HP 9825 Repair - Part 14: Crowbar Protection Circuit

Поділитися
Вставка
  • Опубліковано 23 січ 2025

КОМЕНТАРІ • 208

  • @PCBWay
    @PCBWay 2 роки тому +212

    What a pleasure that we are one part of it! Stellar job as you always do! 👍

    • @ShainAndrews
      @ShainAndrews 2 роки тому +15

      You guys seem to provide a top shelf service to the community. I really appreciate your efforts.

    • @PCBWay
      @PCBWay 2 роки тому +7

      Pleasure to do so 😀

    • @keithkneeland6849
      @keithkneeland6849 2 роки тому +1

      I know a coworker who had PCBway do some circuit boards for a project and they came out perfectly! Great company!

  • @bend1483
    @bend1483 2 роки тому +71

    I’ll admit that I was slightly sad that during the intro the over-voltage wasn’t described as a “big ouchy if you’re a TTL chip.” Hehe. :D

  • @LoMe64
    @LoMe64 Рік тому +1

    Thank you very much for this video!
    I own a HP 9815A for quite a few years which worked fine when I bought it at a fleamarket.
    Now, before powering it up again I will install that crowbar circuit - just in case.
    Great!

  • @captainproton449
    @captainproton449 2 роки тому

    LOTS of memories here! As a young engineer at TI, I had the fortune to build many automated test systems with the HP9825/9835/9845 family and the glorious HP test equipment line. We push HP hard with what we expected and they consistently overdelivered. I told countless people that HP computer controllers, as they were called at the time, were like bricks that compute. They simply worked, no fuss, no problems. Programming them was such a joy. I lost track of time countless times pouring code into those machines and watching them perform magic. They were the reason I morphed from hardware engineering into software where I spent the rest of my career and I always remember my time with the HP 9800 series as the best of times.

  • @StefanoBettega
    @StefanoBettega 2 роки тому +1

    The cleanest crowbar description I've ever seen. Congrats!

  • @rtlgrmpf
    @rtlgrmpf 2 роки тому +2

    Watching Marc draw these little circuits reminded me on my electrical engineering exam ages ago.
    Thankfully this time I didn't have to answer any questions in between.^^

  • @72polara
    @72polara 2 роки тому +1

    This is a good lesson for anyone who uses vintage equipment. This is a really easy circuit to design for your application and easy to test that could save you a lot of trouble.

  • @ojkolsrud1
    @ojkolsrud1 2 роки тому +6

    For a computer I've never used, and will never even see in real life (probably) - this sure has been a cool journey. I'm very happy for you Marc, well done all the way.

  • @brycejeannotte7699
    @brycejeannotte7699 2 роки тому +18

    Glad to hear that all of your 9825s are protected! Great job getting that machine working again.

  • @Eo_Tunun
    @Eo_Tunun 2 роки тому

    Cologne, Germany. 7am. Thrsday, April 21st 2022. I have my breakfast tea and just learnt how to make a crowbar circuit while doing so.
    Life is good!
    Thank you Marc! Another exceptionally well explained and entertaining video! 👍

  • @pileofstuff
    @pileofstuff 2 роки тому

    It's been many years since I ran into a crowbar circuit.
    Thanks for reminding me.

  • @LordPhobos6502
    @LordPhobos6502 Рік тому

    8:45 - I want to thank you for showing us this crowbar circuit, as well as what SCR is needed (I know nothing about SCR's).
    I'm adding these crowbars to my buck converters in a very big project, that I do not want to have to deal with exploded chips if the buck converters give up in the future (or the caps go bad).
    This video should be a masterclass for anyone designing electronics

  • @Damien.D
    @Damien.D 2 роки тому +3

    One of the enjoyment of watching this channel is the "stay tuned for next episode" despite the fact that this one is dubbed as the "final" one ;)

  • @MarcelHuguenin
    @MarcelHuguenin 2 роки тому +25

    I absolutely love how you always want to be as faithful to the time period when recreating items. This one was absolutely gorgeous. Very useful to see it working as well.

  • @jakobhovman
    @jakobhovman 2 роки тому

    I imagine a Crow sitting on top of your 9825 in your workshop...!
    Greetings and Thank You for Videos, Curious Marc, Crew and Crow.

  • @amyshaw893
    @amyshaw893 2 роки тому +4

    you have now successfully protected your 9825 against Gordon Freeman!

  • @MickHealey
    @MickHealey Рік тому

    You have a new subscriber. I stumbled upon this channel, and specifically this HP9825 repair project. I've just spent an entire evening watching parts 1-14. Absolutely incredible Marc, and really fascinating to follow along.

