Ordinary people : "Oh no this piece of electronics died... let's buy a new one!" - CuriousMarc : " * Laughs In HP Equipment * You don't have my permission to die!"
@SteelRodent yeah, nowadays stuff is just made to be thrown away, and the same manufacturers that claim they're "green" and "environment-friendly" do that lol
This is also made from largely off the shelf components. It’s not a proprietary SoC blobbed onto a PCB. I guess we had the choice between repairable or small, and we chose small.
I just repaired my 24" monitor from 2008 with a set of new capacitors. Power supply issues. It was easy and fun to do and I wouldn't have known better to not throw it out if it hadn't been for these glorious nerds. This is a cliffhanger, I loved the HP 9825 hurtling toward the moon bit.
@SteelRodent Up to a point. HP only chips that are no longer made and thus are rarer than hens teeth makes such repairs seem like a quixotic undertaking.
The "2001 - A Space Odyssey" clip reminds me of an Easter egg I discovered in the HP 150 terminal firmware while alpha-testing another HP product. If you send it "Esc&a?" it responds by displaying "My mind is going..."
12 minutes long ? You certainly know how to feed the lions small pieces of meat,when they want the whole wilderbeast! Lol. lol...Its not a critique Marc you've made a tremendous effort to produce this content,it takes time,and you have family,and work as well,we are thankful to you guys.
lol - I get up every morning in anticipation of an update. I love these old HP machines having cut my teeth supporting the Royal Navy's use of the HP 9020 workstation in the 80's. They certainly don't build them that way anymore. Miss the old HP and Digital days
@@guycoder I'm learning a lot from these videos about old HP equipment.The quality of the machines is quite high,they were certainly ahead of the pack in capability,and HP never believed in a straightforward build,there is a lot of complexity there.
I once had a way-too-simple memory test I wrote pass on a prototype board where memory in fact did not work _at all_ (the control lines were connected incorrectly) ... turned out the last value written was just being capacitively retained on the data bus long enough to read it back successfully. Since then if I need a tiny trivial memory test my preferred one is to fill all of memory with the pseudorandom sequence and then verify all of it.
I'm absolutely fascinated in the way you are able to get across the way the system should be and isn't working. I understand electronics at a medium level, far from your level but with your explanations I feel that I could work on one myself but in reality if I had one in front of me my ape like hands would struggle to switch it on let alone repair it. I am just happy to keep watching your videos and living a life of a genius through your eyes. Thank you and keep up the great work.
Nice, this is going to be an epic repairathon. In a few months: "HP 9825 Repair Part 42: Wohoo, we're making progress! We replaced 5 more TTL chips today and the first 10ms of the boot up sequence seem fine now..." ;-)
When trying to debug machines with multiple bus faults I've sometimes used test roms built for the purpose. They first exercise all the rom bus, then do read/writes from various ram areas, then exercise the peripherals, all without using anything more than the CPU's registers. They're simple enough that you can hand-assemble the code.
So, I was debating on for a vintage repair project (a much less rare 1980s Compaq Portable) if I was going to try and repair the PSU or replace it, and well, this series has convinced me that I'm better off w/ a modern replacement PSU, especially since +5v on that PSU board is generated by a single zener diode and bypasses the crossbar circuit on that PSU ...
Well, these failures are going to pop up, Marc. Those TTL IC have all been stressed far passed their maximum operating voltage. I would start with all the bus drivers/bidirectional tri-state IC’s and latches first. Blanket replace all of them and any buffers that are tri-state. Just bite the bullet and replace them. Then you know they are all working. Anyway, be sure to put a crowbar on the +5vdc powersupply.
In the early days, I put 5V on the pcb and with a voltmeter to ground and the + connected with 10k pullup resistor to 5V. Then measure all the pins from each chip. When the result is in the forbidden zone, there is the problem. If done remove the resistor from the 5V and connect it to the ground, repeat all the measurements now with pulldown. It takes time, but it is more easy and safe. Succes.
