HP 9825 Repair Part 8: memory fault finally identified, it boots again! 

6:10 Audio shifts to left channel. I do that all the time in editing.

"obtw I was an Intel fellow once..." Possibly the most understated flex in history ;)

I use to work for Hewlett Packard in the Green Stripe operation doing component level repair on boards for the 9825. We had test ROM and RAM modules that fit into the option ROM slots. These bypassed the operating system and provided working RAM that allowed you to wake a dead system and provide a host of diagnostics to trouble shoot the system.

That "Digi designer" breadboard thing is pretty slick! Good on you for sticking with it and thanks for sharing the process! The test circuit for the new chip really made it clear how it works.

It's alive! Very impressive. Love the troubleshooting of actually understanding the circuit and replacing broken chips, not just throwing darts at the board replacing every part and cap you can find!

Your channel is daily reinforcement of my stupidity. It’s like looking up at the night sky and seeing stars, so far away and incomprehensible. You and the people that help you are brilliant and have my admiration.

Great troubleshooting! Glad it's working again. Geez, two refresh circuits. Like discovering after years that your car had a 2nd steering wheel you didn't know about..

By looking at your efforts, I can't help but to think of the folks in the HP-department who build the 9825. The amount of paperwork, printouts and discussions back and forth just to be able to produce it is amazing.

I would just like to say that not only do you manage to be on the way to a complete repair, you do an impressive job of explaining it to an entertaining level. Such a joy to watch instead of all the cap replacers and alcohol sprayers you have on youtube as well. Thank you.

Thanks for the intermission. A smoke and a not so soft drink and I am ready to figure this all out! Excellent lesson in troubleshooting as always. Troubleshooting skill is something that I really feel some of us are born with, it comes naturally. For everyone else, they need to see and understand the though process a born troubleshooter goes through. You teach this better than anyone, and with a bit of humour.

Success! Now Mr. Fancy pants can use his computer again :D

Very interesting to follow how you are finding faults using logic analysers and other equipment! 😀 An alternate approach, especially since you are lucky to have another machine that works, would've been to acquire a thermal camera and image both the the working and faulty boards. Then compare and look out for chips that are either getting too warm or not warm when they should.

Good job finding that bad memory controller. Having the tools to accurately simulate the hardware logic step by step is invaluable in debugging

Run, rabbit run Dig that hole, forget the sun And when at last the work is done Don't sit down, it's time to dig another one

You did a pretty deep dive into the spaghetti logic. The simulator was a great tools for this. The fun part (for me) is actually re-tracing the design efforts by HP and getting a deeper understanding of how it functions. And now your an expert on internal DRAM refresh systems in HP 9825s :D Cheers,

Very cool. I never knew how dynamic RAM refresh works. It is funny how something like you only have one working word of RAM can escape your attention until late in the game. Lesson is never assume anything. Excellent use of the simulation and breadboarding the controller tester. It must have been a challenge to figure out how to implement the internal counter.

Well, thank you for that masterful lesson on memory refresh. The Digiboard setup actually helped a lot to visualize the whole thing (besides the Logisim simulation). I am constantly astounded by your amazing skills. Thank you Marc for creating these videos!

Really wonderful to follow this. As a junior 'tech engineer' at HP I worked with a 'technical salesman' to sell these, just as they were beginning to be replaced. But for sure I never 'dived so deep' as you are having to go! Bravo!

It lives! This has been an awesome journey and HP really loves to over-engineer their stuff. Logisim is probably my most used program, it's an absolute life saver at times!

I would like to see the face of Mr Fancy Pants when he gets the repair bill 😱

I have followed avidly the saga of the HP 9825. It's very interesting watching CuriousMarc and his colleagues patiently and tenaciously repairing this beloved unit. I cannot wait for part 9.

I've been binge watching this series, I'm remembering stuff I'd forgotten and learning more. Thanks for making the series 🥳

This series feels like when you try to get out of a labyrinth but you have to make sure to go into every dead end before succeeding. But that's actually a good thing, because the dead ends are the interesting parts.

Magnificent trouble shooting - Incredible patience. I've never considered simulation as a repair tool. You uncovered a major flaw in the memory test routine. It's challenging to do a good mem-test that is not mislead by hardware problems. You'd like data width to be as big as address width which you have here. I'll bet HP didn't do production testing with this routine.

To me, this was the most expected video of the week ! Superb job as usual. Well, cant wait for part 9 !

I learn so much about troubleshooting from your videos! It's helped me in my job as a controls and instrumentation engineer!

Congratulations! That makes my long-ago repair of a Keronix CPU board look simple.... I'd been wondering for a while about the nature of the RAM test; now it appears that it is indeed the old "test each cell individually, and don't worry about testing the addressing" approach. By the time I started in the industry, making sure all the address lines were working was kind of standard, but the 9825 is from an earlier era when I guess people hadn't been burned enough by addressing failures to worry about testing for them.

