thought I knew exactly what the issue was but it was hardware related for you.
On the Ti994A we had we used the the generation before that Panasonic tape player (RQ-309DS a touch heavier due to more steel in it). It had a tone control and my brother and I found that you could not have the tone set all the way to low or high but the Ti preferred it in the middle for loading software.
Hey, thanks for sharing your hobbies. It's inspiring.
Again, awesome debugging 😊
One thing about those old cassette players is some had line-level inputs and outputs, while others had speaker-level outputs (like most transistor radios or boomboxes). The only way to tell (if it’s not printed/embossed on the case) is to look at the schematic, or open it up and see if the “out” jack wires go to the speaker or not. I think most 8-bit micros want line-level input, but I could be wrong. 😮
The price difference between the master and electron seems to be represented well in how well the game runs on each system. We are used to this sort of thing with PC gaming but I was a little surprised there would be that much of a difference here on 8 bit Micros.
I really like to watch you troubleshooting computers 🖥️ And the best part of the video is when you find the problem and machine starts to work.😊
Brings back good memories of my Acorn Atom, my first real computer! Loved the repair methodology!
Well done tracking that down! I enjoyed the whole CSI of this this episode.
Amazing. Rather than replacing big chips, just find a dead diode that's not even on the schematics... Congrats on saving this piece of history
those 2 transistors connected like that make them work as diodes, with good hard 0.6v ish drop, and in the feedback loop like that, male that op amp a 'squarer' and amplitude limiter
Excellent professional fault finding Adrian. Well done!
Brings back memories of loading and saving programs on my old Ti99a.
BTW if you want to capture colour it's fairly easy to build a RGB scart lead for the BBC family of machines. Just a few resistors in the back of the scart plug to bring the levels down.
Just seen your video, we use to have a Electron when growing up. We also had Acorn Electron Advanced User Guide, it had more upto date schematic, which does show the D8/D9 diodes on the cassette in/out from the ULA.
What made the BBC tape loading rather slow was because the data is split into blocks with a second or so between them, which you probably heard. The advantage of this is that if an error occurs you can rewind the tape back a little, resume playback, and the computer will retry loading from the block that failed. Most other computers (Spectrum etc) would completely error out if there was a playback glitch, and require you to re-start the tape from the beginning.
Best repair video yet!
The Acorn Electron was my first computer and it started my 25 year career in IT. My favourite games were Snapper and Arcadians. I hope you do a follow-up video where you get it hooked up with colour output and try out some more games! Thanks very much.
I played Commando on my C64 back in the 80s for hours on end. This video really shows how much faster the C64 is compared to the BBC Micro.
Different ring 🎉❤
4:23 I had a little 'ohh NO' moment, when you pressed the play button and the record button got engaged too. 😱 weird concept of having to press the same button for play and record.
One game people should ALWAYS check out on the Electron is Tynesoft's Kastle (with K). That one really, really showcases what the little computer was capable of.
It has some large sprites that are nicely masked, and so on, but there are better platform games. Citadel is probably the one people should start with.
FWIW those two transistors in the op amp circuit were being used as diodes (thus the shorted pins), probably for pulse shaping.
Presumably the two diodes near the ULA were added after that schematic for protecting the input to the ULA.
A very common problem with cassette data is the alignment of the heads on the cassette player. Even though you can hear sound when you play the tape, the computer won't recognize it. The very first thing to try is - using the little screw at the base of the play head, try to realign the tape head. It should go from a muffled sound to a bright sound when you do so, and amazingly it generally fixes the problem. That and cleaning the heads and tape path with isopropyl alcohol.
So satisfying! A component that shouldn't have been there was faulty - I can see why the previous owner changed the ULA, with the schematic you (and presumably everyone) have, it did look like the ULA was faulty or there was a shorted trace. An advantage the ZX Spectrum had over the other machines at that time was that it loaded at 1500bps and didn't pause between data chunks (C64 was 300...). That saved a good chunk of time. Turbo loaders helped a lot too. Impressive how the audio analogue circuitry deals with wildly different volume levels though.
There is a later version of the schematic in the Acorn Electron Advanced User Guide that includes the diode. That schematic pertains to the issue 6 board that Adrian has here.
The two transistors across IC3 form a clamping circuit. When the output voltage is lower than their base to emitter breakdown voltage (typically 0,6-0,7V) they will remain open, so the gain is determined by R9, which is 270K -- basically really, really high. But once the output goes past the breakdown voltage, the transistor will act as a short from collector to emitter, making the feedback resistance zero and limiting gain just enough for the voltage to remain at +/-0,7v. Otherwise the input would be 10vpp from the opamp power supply rails which would damage the chip. A diac or a set of two diodes in anti-parallel would have worked the same way.
D8 and D9 might have been added at a later board revision with a newer ULA that doesn't need a negative supply voltage, so the input signal has to be DC offset to be processed properly.
I have an old CoCo that need restoration. Having a PC that could load programs from cassette is a goal of mine. Thanks for this video.
I remember loading BBC game tapes on my Electron back in the eighties. About 50% of the time they even ran. The main problem was when they needed mode 7 or some other full Beeb function. So no Granny’s Garden for me. Which is a roundabout way of me saying that I’m surprised the Master wouldn’t load the Elk tape or vice versa.
ZX:/ (left, right, up, down) is a common key combination to a lot of Acorn/BBC Micro games, with RETURN to fire/jump/etc. I still use these as the direction keys when I'm playing PC games, instead of WASD, which I can't get on with.
However, I always use AZ,. and space (up, down, left, right) when playing Chuckie Egg, for reasons lost in the midst of time.