I really like these videos, where you go through the heavy but excellent troubleshooting. It I understand its heavy repair.. but please continue, without just changing everything...
That's an interesting idea. We are delving into the area of voodoo debugging, but this is one of the few cases where it might actually work well. Don't have one of these, but maybe we can dig one out from one of our friends.
Very interesting diagnostics process, i would be wasting my time checking all the traces for shorts. But having access to proven boards would save a lot of time!
Marc coined a new word: Fragilized. I'll keep that one around for future use. The first code I wrote as a young engineer was HPL on the 9825 at UltraTech Stepper, brings back happy memories.
Nah, there’ll be another thing along when this is fixed and you’ll have to wait for those videos. Go back and watch the series on restoring teletypes if you want something to watch in the meantime 😀
re replacing chips.... can you use an in circuit chip tester of some kind to detect possible dead chips. I have a vague recollection this was something we did 30-40 years ago when we build stuff with "discrete" logic
Fascinating! You never give up do you? Meanwhile on my side, windows now update without even asking. So I just tell clients my network is gone and I don't have access to software licenses anymore to work on their projects. And they totally understand!... I give up...
Yes Marc,Gene Krantz was priceless! He said things which as an engineer i totally agreed with,but he had a very succinct way of putting things.What a cool troubleshooter he was.My model on how to make complex decisions under real pressure.It was a pretty good movie actually. Well,,that's why he headed up the mission control team. He earned his pennies that day.
Unfortunately I had the same with a programmable PS. Unique chips from USSR were breaking and the board is crap. Beyond digital board repair, there are lots of caps and resistors that should be replaced. I gave up
All this for lack of an SCR OVP. I would be reluctant to replace all the chips without a really really good reason. You might get different performance from new chips that could upset timing. I have put new chips in old designs with unfavorable results. Finding a new chip with decades old behavior is an interesting problem. Even though I have good de-soldering tools I would clip the chip leads and remove one lead at a time. You are getting good mileage from the logic analyzer and you are doing a great job. I feel the stress and frustration and it not even my calculator. I don't have the pants for one anyway.
It's testing fate plugging in known good components (cards) into suspect systems and visa versa. Some times you end up killing your once good components.
@@tigercat3864 short circuited components on suspect cards can make a bad day worse. If there are any CMOS ICs on any of the cards they love a good short. While modern CMOS components are more tolerant chicp of that vintage were not.
I suspect the Bus Transceiver went bad. It converts from the multiplex A/D bus to the separate A and D buses local to the RAM and ROM. Possibly a 74xx245.
Seems like the TTL chips don't like it up em but the NMOS chips survive. I recently accidently ran a 74HC device from 12V and was surprised when it survived.
Check the RAS and CAS signals to the ram and the latch at both sides incase something is pulling the bit low. Have a sock ready incase you need a filter!
There are few more horrendous electronic failures than overvoltage caused by a faulty power supply. When I was learning electronics, many many years ago, PSU Design 101 was "NEVER, EVER allow the output voltage to rise - ALWAYS design your PSU to fail in a safe state with zero output" Lots of ways to achieve overvoltage protection of course, and I was always taught that in high-end equipment, you should always have more than one safety system - a voltage comparator driving a cut-off relay AND a crowbar circuit to blow the fuse, for example. PSUs without at least two overvoltage protection schemes are just an expensive accident waiting to happen, especially with old-school TTL logic :(
With this title I expect that you'll use the Apollo Guidance Computer to tow it home. Write an emulator, put an AGC in the box, bam HP9825 fixed... Easy. :)
@@jlwilliams IIRC they had that already while restoring the AGC, Github to it github.com/virtualagc (had to look around, its linked in part8, dont know if in other Episodes aswell of the series)
Probably an insulation problem with the stirring fans on that memory board . . . We used to say that a program had reached Critical Mass when the repair of one bug tended to introduce 1+ε bugs. Same must be true of hardware at some point.