  • @hstrinzel
    @hstrinzel 2 роки тому

    FABULOUS REPAIR! Or PROTECTION from needing similar repairs! Amazing. NOW, PLEASE, PLEASE, show some more wizardry that you can DO WITH THE 9825.... THANK YOU!

  • @theafro
    @theafro 2 роки тому +2

    I've been inspired! I'm now waiting for my own design PCB to arrive for using with my old Acorn 8-bit machines. They use a switch-mode supply that is usually very reliable but there's always a chance it decides to send all the beans direct to the board.

  • @Ranger_Kevin
    @Ranger_Kevin 2 роки тому +15

    Such a simple, yet useful circuit. Even I could follow along for once :-)

    • @8BitNaptime
      @8BitNaptime 2 роки тому

      Yet not used at all in modern electronics. It is a crutch because of the unreliability of earlier components and designs. It is designed to prevent fires, not really protecting the circuit. It is not a precision circuit. Modern power supplies are far more reliable and have much better ways to really protect the device, like over voltage protection, that is a precision approach and actually disconnects the load, no blowing fuses, etc. SCRs and blowing fuses is a vintage thing...

  • @zxborg9681
    @zxborg9681 2 роки тому +3

    Been waiting to see this upgrade. I love that you tried to colour-match the original components too.

  • @pauljrogers2885
    @pauljrogers2885 2 роки тому +1

    An outstanding explanation of a crowbar circuit. Probably the best on youtube.
    I always wondered how they worked and now I know. I never guessed they were so elegantly simple.
    Thanks.

  • @mariodistefano2973
    @mariodistefano2973 2 роки тому +1

    Hi Mark, you did a Wonderful ADD-ON on that *delicious* machine where I first learned how to program!!!!
    Anyway, in the evil fate, when the crowbar will activate (in another evil fault case), I find the blown fuse really misleading:
    The blowing of the fuse WILL NOT tell you where the fault occurred, on the +5V or on the +12V.
    Any non-technician guy who should happen this, finding the blown fuse, will -at first- try to replace it giving another "kick" to the power supply circuit and maybe the rest of the machine.
    Very nasty, especially if the SCR will be damaged for the first excess of current.
    Personally, I would like to add some other circuitry to "store" in some way, where the fault first occurred, so one can have a "path" to the eventual fault search.
    Anyway, that's just my opinion.
    Thanks, Mark for these wonderful "lessons" :-)

  • @bennylloyd-willner9667
    @bennylloyd-willner9667 2 роки тому +1

    I was amazed by the SCR in my really early teens. I loved that I could use a momentary switch to turn on my little experiments, I used an N.C. momentary switch to turn off. It was so much more satisfying than using a normal switch. God, how nerdy I was even that young 😂

    • @absurdengineering
      @absurdengineering 2 роки тому +1

      I had an obscene fascination with our brand new (then) Hungarian city buses and their push-open push-close door control panel. The button indicators were controlled by the door state, with a wonderful delay as the doors operated. To open the door, the driver would press the red button, and in a second the green light goes out and the red one comes on. Press the green button to close the door, and in a second the red light goes out and the green one comes on. My child brain was taken over by that thing. I labored intensively to put some ragged looking equivalent with colored light bulbs and toggle switches I had on hand.

    • @bennylloyd-willner9667
      @bennylloyd-willner9667 2 роки тому

      @@absurdengineering is amazing how details can mesmerise a geek brain 😂. Nowadays it's almost cheating, just throw a microcontroller at any problem . I just got into smart home gadgets and remembered my first attempt at making a touch control. I was 12 or something, and I soldered a Darlington coupling together and connected it to a large piece of tin foil by the door to my room. I could never get it to work since I couldn't adjust it properly, it detected me over a meter before getting to the room 😂. Now I'm thinking of using that failure as a cheap room presence detector for Home Assistant 😁

    • @theoldbigmoose
      @theoldbigmoose 2 роки тому

      We all had amazing times in our childhood! We were not distracted with mindless social media. We studied real technology, and were driven to create and engineer. Oh to have that body now with the mind we all have now stuffed full of great knowledge and experience!

  • @wdavem
    @wdavem 2 роки тому

    Wow so now you've way better then totally fixed that thing. I've never seen anyone do a repair job like that! I am in awe!

  • @leon545b
    @leon545b 2 роки тому +1

    Glad to see you used the classic hp gold flash on *all* the traces. ;-) Nice work!

