HP 9825 Repair - Part 14: Crowbar Protection Circuit 

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

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

you have now successfully protected your 9825 against Gordon Freeman!

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

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.

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.^^

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.

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

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 ;)

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

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.

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!

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!

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

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.

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.

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.

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!

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 😂

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.

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! 👍

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!

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" :-)

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

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.

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)

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

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

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.

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

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

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

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!

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 :)

Bravo! Thank you for explaining how the crowbar works.

Great job as always, Marc!👍

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

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

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.

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).

Congratulations!!!

Cool circuit! Great demo!

(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.

Nice work, thanks for sharing ☺

Yay, nicely covered.

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.

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.

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.

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.

Looks original. Great work.

I love this series, hope for new one ;)

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.

Great video!

Beautiful!

The capacitor discovery is especially satisfying :D

such a simple but effective solution!

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.

Great video, thanks for sharing

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.

that was a great explanation

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

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

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

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.

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

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

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.

Nice work to protect those HP's.

absolute perfect !

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.

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

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...

Wonderful!!

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!

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!

What a miracle this youtube That lets mere mortals as i Sit at the feet, of these giants of intellect And drink deep, from the font of knowledge

Magnifique!

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

Awesome

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.

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

Surely it should be labeled 'fp' for fancy pants rather than Hewlett Packard.

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

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.

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.

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.

Marc says "crowbar", but shows a prybar. Didn't thing we would notice, but we did!

In the Right of the Crow-bar-Pcb I See an original MPP-Core from magnetics 55208? It‘s amazing, See this Part in a an antic machine. You can buy this cores today, for switching Power-Supplys this cores work very fine, I think, there is Nothing better as iron-powder- cores MPP from magnetics in this days.

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

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}

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.

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?

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

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

The power dissipation of the lone Zener would be 21W, not 10W. :)

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.

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

need mod, I would have figured a zener with a fuse would be good enough but this is way better.

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).

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