Amazing debugging technique. I really loved the simulation you created to understand the memory board.

Really good channel. I have no idea where you find the time to produce video's like these and troubleshoot this gear! Bravo!

Think this is best digital troubleshooting I have ever seen.

Figures you were able to get it going again. I mean, if anybody could, it would be you and your colleagues. After your work with the AGC, it’s not surprising at all. 😉

Can barely wait for parts 9-15 of this excellent series! 👍😉

Oh man, this was immensely satisfying to see. The tester for the Intel chip was an excellent example. I can excuse the mono audio for the W on the dram controller. Hooray!

On one hand it sucks that the HP 9825 got zapped...but on the other hand, the silver lining is that it has provided an excellent series of videos on how the machine works and how to repair it.

Hats off to you Marc for making us understand this beautifull logic incorporated in those nice old machines. For me a bridge too far (my brain is analog lol) but I can follow everything you do.

That logic sim was awesome. Love the breadboarding too. Really breaks down the woo woo magic of the chip for us :-) Nice to see it running again. A treasure for sure. It makes me want to troubleshoot my Apple IIe that I've been putting off

Heck yeah! This episode has me grinning, and thinking of potential real world uses for my 4 digit BCD/Hex to Decimal display I built in college. Good work, keep it up!

Are you going to decap the Intel chip, to see where exactly the magic smoke escaped?

Nobody repairs the device as you do. My friend do repair industrial stuff this way: he writes down all the chips, buys all of them, replaces all of them and it always works. But this type of hp gear have got the heart and I fully understand your chase to understand how it works, how it thinks, how it lives. My friends way: cut a brain and replace whole dead body. Your way is to replace only the dead lungs (or something)...

You're developing some Ben Eater type breadboard skills here ;-)

I can hear Doc Brown shouting in the street: "IT WORKS!!!"

Marc - you are now Mr. Fancy Pants 2021 :) Well done - I've really enjoyed following your journey on repairing the HP 9825 so far.

where were you a fellow at? I was on a tiger team in phoenix. I remember the original plotter for the optical stepper masks that was in san jose, it was in a halon room with emergency breathing apparatus, did you ever see it? I seem to remember it was 20x20 feet, but I don't remember exactly.

You know CuriousMarc is a tech guy first and a youtuber second when the test string he uses is "HELLO WORLD" instead of "LIKE AND SUBSCRIBE".

Worked in the seventies on DGC Nova line of computers. Large boards with about 150 IC's and no Logic analyzer, just a logic probe and analog scope.

Nice work, a logical approach is usually the key to solving these complex problems.

Really enjoying this series! So close to getting it all running again!

Last time I was this early, CuriousMarc didn't have his Magic Fancy Pants.

Intermission Time really made me hungry and I started smoking.

Great Video! one of the best ones I've seen to date!

I probably understand 10% of this at most (but more as I follow along) nevertheless I find this fascinating. Bravo Marc!

If this series was an action movie this episode would be the scene where the hero is almost beaten, he is stumbling and about to go down. Them somehow he digs deep, summons some sort of inner strength and gets to his feet and defeats the bad guy leaving the audience cheering.

Very very happy you get out of that mess! Congratulations Marc! But you still don't use tulip sockets after removing ic's and I think *that's a BAD THING* if you have to put your hands there inside again!

You guys might invest in one of the Retro IC Testers. Can test TTL, RAM, etc. Very handy.

Yeah! Great progress by now.. Only few steps left until all boards will be repaired.. 👍

Immense respect for sticking with this repair. Would that I had your magic powers, my pile of buggered test gear would be smaller.Thanks for another mostly incomprehensible video, logic to me is illogical, glad someone knows what it all means.....DA.

An Intel Fellow! Now we want to know more Marc!!

Marc , vous etes trop fort ! Le matériel HP des années 80 était au top niveau ! cher mais top niveau ! Your channel is really great !

That was a deep rabbit hole, if I've ever seen one.

It's so interesting how you can debug and examine these older machines. Just before things became so integrated that you could no longer really do so. As a software dev I have little idea how modern hardware engineers can design and debug modern hardware. There must be sooooo much reliance on simulation and using larger and larger blocks of known good designs. Probably similar to software dev in that regard.

I am overwhelmed and mind-numbed by your heroic effort in repairing this ancient system. Quite an amazing amount of work. So I confess and apologize that I don't watch every minute of your videos. Thus have a tiny, tiny question. Is it a requirement that you must use this MEMORY CIRCUIT BOARD? In other words, fix it? I imagine you could replicate the entire function of this 2K monster with a raspberry pi zero. ($5)

I think that's why TRON "Trace On" is a famous movie. It helps you out of a big sticky situation. Fascinating. Thanks.