  • @TheRealMisterProtocol
    @TheRealMisterProtocol 2 роки тому +1

    I have absolutely no use for this computer, but it's so damn cute, and the programming interface is so intriguing, that I want one anyway! Well done!

    • @thesteelrodent1796
      @thesteelrodent1796 2 роки тому

      A lot of the technology and programming language from these old machines was actually carried over into their more modern sibling "pocket" calculators from the 70s, 80s and 90s, even though the controller aspect vanished as the machines became pure number crunchers. Things were shrunk down and optimized, but in many ways stayed the same. Wouldn't be surprised if you can make these old machines talk to a HP 48 (the last calculator HP made before shutting down) without much effort

  • @DantalionNl
    @DantalionNl 2 роки тому +4

    Just to add, an SCR is a current driven device not voltage as was suggested in the elevator music explanation!

  • @UsagiElectric
    @UsagiElectric 2 роки тому

    Excellent job and great explanation of a crowbar circuit!
    Though I can't help but think that we could totally build it with a big mercury thyratron so you get a nice blue glow indicating when the the crowbar has been tripped, haha.

    • @CuriousMarc
      @CuriousMarc  2 роки тому

      Go for it, Thyratrons are awesome! Although I’m not sure tube circuits are that worried about over voltage ;-)

  • @JamieWhitehorn
    @JamieWhitehorn 2 роки тому +1

    Awesome video and a great explanation and demonstration of how the protection circuit works. Simple but very effective.

  • @PlasmaHH
    @PlasmaHH 2 роки тому +7

    What I would be interested in is a scope trace of the 5V line when suddenly putting in various voltages on the input, i.e. how quickly it shuts down and how much the voltage really rises (you should probably keep some input caps there for extra realism)

  • @anthonytidey2005
    @anthonytidey2005 2 роки тому

    When designing circuit at several of the companies I work at eg Computer terminals and Best Inspection in the first company we put crowbars on every power supply on all larger circuits. Especially to protect the 7400 series circuits.
    Thanks for the video.

  • @TR3A
    @TR3A 2 роки тому

    Bravo! Thank you for explaining how the crowbar works.

  • @krypton2k
    @krypton2k 2 роки тому +2

    Interesting that the power supply you used to test also has a crowbar setting.

  • @Yrouel86
    @Yrouel86 2 роки тому +3

    Given the cutout for that capacitor it's likely HP designed the board for the 9825 and I bet they included it in the ones they had to fix (and that triggered the need for that modification in the first place) and then continued to add it by default in the subsequent model(s).

    • @ferrumignis
      @ferrumignis 2 роки тому

      I should think the models it was used on already existed when this daughter-board was designed, otherwise it would have been much more cost effective to integrate the crowbar into new designs.

  • @DKTAz00
    @DKTAz00 2 роки тому

    The capacitor discovery is especially satisfying :D

  • @JensAndree
    @JensAndree 2 роки тому +16

    You need to break something else now so that we get another 14-part instalment... ;)
    Jokes aside this was an interesting repair since there were so many blown components, but luckily the term "beyond economic repair" doesn't apply if you can repair your own stuff!

    • @CuriousMarc
      @CuriousMarc  2 роки тому +8

      Not to worry, things break every week on their own in my collection of vintage elecro-treasures. I can hardly keep up with it. Surely something greatly interesting will break by the time I finish this reply. Stay tuned for the next episode!

  • @obd6HsN
    @obd6HsN 2 роки тому +16

    I don't know whether this is the actual etymology, but railways with electrified third lines have always used - still do! - a crowbar in an emergency to isolate traction power. A worker can place the crowbar to create a short circuit between 3rd rail and grounded running rails to cause the fuses to rupture.

    • @greendryerlint
      @greendryerlint 2 роки тому +3

      I'll bet it makes for a nice light show too..

    • @phipli
      @phipli 2 роки тому +2

      If you drop a bar between the two running rails on a non-electrified track here it registers it as an axle, decides there is a train in section and switches the previous signal to danger.

    • @vinny142
      @vinny142 2 роки тому +2

      I'd love to know more about that because it sounds like a great way to end your life in a hurry, the amount of power going through the rails is not insignificant.
      My guess is that you're mistaken with the custom of using a crowbar to short busbars when they are powered down, so as to have a safety in case somebody activates power without permission while somebody is working on it.

    • @Broken_Yugo
      @Broken_Yugo 2 роки тому +1

      @@vinny142 Yeah, sounds like a hardcore arc flash event to me, maybe on the end of a 30 foot fiberglass pole?