My guess, one of the ROMs ICs fried and is pulling the data bus low on that bit. The RAM refresh clock might be off-spec too. But the proprietary ICs are fine, so it can be repaired!
It seems what’s needed here is more sophisticated instrumentation, maybe a full main bus interface so you can disconnect the processor and drive the system manually and test every subsystem.
Prepared from the last part with shampoo and still it waits for your victory. Cascade of problems shouldn't trace you away, stay on track and with the help of bright minds the problem will be solved. Hope so..
I recall seeing a comment in an earlier episode (which I tend to agree with) that mentioned that even if you repair this unit, other chips which might be working now will start to fail sooner because they are damaged, thats probably what Marc is seeing here, some chips are now marginal and starting to fail. Will be interesting to see what happens here. whether it ends up as a modular replacement or a component replacement repair
Sounds like buffers and/or muxes to me. I'd expect the ram and probably rom to hold up a bit better. But with 13V for a while, I guess it could be anything.
I wish I had something more constructive to say other than, "Wow, this is so damn cool." But it is, and I find this channel extremely engaging... if you get what I'm saying. So here is a commant.
Ah, it's misleading. RAM starts from the top at 77777 and it stops finding any below 60,000. So it sees 8KW, but these are 16 bit words, so that makes 16KB. It's still missing the lower 8 KW on the same board (should go to 40,000), so clearly an address decoder fault, not a memory chip problem, since all that is in the same 16Kx1 chips.
@@CuriousMarc Ahhh okay, apple ogees.. i understand now.Made mistake earlier, i suggested to someone you impedance test input/output of logic on processor board to find anomalies,i meant memory,as processor seems to be golden as you would say! Well,, you guys did a gate by gate check on the AGC!! Don't know if you're up for it,Carl looks as if he's ready to buy boards..
check if the data bus has 74245 bus buffers / switches. That's where you will find your missing bit... I've restored many an old HP machine that had 245's fried. Since you had a big overvoltage , at this point i would just strip the boards, put in sockets and simply swap all the chips. Many will have had a sufficiently large hit that they will die in the next 500 hours of operating time. You will be trouble shooting this thing for years to come. And dump the roms into eprom too.
I'm no electronics wiz by any means, but could the 0's be down to a broken / blown trace on the mobo just in front of the first ram chip? i'm guessing the data comes out in parallel (and going by the schematics)
It is fun watching other people solve their nasty problems. I would not have such a great sense of humor as you do filming myself attempting to fix mine ;)
This series has been fascinating from the begining, that's great content there. Thank you very much. But I think that, at this point you are not far from being victim of the sunk cost fallacy. Maybe you should call it quit on this one, and use it for spare pieces? How valuable is it? Is it really worth it?
Marc&crew fixing old computers is honestly one of the most entertaining things I watch on YT. Thanks for all the awesome videos!
Same. The fact that they aren't over the top is what keeps me coming back. The ones that carry on like a banana have me scrambling to close it.
@@ImmortanJoeCamel Is that a Tac2 joystick in your profile picture?=)
@@ojkolsrud1 Yes. Yes it is. Nicely spotted.
"Repair it faster than it breaks" is a good maxim!
....that's how management works where I work sadly
I had a car like that once
I wish Microsoft could manage that with their software.
Ordinary people : "Oh no this piece of electronics died... let's buy a new one!" - CuriousMarc : " * Laughs In HP Equipment * You don't have my permission to die!"
@SteelRodent yeah, nowadays stuff is just made to be thrown away, and the same manufacturers that claim they're "green" and "environment-friendly" do that lol
This is also made from largely off the shelf components. It’s not a proprietary SoC blobbed onto a PCB. I guess we had the choice between repairable or small, and we chose small.
I just repaired my 24" monitor from 2008 with a set of new capacitors. Power supply issues.
It was easy and fun to do and I wouldn't have known better to not throw it out if it hadn't been for these glorious nerds.