Hi Marc, while seeing your repair, HP 5004 signature analyzer came to my mind. Couldn't this tool be used for fetching good signature form working boards and compare then to the ones of the broken, to narrow down the faulty regions? But I think, to work with signature analyses, defined signals for triggering and gating have to be supplied by DUT. In addition, understanding what goes on is good any time. This logic simulation tool is super, thanks for your intro.

Marc, you have to wear the fancy pants again, Bravo!

I love those small red dot matrix displays that hp used. Haven’t seen anything similar these days and I don’t consider the lcd’s a close runner up.

I have an old logic trainer which is considerably bigger than that one, but I don't know if it's fully working or not and it has no brand on it either so I don't know who made it. It has enough switches and LEDs on it to control an 8-bit microprocessor plugged into the breadboard.

20:45 Great Elmer Fudd impersonation there

@10:19 i see.. that refresh sound-cycle sounds different (longer) than the other ones before... @14:20 Just noticed that A. P. Products logo. I've got myself a PowerAce 102 breadboard, which is where i recognized it from! @20:45 Where'd you get those octal wheel-thingies from? @22:39 I think the closed captions got messed up again?

Really wonderful

Some may see all this work as a sisyphean task, but I love it ! Catching the 'look' of the dueling refresh logic was great insight! But my god all the test leads ... I'm curious how long it took to reset the testing apparatus after swapping out the chip?

I'm surprised that the calculator still has the original chiclet keyboard. They were replaced with a much nicer keyboard in the early 1980s. I worked with the 9825 and its successor the 9816 running 3060 board testers. They were almost indestructible and worked for decades. I wouldn't be surprised to still see them in production. They only weak point was the tape drive which had a tiny grain of wheat bulb used to detect the end of the tape. This would burn out and the tape would spin off the reel and it was a real pain to get the tape, and the elastic band driving and holding the tape on, back together.

Noodle Logic? Think that was one of my favourite Steely Dan albums back in the day.

Hurrah it boots!!

I know what my summer project will be now! Designing my own Digi Designer because that thing is awesome!

What a saga, brilliant work Marc! Noodles for lunch it is :)

Great fix and with an inexpensive chip!

Super result so far! but did you fit the overvoltage protection crowbar circuit mod to the power supply yet?

2:43 at this very moment when you revealed that there is an ASIC in the circuit on top of the million standard gates, I thought "I bet this is the one which is dead, just because it is more difficult to obtain" and see, there it is... Why always the unobtainable fails? Recently I was repairing an amp, and there is a complementary JFET input. One of the JFETs is still available for purchase, the other is not. Guess which one was dead...

I found the Digi Designer demonstration very inspiring. Is the breadboard on top your own enhancement?

I've never been this early for a Curious Marc video!

This is like watching a newly released season of "24" and just dreading the wait until the next episode! =D You end up binge-watching all episodes of the old seasons just to pass the time! At the moment I am rewatching his HP85 videos where he is refurbishing gummy capstan wheels in the tape drive! :P

Now I'd like to see Ken fix that controller chip.

I love that your channel is "Debugging best practice" Marc

Finally some positive progress! I was beginning to think this project was permanently stuck in reverse!

I’m finally starting to grasp the complexities of this troubleshooting after years of watching

Great job! Fun and interesting stuff!

Marc, is there a reason you've been preferring octal in this series? It seems like that machine would be more natively represented in hex. It's getting late now, but it seems like the hex values and addresses would be more helpful in interpreting the behavior -- things like reset vectors, special boundaries like RAM versus ROM, or boot ROM, that kind of stuff.

I have a bunch of NOS Bi-Polar PROMS and a Data I/O 29B. Happy to burn a ROM if needed!

Interesting. Remember that Woz worked for HP. The Apple 2 interleaves ram refresh, processor and video with a minimal chip count.

Learning so much on this series. Thank you.

Wow, what a journey!

What an intense journey this has been! Thanks for sharing it. If that board never fried, we wouldn't have had such an in-depth knowledge of the intricacies of Noodle Logic. Maybe you should take it easy for your next project, and do something simpler like, you know, landing a man on the moon =p

Another STELLAR series! I hope that something else in your museum breaks immediately after this series is complete! ;-)

25:23 Ken wanted to decap it eh :)

Keep up the great work!

There are still parts to fix, right? When should we expect a part 9?

Fantastic work!

I’m glad it finally boots!

Mr. Fancy pants would be happy!!!

24:08 A more fitting print program would had been "Hello doctor name continue yesterday tomorrow"