    • @obd6HsN
      @obd6HsN 2 роки тому +5

      @@vinny142 I don't know whether the "My guess is that you're mistaken..." comment is aimed at me, but if so feel free to read NR/WI/ELP/27173 "Application of a Network Rail Standard Short Circuit Bar in an emergency". The "Short Circuit Bar" is known as a crowbar in the industry and has of course evolved from the item you'd buy at a DIY shop, over the years, but is designed to be applied in a live working situation. Hope that helps.

  • @wiregold8930
    @wiregold8930 2 роки тому

    I've still got an HP 85 that worked last time I plugged it in ...~1994. It used to run my test bench at HP up until then.

  • @TheDiveO
    @TheDiveO 2 роки тому

    one of those rare cases where throwing a spanner (or crowbar)into the works actually saves the day.

  • @carpetbomberz
    @carpetbomberz 2 роки тому

    Crowbar Protection Circuit = no more over-voltage TTL ouchies. But I will say I have learned a lot along the way. And we got to meet Mr. Fancypants from the HP catalog, very auspicious.

  • @Broken_Yugo
    @Broken_Yugo 2 роки тому +2

    This is the kick I needed to design my own crowbar for all the 7805 regulated game consoles and stuff I own. They can, and do, fail short without warning or reason from time to time, and nuke whatever logic board it was powering. I've had it happen twice now, with vintage and modern 7805s.

    • @volodumurkalunyak4651
      @volodumurkalunyak4651 2 роки тому

      7805 probably fail from an power transistor secondary breakdown. SOA protection in 7805 isn't as good as it should be. There is an simple and reliable fix: replace it with a switching regulator. Switching regulators operate power transistors primarly in a saturation and cutoff regions, away from an dangerous secondary breakdown. Feeding an 7805 with an 9V instead of 20V might also help.

    • @Broken_Yugo
      @Broken_Yugo 2 роки тому

      @@volodumurkalunyak4651 slapping in a switcher tends to come with it's own problems. I want a bolt on fix, not a re-engineering of a power supply three or four times. Both failures were reasonably heatsinked, fed about 13 volts as designed, not running hot.

  • @edwinschuddemat8280
    @edwinschuddemat8280 2 роки тому

    Great job as always, Marc!👍

  • @pdrg
    @pdrg 2 роки тому +11

    Something to maybe carry over from building restoration, since you're archiving our history(!) is that modern additions need to look modern, so there's no risk in 100 years of confusing the future history researchers! That means in this case for instance you might make the board an unusual colour or just use the rear silk screen to give the date and history and context. And your very deserving name, too :)

  • @campbellmorrison8540
    @campbellmorrison8540 2 роки тому

    Wow I love that PCBway board, wish we had a service like that in NZ.

  • @MeriaDuck
    @MeriaDuck 2 роки тому +1

    13:00 that power supply has 'crowbar adjust' on it, that's no coincidence I presume 🤣

    • @Broken_Yugo
      @Broken_Yugo 2 роки тому

      Purely coincidental, nicer bench supplies often have some sort of over voltage protection feature to prevent accidental adjustment to a damaging level when working with sensitive parts.

  • @tony359
    @tony359 2 роки тому

    such a simple but effective solution!

  • @jensschroder8214
    @jensschroder8214 2 роки тому

    Also useful for the C64, for example.
    The original power brick can sometimes break through the 7805 as well.
    9 volt AC rectified then you are also quickly on 12 volts DC instead of 5 volts.

  • @JohnRineyIII
    @JohnRineyIII 2 роки тому

    I wonder why they put those thru-holes in the edge connector.

  • @erikdenhouter
    @erikdenhouter 2 роки тому +3

    (This text is obsolete, see my comment below)
    Pay attention to the hold current of the SCR (thyristor): when the gate current stays under that specific value, it will short, but it won't clamp, it will return open when the voltage (gate current) goes to zero. You could have that situation when one of the monitored voltages rises very slow. Oscillation could or will occur then.
    You can make use of this property so in specific circuits you don't have the need to shut off current to un-clamp the SCR, but in this case you do not want it.

    • @absurdengineering
      @absurdengineering 2 роки тому

      There’s a way around the slow rise problem: sample the output voltage through a diode and a capacitor peak holder, and use a negative resistance circuit to ensure the SCR gate gets driven hard as the reference droops due to gate current load. As the monitored output collapses, the circuit has its own power to keep the gate on until it latches on. It takes just milliseconds anyway. Ted Yapo put a two-BJT negative resistance circuit up on Hackaday. That circuit itself could act as a voltage sensitive trigger - no zeners needed :)

    • @erikdenhouter
      @erikdenhouter 2 роки тому

      @@absurdengineering Sorry, I made an mistake. The hold current is not the current through the gate, the hold current is the minimum switched current through Anode-Cathode. As long as that current is above the hold current (40mA at 25 °C for the 2N4441) the device will latch. In this case it is a full power supply with many amps that will be shorted, so enough current. After that, the fuse must have taken over.
      If still in doubt about a slow rise response, you could also think of two SCR's, one small one to switch the gate of a power SCR.