This is a cliffhanger, I loved the HP 9825 hurtling toward the moon bit.
@SteelRodent Up to a point. HP only chips that are no longer made and thus are rarer than hens teeth makes such repairs seem like a quixotic undertaking.
The "2001 - A Space Odyssey" clip reminds me of an Easter egg I discovered in the HP 150 terminal firmware while alpha-testing another HP product. If you send it "Esc&a?" it responds by displaying "My mind is going..."
12 minutes long ?
You certainly know how to feed the lions small pieces of meat,when they want the whole wilderbeast!
Lol. lol...Its not a critique Marc you've made a tremendous effort to produce this content,it takes time,and you have family,and work as well,we are thankful to you guys.
lol - I get up every morning in anticipation of an update. I love these old HP machines having cut my teeth supporting the Royal Navy's use of the HP 9020 workstation in the 80's. They certainly don't build them that way anymore. Miss the old HP and Digital days
@@guycoder I'm learning a lot from these videos about old HP equipment.The quality of the machines is quite high,they were certainly ahead of the pack in capability,and HP never believed in a straightforward build,there is a lot of complexity there.
Next episode will be 5 mins long :P
I once had a way-too-simple memory test I wrote pass on a prototype board where memory in fact did not work _at all_ (the control lines were connected incorrectly) ... turned out the last value written was just being capacitively retained on the data bus long enough to read it back successfully. Since then if I need a tiny trivial memory test my preferred one is to fill all of memory with the pseudorandom sequence and then verify all of it.
I'm absolutely fascinated in the way you are able to get across the way the system should be and isn't working. I understand electronics at a medium level, far from your level but with your explanations I feel that I could work on one myself but in reality if I had one in front of me my ape like hands would struggle to switch it on let alone repair it. I am just happy to keep watching your videos and living a life of a genius through your eyes. Thank you and keep up the great work.
Marc I'm on the edge of my seat here. Please release part 6 so we can see how you dig yourself out of this hole.
Nice, this is going to be an epic repairathon. In a few months: "HP 9825 Repair Part 42: Wohoo, we're making progress! We replaced 5 more TTL chips today and the first 10ms of the boot up sequence seem fine now..." ;-)
When trying to debug machines with multiple bus faults I've sometimes used test roms built for the purpose. They first exercise all the rom bus, then do read/writes from various ram areas, then exercise the peripherals, all without using anything more than the CPU's registers. They're simple enough that you can hand-assemble the code.
When the HP is fixed, you should make a video about your HF station sometime :)
Apollo 13 made it home. I'm sure you will get this HP 9825 home! 😁
Loving this series. Don't give up!
Feel sad for the machine, but I dont feel sad that I get to watch you try and bring it back :D
So, I was debating on for a vintage repair project (a much less rare 1980s Compaq Portable) if I was going to try and repair the PSU or replace it, and well, this series has convinced me that I'm better off w/ a modern replacement PSU, especially since +5v on that PSU board is generated by a single zener diode and bypasses the crossbar circuit on that PSU ...
Well, these failures are going to pop up, Marc. Those TTL IC have all been stressed far passed their maximum operating voltage.
I would start with all the bus drivers/bidirectional tri-state IC’s and latches first. Blanket replace all of them and any buffers that are tri-state. Just bite the bullet and replace them. Then you know they are all working.
Anyway, be sure to put a crowbar on the +5vdc powersupply.
Mr. Fancy Pants gets me every time!
Could be why this 9885 bit the dust? Marc isn't wearing plaid bell-bottoms.
I liked the ham radio segment. 👍🏻
Really enjoying this series. I have faith you will get things back on the ground with no loss of life.
I see the persistence and enjoy it. as a retired software engineer it is so nice to watch.
In the early days, I put 5V on the pcb and with a voltmeter to ground and the + connected with 10k pullup resistor to 5V. Then measure all the pins from each chip. When the result is in the forbidden zone, there is the problem. If done remove the resistor from the 5V and connect it to the ground, repeat all the measurements now with pulldown. It takes time, but it is more easy and safe. Succes.
