HP 9825 Repair Part 5: We have an Apollo 13 disaster on our hands 

Marc&crew fixing old computers is honestly one of the most entertaining things I watch on YT. Thanks for all the awesome videos!

"Repair it faster than it breaks" is a good maxim!

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.

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!"

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.

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..." ;-)

Apollo 13 made it home. I'm sure you will get this HP 9825 home! 😁

When the HP is fixed, you should make a video about your HF station sometime :)

Mr. Fancy Pants gets me every time!

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.

i'm sure the ppl on 80M are glad to hear the processor is fine lol. 73s

Oh no, looks like Data when trying to save Lal on the enterprise 😭

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.

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.

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.

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. :)

Feel sad for the machine, but I dont feel sad that I get to watch you try and bring it back :D

I wish I discovered this channel *after* this series was completed!

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.

Please, let's join forces and find Mr. fancy pants. Would love to see him cameo the channel :D

New video in your channel, let's stop everything and coffee time here :)

Loving this series. Don't give up!

I think you need to replace the CO2 filters!

I liked the ham radio segment. 👍🏻

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 :(

Good to know I might be able to get sneak peeks of an upcoming video by diligently monitoring 75 meters.

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.

absolutely love the crew hide in processor remark as it is still good !

"I bet you my weekly allocation of bananas" is now one of my favorite sayings.

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.

Love this series, even if it makes me feel wholly inadequate from a repair skill perspective.

HP2225A : the first inkjet printer i have ever used! Great series of videos. The title of this one makes me sad

You guys will get it! With all the other things you have fixed, I know this one will turn out good, too.

Marc, you and the crew are 1 of a kind

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 ...

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.

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...

I see the persistence and enjoy it. as a retired software engineer it is so nice to watch.

Really enjoying this series. I have faith you will get things back on the ground with no loss of life.

loving this series. love fault finding. much love from uk

Ok, we have to put this (life) back into this (dead HP 9825) using this (a bunch of antique bits and HP bops).

Oh Marc this is a deep rabbit hole! I wonder if is not faster to replace all the 74x logic at once ;)

well you knew it was going to be bad, over voltage is never good! but your getting there. Don't give up!

Truthfully, I might would be evaluating this 9825 for its gold content at this point.

Thumbs up for the 7300 action in there. 73s from VK6

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...

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!

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.

I find these videos very entertaining and educational. Thanks for the show Marc!

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

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 ;)

Please continue to fix this. This is honestly amazing content

Marc,since you have a good board and a bad board ,you can use a Huntron tracker for fast faultfinding ,

This is absolutely gripping content

I absolutely loved that small ham radio clip, maybe somebody even read you, not sure

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 !

After something like this and swapping parts between the machines my fear is always that by the time you get to the end and it works, all the good parts will be on the bad machine.

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. 🤗👍

Never give up, never surrender! Who knew Marc was a ham! 73, KC1PAN.

Indeed, I do feel welcome!!

Ah, worked with this machine for years as part of a HP 3060 board test system (some time ago now :) )

Great series so far! I am really enjoying watching it! Good luck fixing the ship of Theseus.

cpu works, and fancy pants are still fancy, so all is good

Thanks for keeping us informed of your progress. Good luck!

don't give up, Houston!

"Let's think about this from a standpoint of status: What have you got in the calculator that's good?"

Best series on youtube!!!!

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 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.

Keep going! Keep going!

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.

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..

The biggest problem those thinkjet printers had is that the ink cartridges would weep. If you didn't use it for a while you'd end up with ink all over the bottom of the printer. It was a real mess. Otherwise they were great at the time cause they were pretty small for a printer that would use letter size fanfold paper and a hell of a lot quieter then a dot matrix.

Have you tried praying to the machine spirit and applying the sacred oils?

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.

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!

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!

Just keep breaking it until it works!

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

I think it’s the tri-state bus buffers. Probe both sides to confirm if that’s where you’re losing the bit.

when you good at problem solving, life gives you more problems...

LMAO @ need to repair it faster than it breaks! Really enjoying this series!

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

With CuriousMarc's HP equipment, "Failure is not an option!"

When MTTR exceeds MTBF. (And I sure hope you were transmitting into a dummy load.)

Soon the #1 Google image search result for HP 9825 will be a picture of Mr. Fancypants

Two days and still no update! :( I feel like I'm stuck waiting outside the surgery for a loved one going under the knife.

Oh... new CuriousMarc video on his unfortunate HP 9825..... exciting.. Wil he be able to repair it????

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.

Marc, I'd have never thought I'd hear ya on 80 meters.

use an ir heat sensor to see which chips are getting hot. or randomly replace chips until it works

I think Mr. Fancy Pants would approve!

Jeez. You guys have some patience lol Now I wonder if my 85B has power supply protection...

Ah nice vignette from the ICOM 7300.

Trous de mémoire, it happens to the best of us. Could it be the multiplexer? I thought you said it does address and data on the same bus by multiplexing? "We've never lost an Hp machine in my lab; we're sure as hell not going to lose one on my watch."

I love these long episode series, it's crisp and delicious like good candy ! La suite Marc... vite :)

He gets bananas! I'm still working for peanuts...