    • @erikdenhouter
      @erikdenhouter 2 роки тому

      @@absurdengineering "That circuit itself could act as a voltage sensitive trigger - no zeners needed :) " Found it with that line.
      Need to think hard to understand it, but I will try.

  • @noelj62
    @noelj62 2 роки тому

    Looks original. Great work.

  • @Nighthawke70
    @Nighthawke70 2 роки тому +2

    Any vintage system worth the time and energy should have a crowbar board put in it to preserve it. Commodore Business Machines built a lousy power supply for the legendary C=64. The diodes in it would break down, causing +5V to become +12V, bricking a fun piece of history. For those who could not afford a replacement power supply (or wanted a challenge), they bought (or built) a crowbar to go between the potted and sealed power supply (it looks cool too) keeping the valuable components inside from frying. Others simply bought a replacement supply, or gutted the sealed power pack and built their own. A couple of vendors sell a multi-plug power supply for the Commodore computer family, since they all required the same voltage ratings, just differing in the power socket shapes and pinouts. The C=128 used the same voltages and pinouts the C=64 did, it merely drew more amperage. Any decently built power supply these days can shoulder the loads these systems require, so there you go. This goes the same for any of the CBM models, save for the 128D which used an internal power supply. And, yeah, the Amiga family, which have their own caveats.

  • @samuellourenco1050
    @samuellourenco1050 2 роки тому

    Crowbar circuits are very effective, and when using them along with a TVS diode in parallel and a fuse in series with the whole thing, or even better, a PPTC fuse, it makes the circuit virtually indestructible, unless you use mains voltage. I usually use a PPTC, a TVS diode in parallel, the crowbar and then a Schottky diode in series.

  • @TeslaTales59
    @TeslaTales59 2 роки тому

    Nice work to protect those HP's.

  • @acmefixer1
    @acmefixer1 2 роки тому

    Do you think that this concludes the 9825 repair series?
    This is a very commendable project.
    What I would like to see is an assessment of the power supply and how to change it to make it less prone to failure and also make it less wasteful of power. What I'm trying to say is this protects the 9825, but it still doesn't drop the voltage before the pass transistor (that failed) down, from 13V to a more sensible 7 or 8 volts, like most 5V power supplies of that era. My thought is that lowering the input voltage to the pass transistor would lower its power dissipation and reduce the chance of it failing.
    Thanks, Marc - stay curious!

  • @castlecodersltd
    @castlecodersltd Рік тому

    Nice work, thanks for sharing ☺

  • @cannotbeleftblank6027
    @cannotbeleftblank6027 2 роки тому +1

    10:17 Why not choose thicker copper for the board? 30 Amps is a lot for those tracks, even if they are wide and (hopefully) only need to carry the current for a short time.

    • @craigruff8916
      @craigruff8916 2 роки тому +1

      It only has to sink that current long enough to blow the fuse on the AC input to the transformer. This will typically happen quickly enough that the tracks won't heat up enough to be a problem.

  • @OmeedNOuhadi
    @OmeedNOuhadi 2 роки тому

    Congratulations!!!

  • @bfx8185
    @bfx8185 2 роки тому

    I love this series, hope for new one ;)

  • @crazyirishman121
    @crazyirishman121 2 роки тому

    What note book are you using at 11:00 ?

  • @SomeGuyInSandy
    @SomeGuyInSandy 2 роки тому

    Cool circuit! Great demo!

  • @forbiddenera
    @forbiddenera 2 роки тому

    Looking at your repro board, reminds me I just saw a post on hackaday for rounded/curved traves plug-in for kicad to make boards look like old hand laid ones. Maybe 👀 something you could use for repros

    • @CuriousMarc
      @CuriousMarc  2 роки тому

      Yes I have that plug in! I used it int the HP 8082 repair episode 2. Nice idea. But the HP boards were plotted out, so they have nice geometrical traces.

  • @davidmcgill1000
    @davidmcgill1000 2 роки тому

    So what's that crowbar adjust on the bench power supply about?