You guys will get it! With all the other things you have fixed, I know this one will turn out good, too.
I really like these videos, where you go through the heavy but excellent troubleshooting. It I understand its heavy repair.. but please continue, without just changing everything...
I find these videos very entertaining and educational. Thanks for the show Marc!
Thanks for keeping us informed of your progress. Good luck!
absolutely love the crew hide in processor remark as it is still good !
Marc, you and the crew are 1 of a kind
Great series so far! I am really enjoying watching it! Good luck fixing the ship of Theseus.
I think you need to replace the CO2 filters!
Marc,since you have a good board and a bad board ,you can use a Huntron tracker for fast faultfinding ,
That's an interesting idea. We are delving into the area of voodoo debugging, but this is one of the few cases where it might actually work well. Don't have one of these, but maybe we can dig one out from one of our friends.
@@CuriousMarc it's nothing more than a two-node curve tracer. You have a Tek one, don't you ?
Very interesting diagnostics process, i would be wasting my time checking all the traces for shorts. But having access to proven boards would save a lot of time!
Love this series, even if it makes me feel wholly inadequate from a repair skill perspective.
loving this series. love fault finding. much love from uk
Marc coined a new word: Fragilized. I'll keep that one around for future use. The first code I wrote as a young engineer was HPL on the 9825 at UltraTech Stepper, brings back happy memories.
Complete nerd out..... awesome content Marc thanks alot.I take it you denied access to the scrap yard really hope you get it repaired good luck sir !
I wish I discovered this channel *after* this series was completed!
Nah, there’ll be another thing along when this is fixed and you’ll have to wait for those videos. Go back and watch the series on restoring teletypes if you want something to watch in the meantime 😀
could go watch his apollo agc rebuild first
Go watch the Soyuz Clock Repair series. I found those extremely interesting
re replacing chips.... can you use an in circuit chip tester of some kind to detect possible dead chips. I have a vague recollection this was something we did 30-40 years ago when we build stuff with "discrete" logic
Fascinating! You never give up do you? Meanwhile on my side, windows now update without even asking. So I just tell clients my network is gone and I don't have access to software licenses anymore to work on their projects. And they totally understand!... I give up...
Oh no, looks like Data when trying to save Lal on the enterprise 😭
well you knew it was going to be bad, over voltage is never good! but your getting there. Don't give up!
Yes Marc,Gene Krantz was priceless! He said things which as an engineer i totally agreed with,but he had a very succinct way of putting things.What a cool troubleshooter he was.My model on how to make complex decisions under real pressure.It was a pretty good movie actually.
Well,,that's why he headed up the mission control team.
He earned his pennies that day.
Indeed, I do feel welcome!!
HP2225A : the first inkjet printer i have ever used! Great series of videos. The title of this one makes me sad
Unfortunately I had the same with a programmable PS. Unique chips from USSR were breaking and the board is crap. Beyond digital board repair, there are lots of caps and resistors that should be replaced. I gave up
Please continue to fix this. This is honestly amazing content
All this for lack of an SCR OVP. I would be reluctant to replace all the chips without a really really good reason. You might get different performance from new chips that could upset timing. I have put new chips in old designs with unfavorable results. Finding a new chip with decades old behavior is an interesting problem. Even though I have good de-soldering tools I would clip the chip leads and remove one lead at a time. You are getting good mileage from the logic analyzer and you are doing a great job. I feel the stress and frustration and it not even my calculator. I don't have the pants for one anyway.
yep. while I never had to do old school repairs like this, I watched them over other people's shoulders.
I'm hoping that ROMs are okay, maybe just a bad multiplexer chip. When it's fixed, I hope we see a crowbar overvoltage protection circuit added. 🤗👍
Good to know I might be able to get sneak peeks of an upcoming video by diligently monitoring 75 meters.