Some better games to showcase the Electron would be the aforementioned Chuckie Egg, plus the Repton series.
Loading games from cassette tape... the struggle was real. We were so spoiled here in the U.S. Floppy disk drives weren't too uncommon for many "well to do" families.
Even then, my Commodore 64 friends had to wait longer, if they didn't have the Fast Load cartridge. Ahh... good times.
Great work as always!
I watched this like a movie) Thank you Adrian!
Excellent methodical trouble shooting. Every time you cut from the schematic I expected to see a shot of surface mount stuff under a microscope, not the glorious gigantic old school components. Fun stuff, congrats!
That was plain awesome troubleshooting.
This really takes me back - I was shouting *tape at the screen early on. 😁 My Dad had a BBC Micro when I was a kid which I 'permanently borrowed' 😀. The machine had a great feature which allowed you to type 6502 assembly language straight in from within even a basic program (I think it was within brackets but could be wrong - it was a long time ago!). It got me programming assembler when I was 12 so I could write my own games and lead to a long and successful career as a developer.
This brings back memories…. My first computer was the electron, I remember for loading games the volume level was very important. somewhere between 3/4 and full depending on game then the dreaded syntax error….. happy days
Great video. Some Electron tapes will load on a BBC B or Master. I loaded up the electron version of Chuckie egg on BBC B. Also to load a tape you can also use the shortened command of CH.""
Also at least with Commodore Datasets I always found it best to get past the tape leader, sometimes that will cause bad loading.
The transistors in the feedback loop are diode connected which forms a clipping circuit. It's basically a sine to square wave converter.
The Acorn Electron, BBC Micro and Master are all compatible with their cassette recordings but may not be able to run the code due to machine differences. BASIC Programs using modes 0-6 will run. Also programmes are recorded in blocks so you can do *CAT and play the tape at any point and it will tell you what’s there. Also if you have an error you can rewind the tape to before that block and try again from that point. The tape is stopped and started between each block so there’s a gap in the carrier tone. It’s why the loading feels slow even though it’s not. The data to block over head means there’s more data than just the programme. Fast loaders ditched the block loading at the cost of reliability.
In fact, the BBC Micro supports a 300 baud transfer mode as well, which is not supported by the Electron, as far as I am aware. Modern experiments with the cassette interface have achieved some interesting results, however, so I am not ruling anything out.
Lowering Page basically reclaims the memory that a DIsc Filing System would use up. On a BBC B Page on a cassette non DFS system is E00.
Love these old computers. I had a Vic 20, CBM 64, Spectrum , Atari 400, Amiga 500, Amiga 1200 - I wish I still had all of them rather than Binning them 😞...well done you
15:23 I prefer watching the grass grow ;) I remember trying to figure out how to play some games on c64. Sometimes it required changing the joystick port or pressing a specific key, and sometimes I just needed to press almost every single key on the keyboard. I'm glad that those days are long gone.
Given my 8-bit experiences when young were solely loading form tape, yeah, the frustration of it taking so long was often problematic, cos you'd set the computer off loading, go off to do something else, forget it was loading, wonder why you hear noises from some other part of the house, then suddenly remember about that game you were loading over an hour ago... :P
That was a really interesting one, Adrian! In your situation, where the inherited machine arrived with partially de-soldered ULA, suspicion immediately falls to that device.
Very glad to see that in the final analysis, it was a readily replaceable discrete component that had failed.
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P.S.: Welcome to loading from cassette tape hell! You "missed out" on this growing up with an Apple //c with integral FDD. My impression is that the majority of folk in the USA who had 8bit machines tended to have FDD drives.
-> In the rest of the world, most of us at least started out with cassette as the data storage medium on 8bit machines and the FDDs usually cost as much or more than the computer itself did!
tape programs brings back a flood of great memories !!
2 points.
Point 1: The input of an Opamp is always near zero unless the Opamp clamps.
Example: on input you have a signal of say 200mV via 10K to the minus input of the Opamp. This will make the output shoot down (a theoretical Opamp has infinite amplification, a real Opamp several hundred thousant times). But if there is a 100K feedback then then the output reaches -2 V (Opamps usually have a negative power as well) then the input is in balance at 0V. The ratio between the input resistor and the feedback resistor determine the amplification ratio, that's how all these work, in this case the amplification is 10 time (100k/10k).
Unless the output is clamping to the plus or minus rail, the minus input will always be around zero (exactly 0 for a theoretical Opamp). What they did here is also have 2 transistors in the feedback, at low volume the amplification is determined by the input and feedback resistors, but as soon as 0.6 V is dropped over the feedback resistor the transistor start conducting and the amplification ratio plummets to a very low ratio. One transistor for +0.6V and the other for -0.6V.
Not all Opamp circuits have a negative voltage rail but then there minus input just has a bias voltage. The DC voltage on the minus input is not 0 volt in that case but still there is almost no AC signal because the current from the input and the current from the feedback are in balance. I hope this helps understanding these circuits better and why there is almost no signal on the minus input. If there would have been a large signal then that would have indicated a problem...
Second point:
The 2 diodes on the ULA cassette input are most likely 2 anti-parrallel diodes. They just clamp the signal to +/- 0.6 V for the input of the ULA, protecting it from a too high voltage on the input, destorying it. Very likely the added them afterward to protect the ULA better. You already saw what happened when the transistors were removed from the feedback. Without the diodes that may have been the dead blow for the ULA. In your case not because it has the diodes and one was even a short. But without it, it could easily be the end the ULA which is internally a low voltage sensitive component.
Very satisfying to see this repair. A shorted diode!
just watching you use a multimeter kind of shows maybe I was using mine correctly so I learned something here