  • @KeritechElectronics
    @KeritechElectronics 2 роки тому +1

    A crowbar circuit prevents the unforeseen consquences of electronic engineering from jumping on your head :)
    "Pretty bad Electroboom moment" haha, that was Fran-tastic!
    Good ideas on crowbar circuits, though I'm thinking in terms of a N-MOSFET as a shunt device rather than SCR.

    • @siberx4
      @siberx4 2 роки тому

      You can in theory use any power switching element as your "crowbar", the SCR just has the convenient property that it self-latches once triggered. Without this, your circuit might oscillate around the trigger point depending on the particulars of the failure you're trying to protect against, so you need to account for that if you're using something like a MOSFET.

  • @james77011
    @james77011 2 роки тому

    What is the purpose of the capacitor?

  • @byteborg
    @byteborg 2 роки тому

    Beautiful!

  • @twobob
    @twobob 2 роки тому

    Yay, nicely covered.

  • @tocsa120ls
    @tocsa120ls 2 роки тому +3

    2:57 how to use a Zener as a really crappy LED
    6:00 and that's almost exactly the circuit my "cheap in the 70s, expensive now" Pioneer amplifier uses to clamp the output should some power transistor decide to send 50V DC to the speakers

    • @der.Schtefan
      @der.Schtefan 2 роки тому

      All diodes can emit light. ;)

    • @SeanBZA
      @SeanBZA 2 роки тому

      Yes, should have a 22R resistor in series with the zener diodes, plus another forward biased diode, because if the 12V or 5V rail is not present, and the other rail goes to high voltage the crowbar will probably not trip, but instead just burn out the one zener diode.

  • @martinsweltdertechnik1868
    @martinsweltdertechnik1868 2 роки тому

    You could also use a suppressor diode (TVS). No PCB needed.

    • @CuriousMarc
      @CuriousMarc  2 роки тому

      Nope, can’t do that. Completely different devices for a completely different usage case.

  • @dentakuweb
    @dentakuweb 2 роки тому

    that was a great explanation

  • @andrewhall2554
    @andrewhall2554 2 роки тому +1

    I thought it was kind of funny that the 6727B power supply used for testing the crowbar circuit has "Crowbar Adjust" adjusting knobs on it...

  • @albinklein7680
    @albinklein7680 2 роки тому

    Damn. I had a similar odyssey a few weeks ago. Someone plugged a 20V center positive notebook-power supply in an very, very expensive allen&heath audio mixer which needed 12V (center negative, of course) The transistors in the internal supply failed short and everything which rudimentarily resembled a semiconductor was blown to pieces.

  • @Hiphopasaurus
    @Hiphopasaurus 2 роки тому +1

    Marc: "Works fine"
    HP: "error 07"
    Classic.

    • @CuriousMarc
      @CuriousMarc  2 роки тому +2

      That’s because I just entered random text to print, not a proper command.

  • @skfalpink123
    @skfalpink123 2 роки тому +3

    How I miss that world, where electronic components were large enough to be picked up with two fingers, had multi-coloured markings for identification, and were mounted through holes in a PCB.

    • @absurdengineering
      @absurdengineering 2 роки тому +1

      One has to love the low parasitics of surface mount packages, though. And the reduction in resource use that goes as cube of linear dimension. Given the omnipresence of electronics, just the savings on plastic and copper from not having to package chips in DIP and PLCCs are enormous. Stationary appliances are not size constrained, and in principle we could put control boards with DIP and through hole components into those. But it would take so much more material on a world scale. There’s plenty of 0.1” SO chips available, and 1206 packages for passives are easy to work with as well.

  • @AlphasysNl
    @AlphasysNl 2 роки тому +1

    Neat little circuit. Would've saved my Atari bigtower build. The component you're calling an SCR, is it the same as a Thyristor? The schematic symbol seems to suggest that.

  • @chupathingy5862
    @chupathingy5862 2 роки тому +47

    Here's an amusing idea; Have you considered decapping the dead chips to observe the failure mode? Like, what overvoltage physically did to the die? It might be interesting to see how they've been mangled.

    • @CuriousMarc
      @CuriousMarc  2 роки тому +44

      Yes we did decap the Intel 3242 memory controller that was totally dead.But visually, we could not see anything wrong with it.

    • @rogervanbommel1086
      @rogervanbommel1086 2 роки тому +2

      @@CuriousMarc did you decap any others, like the inverter that is totally dead

    • @projectartichoke
      @projectartichoke 2 роки тому +23

      Moderate over-voltage in TTL (and other) integrated circuits and transistors (anything with P/N junctions) first results in junction breakdown. There isn't much to see without an extremely powerful microscope because the breakdown happens on a molecular level. With high enough voltages and currents in integrated circuits extreme heating can occur that can melt bond wires or produce visible scorching or carbonization and even detonation. With a high number of integrated circuits on a circuit board, each will be sinking voltage and current in parallel which will have a tendency to reduce any kind of spectacular failure modes.