It's testing fate plugging in known good components (cards) into suspect systems and visa versa. Some times you end up killing your once good components.
My heart nearly stopped when he started substituting parts from the good one.
Yes,i concur, it is risky,it may give you quick answers,or it may give you a coronary,a gamble for sure,at this stage things are looking bad.
Nah, he fixed the power supply problem that was wrecking parts. He might have added an OVP crowbar too, I can't remember.
@@tigercat3864 short circuited components on suspect cards can make a bad day worse. If there are any CMOS ICs on any of the cards they love a good short. While modern CMOS components are more tolerant chicp of that vintage were not.
New video in your channel, let's stop everything and coffee time here :)
Same here xD
My thoughts exactly!
I suspect the Bus Transceiver went bad. It converts from the multiplex A/D bus to the separate A and D buses local to the RAM and ROM. Possibly a 74xx245.
i'm sure the ppl on 80M are glad to hear the processor is fine lol. 73s
Except I didn't hear him identify...
@@xenoxaos1 Sigh just a matter of time before the radio police showed up ...
I have nothing to say to you about this other than "go away".
@@detaart I guess he could have been shouting into a dummy load😂
Hello marc do you have any experience with early 1970s Pmos logic as I’m struggling with a Compucorp calculator. Thanks regards Chris
Thumbs up for the 7300 action in there. 73s from VK6
Seems like the TTL chips don't like it up em but the NMOS chips survive. I recently accidently ran a 74HC device from 12V and was surprised when it survived.
Check the RAS and CAS signals to the ram and the latch at both sides incase something is pulling the bit low. Have a sock ready incase you need a filter!
8:44 What's your Call sign?
This is absolutely gripping content
There are few more horrendous electronic failures than overvoltage caused by a faulty power supply. When I was learning electronics, many many years ago, PSU Design 101 was "NEVER, EVER allow the output voltage to rise - ALWAYS design your PSU to fail in a safe state with zero output"
Lots of ways to achieve overvoltage protection of course, and I was always taught that in high-end equipment, you should always have more than one safety system - a voltage comparator driving a cut-off relay AND a crowbar circuit to blow the fuse, for example.
PSUs without at least two overvoltage protection schemes are just an expensive accident waiting to happen, especially with old-school TTL logic :(
Have you thought of redrawing the schematics / relayout of PCBs in some contemporary software? Or does that violate a copyright?
With this title I expect that you'll use the Apollo Guidance Computer to tow it home. Write an emulator, put an AGC in the box, bam HP9825 fixed... Easy. :)
Could Mike S. make a gate-exact replica of the failed boards using an FPGA?
@@jlwilliams IIRC they had that already while restoring the AGC, Github to it github.com/virtualagc (had to look around, its linked in part8, dont know if in other Episodes aswell of the series)
@@jlwilliams With enough cheetos.
Ah, worked with this machine for years as part of a HP 3060 board test system (some time ago now :) )
BTW I have gone that route replaced all chips on a board it still wouldn't work. You may have melted a trace in the PCB somewhere.
Oh Marc this is a deep rabbit hole! I wonder if is not faster to replace all the 74x logic at once ;)
"I bet you my weekly allocation of bananas" is now one of my favorite sayings.
For sure let’s both use that soon.
Probably an insulation problem with the stirring fans on that memory board . . .
We used to say that a program had reached Critical Mass when the repair of one bug tended to introduce 1+ε bugs. Same must be true of hardware at some point.
My guess, one of the ROMs ICs fried and is pulling the data bus low on that bit.
The RAM refresh clock might be off-spec too.
But the proprietary ICs are fine, so it can be repaired!
It seems what’s needed here is more sophisticated instrumentation, maybe a full main bus interface so you can disconnect the processor and drive the system manually and test every subsystem.
I'm on the edge of my seat! But if the keyboard chip is bad, this one might be a goner. Depending on your views about the Ship of Theseus that is!
use an ir heat sensor to see which chips are getting hot. or randomly replace chips until it works
Prepared from the last part with shampoo and still it waits for your victory. Cascade of problems shouldn't trace you away, stay on track and with the help of bright minds the problem will be solved. Hope so..