    • @SeanBZA
      @SeanBZA 2 роки тому +3

      I had a 5406 PM3 decap itself, and it did look a little discoloured, and had failed outputs that were always on. Ironically a device with 30V rated open collector outputs, and the only device harmed when the pass transistor, or the power supply, decided to be unhappy (did not check, replaced every part in power supply, including the resistors) and the crowbar circuit had quietly failed as open circuit. That also was replaced in total, all new parts. Rest of the 500 or so TTL, MOS and NMOS parts, including some that are made from severe obsoletium, in that both the manufacturer (Mostek) no longer exists, but datasheets for the device are also unobtanium, and the only reference you get to the MK4007 are for uncommitted CMOS transistors, not the actual serial CCD memory device.
      Did not even change the IC, despite having a lot of them in the flatpack format, as there were even more complete boards in stock, brand new, never been used, just had to fix the standard failed wet slug, rubber seal tantalum capacitors, that were long dead. Probably changed at least 5kg of them in one year.

    • @letsgocamping88
      @letsgocamping88 2 роки тому +2

      I wonder if an electron microscope would do it? maybe one for applied science to look at?

  • @esra_erimez
    @esra_erimez 2 роки тому

    I'm glad to hear that Mr. Fany Pants didn't die

  • @stinchjack
    @stinchjack 2 роки тому

    Ah cool I'm now planning to add one of these to my project in case my LM7805 fails :D

  • @floeki-jekker
    @floeki-jekker 2 роки тому

    absolute perfect !

  • @thomasives7560
    @thomasives7560 2 роки тому

    Tektronix has a similar one-supply circuit, PN: RA3241-00. That part uses the 2N4441 SCR and a similar zener/resistor setup, but it is *FUSED*, which makes it a nice drop-in for various equipment. The Tek part is designed to sit on the side of the main board and attach with a couple of small wires across the supply, so it is adaptable to almost any installation. If anyone is interested, let me know and I will send pics and a schematic, or just put them on my YT channel for everyone. Cheers!

  • @wdavem
    @wdavem 2 роки тому

    I'm nowhere as experienced as you but I have seen things like time base correctors get overvolted such that they are shorted all over the place that are given up on if they aren't otherwise good enough. When I get the total lost cause like that I just want to put a car battery across it on it's way to the e waste.

  • @BlueSkyScholar
    @BlueSkyScholar 2 роки тому

    I still have a number of mystery HP zeners I wish I could get a cross on, Ahem.
    1902-3190 is 13V I have on hand, @2:20 I see 020 952 (1902-0952) 5.6V which is a hole in the Keysight Find-A-Part, but I found else where and have on hand too. I also have green CGW .01 capacitors if you really want to go for authenticity.

    • @CuriousMarc
      @CuriousMarc  2 роки тому

      Since the numbers on the sheets are in complete disorder, it takes hours sifting through them to find one part number. For Zeners you should just measure them, takes a minute. I have not found a good way to digitize the fiches yet, but I'm working on it. Then everyone will be able to do a search!

  • @annwan9557
    @annwan9557 2 роки тому +1

    Oh I've heard of crowbar protection circuits but never knew how they worked
    Well now I know
    Awesome video as always

  • @ruperthartop7202
    @ruperthartop7202 2 роки тому

    Great video, thanks for sharing

  • @silverXnoise
    @silverXnoise 2 роки тому

    I don’t think I have ever considered describing the troubleshooting and repair of an obsolete albeit fancy calculator as f#!&ing *epic* but here we are.

  • @lordskeletorde
    @lordskeletorde 2 роки тому

    Great video!

  • @tekvax01
    @tekvax01 2 роки тому

    Marc, Just curious, could you maybe use an automatically resetable polyfuse instead, in some manor, as to not sacrifice the main primary fuse?

    • @Broken_Yugo
      @Broken_Yugo 2 роки тому

      Maybe if you expected the thing to blow frequently, like in some sort of test setup, not the case here.

  • @Shadowdncer
    @Shadowdncer 2 роки тому

    Doesn't that circuit, at least in theory, provide a path connecting the +12V supply to +5V? I'd maybe put some regular diodes back to back with Z1 and Z2.

    • @volodumurkalunyak4651
      @volodumurkalunyak4651 2 роки тому

      No. SCR contains a diode, gate to cathode. If you forward bias it:
      1. Gate - cathode voltage is approximately same as in 1 silicon diode in forward direction.
      2. SCR fires and starts conducting anode-cathode.

  • @wdavem
    @wdavem 2 роки тому

    I want to start thinking about putting crow bar circuits in the old video devices I work on (that don't have them already). The first time I ever successfully fixed a power supply in my life it was the crow bar circuit that had failed on a big 'old sony umatic vo2610, so I remember that sort of scr layout { IF I REMEMBER RIGHT anyway}

  • @travishayes6678
    @travishayes6678 2 роки тому

    Part CGW-CAC03Z5U104M050A teal 0.1u axial ceramic caps are still available on the surplus market. I ordered some recently, somewhat hoping they would be red (which seems to pre-date the teal coating).

  • @scowell
    @scowell 2 роки тому

    Yes! I can sleep now. Especially amusing that the board was designed to fit the older machine... yet we don't hear of any 9825's with this board in them?

  • @GadgetUK164
    @GadgetUK164 2 роки тому

    Wonderful!!

  • @Rob2
    @Rob2 2 роки тому

    Here we usually call that a Thyristor. Although some sources suggest they are not the same thing.

    • @davidv1289
      @davidv1289 2 роки тому

      Here we call them Thyristors as well, but usually when referring to a group of devices - SCRs, Triacs, etc. When referring to an individual device we tend to say SCR or Triac. Interestingly, the vacuum tube (valve) equivalent is called a Thyratron. Regards, David.

    • @Rob2
      @Rob2 2 роки тому

      @@davidv1289 Yes, the origin of the name Thyristor is a combination of Thyratron and Transistor.
      I am used to calling them Thyristor and TRIAC (the AC version).

  • @soundcheck6885
    @soundcheck6885 2 роки тому

    Hope the brown wire doesn't blow before the fuse in case of an overvoltage fault! I'm paranoid enough that I would have used two brown wires (15A each) as there seem to be extra ground terminals on the connector. 😜

    • @craigruff8916
      @craigruff8916 2 роки тому

      The input fuse is only 0.75 A, so it will blow long before the 18 ga (or there abouts) wire will.

    • @CuriousMarc
      @CuriousMarc  2 роки тому +1

      That’s because it only needs to support the current for a few milliseconds or so, not continuous (same for the SCR by the way). No large wire nor large SCR required.

  • @flugschulerfluglehrer
    @flugschulerfluglehrer 2 роки тому

    For once I was able to understand what Marc is explaining. This is a first. 😆

  • @TilmanBaumann
    @TilmanBaumann 2 роки тому

    I'm surprised you clip at the exact voltage and not a few ten milivolt over?

    • @TilmanBaumann
      @TilmanBaumann 2 роки тому

      Actually, you say 6 and 12v but the parts are not (at first glance) nevermind then.

  • @alexlang5308
    @alexlang5308 2 роки тому

    Didn't call that SCR part thyristor?

    • @CuriousMarc
      @CuriousMarc  2 роки тому

      Both are related. A Thyristor is sort of a double SCR that you can use for switching AC. A SCR is for DC. [EDIT] No you are right! They are the same. The Triac is the AC version!

  • @Gengh13
    @Gengh13 2 роки тому

    I prefer to use the TL431 instead of a zener for easier voltage adjustment.
    It is a good idea to have a crowbar circuit if there is any chance of a higher voltage being injected in a rail with multiple/expensive ICs.

    • @RobertSzasz
      @RobertSzasz 2 роки тому +1

      And make sure you have fuses in the right spots, or things can get interesting and smelly

  • @edgeeffect
    @edgeeffect 2 роки тому +7

    Not tempted to put a "CM" logo on it????? (in the style of the original "HP" logo) ;)
    Great to see the 9825 alive again.

    • @CuriousMarc
      @CuriousMarc  2 роки тому +4

      Oh yes I am. And I have. Have you ever heard of the CM 59401M parity generator? Here: www.curiousmarc.com/more-projects/fpga-hp-ib-parity-generator

    • @mrnmrn1
      @mrnmrn1 2 роки тому

      @@CuriousMarc The link does not work.

    • @CuriousMarc
      @CuriousMarc  2 роки тому

      @@mrnmrn1 It didn’t! I edited it, try it again now.

    • @mrnmrn1
      @mrnmrn1 2 роки тому

      ​@@CuriousMarc Thanks! Nice stuff. Weird that the Numato Lab logo also somewhat resembles the old HP logo.

  • @someoneoutthere7512
    @someoneoutthere7512 2 роки тому

    Marc, Where did you get those pins for the PCB?