Keep going! Keep going!
I recall seeing a comment in an earlier episode (which I tend to agree with) that mentioned that even if you repair this unit, other chips which might be working now will start to fail sooner because they are damaged, thats probably what Marc is seeing here, some chips are now marginal and starting to fail. Will be interesting to see what happens here. whether it ends up as a modular replacement or a component replacement repair
Sounds like buffers and/or muxes to me. I'd expect the ram and probably rom to hold up a bit better. But with 13V for a while, I guess it could be anything.
Is the assembler mnemonics for the HP processor rather looks like 6502 mnemonics?
Did you have to put a square chip in a round hole?
I absolutely loved that small ham radio clip, maybe somebody even read you, not sure
Best series on youtube!!!!
Two days and still no update! :( I feel like I'm stuck waiting outside the surgery for a loved one going under the knife.
I think it’s the tri-state bus buffers. Probe both sides to confirm if that’s where you’re losing the bit.
I wish I had something more constructive to say other than, "Wow, this is so damn cool." But it is, and I find this channel extremely engaging... if you get what I'm saying. So here is a commant.
don't give up, Houston!
LMAO @ need to repair it faster than it breaks! Really enjoying this series!
No room in the board sandwich to socket the TTL chips?
Amazing. But wouldn't 060000 octal be 24k not 16k of RAM? Is there ROM in the lower 8k?
Yeah i got that too.....Its 24,576 he got his calcs wrong
Ah, it's misleading. RAM starts from the top at 77777 and it stops finding any below 60,000. So it sees 8KW, but these are 16 bit words, so that makes 16KB. It's still missing the lower 8 KW on the same board (should go to 40,000), so clearly an address decoder fault, not a memory chip problem, since all that is in the same 16Kx1 chips.
@@CuriousMarc Ahhh okay, apple ogees.. i understand now.Made mistake earlier, i suggested to someone you impedance test input/output of logic on processor board to find anomalies,i meant memory,as processor seems to be golden as you would say! Well,, you guys did a gate by gate check on the AGC!!
Don't know if you're up for it,Carl looks as if he's ready to buy boards..
You'll fix it. I'm confident.
I love these long episode series, it's crisp and delicious like good candy ! La suite Marc... vite :)
KEEP GOING KEEP GOING KEEP GOING!!! this is very interesting stuff!!! don't give up!
Jeez. You guys have some patience lol
Now I wonder if my 85B has power supply protection...
Ok, we have to put this (life) back into this (dead HP 9825) using this (a bunch of antique bits and HP bops).
Just keep breaking it until it works!
check if the data bus has 74245 bus buffers / switches. That's where you will find your missing bit... I've restored many an old HP machine that had 245's fried.
Since you had a big overvoltage , at this point i would just strip the boards, put in sockets and simply swap all the chips. Many will have had a sufficiently large hit that they will die in the next 500 hours of operating time. You will be trouble shooting this thing for years to come. And dump the roms into eprom too.
I'm no electronics wiz by any means, but could the 0's be down to a broken / blown trace on the mobo just in front of the first ram chip? i'm guessing the data comes out in parallel (and going by the schematics)
Much more likely a broken/blow transistor inside one of the chips. PCB traces are not the weakest point in these things, by a large margin.
It is fun watching other people solve their nasty problems. I would not have such a great sense of humor as you do filming myself attempting to fix mine ;)
What's your Amateur Radio Call sign?
nice work
This series has been fascinating from the begining, that's great content there.
Thank you very much.
But I think that, at this point you are not far from being victim of the sunk cost fallacy.
Maybe you should call it quit on this one, and use it for spare pieces?
How valuable is it? Is it really worth it?
Do you document simple repairs, or do these things just not happen to you?
I love this channel.
Did you purchase the extended warranty?