9810 Power Problem?


My 9810 has gotten flaky. The problem seems to be somewhere in the power supply circuit. There seems to be some sort of power-is-good-boot-the-CPU function in the calc because after you turn it on, it normally takes a few seconds before it comes to life. Recently my machine isn't getting this "boot" signal as it should, I think. Sometimes it does work, but then goes dead after a minute or two. Other times it just sits there with all LED's off or some of them on but with no keyboard response. But the fan always works and all 5 marked power supplies (+12, -12, +16, +20, +24) measure good and are clean (no noise).

Does anyone have any experience with this or any idea about where this supposed boot-up circuit might be located in the 9810?




There is a power-on reset circuit, which pre-loads all the microcode PC flip-flops with 1's to start the CPU at the right point.
The reset circuit is on the 5V PSU card (09810-66581), andis built round a '121 monostable chip. The A inputs are grounded, the B input comes from an RC network (3k3 to +5V, 1uF to ground). The monostable has timing components of 27k and 1uF. Finally, the Q output of this monostable is buffered by a simple transistor stage and is fed out of the rearmost pin of the front card edge connector.


I don't think that the Problem comes from the power supply since when e.g. an overvoltage is detected the 5V line is completely shut down. The same occurs when you switch the power off and on again too quickly.
The symptom appears to come from a weak TTL IC. Some months ago I had a similar problem in my 9830: it hang up every time after a few minutes and did'nt start up when switched off and on again. I had to wait about 5 minutes before the machine worked again. I localized the thermic fault by means of cold spray. It was IC6 (74H74) on board 9810-66512.

Good look,


Thank you both for that great information. I'll check out the monostable circuit that you described, Tony, to see if it's working correctly. If that's not the problem I'll try the cold spray technique, Achim.

Just one question for either (both of you)? How did you get access to the boards while the machine is running? Do you use card extenders? If so, how to you get to the boards in the stack positioned in front of the fan? Right angle extenders? HP must have made a special test jig for servicing these machines. Of course all that the field techs would do would be board swapping, but what did the real service techs use?



Hi Katie,
I did build two simple bus extenders for the 2*18 nad 2*15 connectors of the CPU boards. I made them from small PCBs with gold contacts, the matching PCB connectors, and ribbon cable. If you are interested, I can mail you some photos.

To access the boards in the memory cage is principically the same, but for some boards you need one more bus extender with 2*10 contacts. By using ribbon cable which is long enough you can bend it out of the card cage and lay the card under test on top of the calculator.



Firstly a couple of points from your first message. The fan doesn't tell you a lot -- it's an AC fan running off the mains side of the transformer (in you case (120V), straight off the mains, in my case (240V), the mains transformer primary acts as an autotransformer). Secondly, you say you tested the 5 labelled PSU rails, but presumably you didn't test the 5V line (which is, of course, the supply for all the logic). There's a convenient capacitor to use for this on the display PCB (I think the ends are even marked!), or you
can check it on any TTL IC in the unit. Of course noise on this line causes problems!
I don't have a set of extenders -- I wish I did. One problem is that the CPU boards all have 15 and 18 pin DS connectors, but at different positions on the 4 boards. This is great for failed circus, who;d find it difficult to get the PCBs in the wrong slots (I won't say impossible, I've met failed servoids..). It does mean you either need a different extender for each PCB, or you need to use 2 little boards, one for each connector, which is not mechanically too rigid.
I guess if you want to work on boards in the memory cage with extenders, you'd put the entire cage on an extender (you only _need_ the rear connecotr, the front one is for the expansion ROM cartridge signals only), and then use another set of extenders to put the boards outside the left side of the cage. If I was going to do this, I'd make up some metal rods that would screw into the memory cage mounting holes, aand which would then support the cage on the extender.
Anyway, what I actually do is to either use the wonderfully slim probes of the LogicDart, which will get to wquite a few IC pins without extenders (!), or to pul lthe card in question, solder wires onto useful testpoints, put the card back in, and test the free ends of the wires. It's a bit slower than using extenders, but it does work.
Tracing a 'no display' fault on a 98x0 is non-trivial. The display is scanned in software (well, firmware), so for the display to come up, the entire CPU, memory controller, ROM, RAM, I/O controller and display circuitry have to be working. I am trying to work out ways to really trace faults -- HP kindly provided a test connector on the CPU control card (09810-66513) which carries the microcode address signals, and connectors o nthe memory box/memory control cards (depending on the model) that carry the address/data bus. To use the latter, you'd want to trigger a logic analyser on a particular micrcode address -- say just after the instruction fetch.
All this is possible (and I have a lot of notes I've made on how it works), but it'll have to wait until after the HPCC mini-conference.


Wow Tony, where do you find the time to do all this hacking? I don't know if I'd have the patience to do the the kind of debugging that you do with your Logic Dart and wires soldered to the boards. I really need to use the board extender approach so that I can move from circuit to circuit quickly. I also have a really hard time working on these large machines without schematics. It's one thing to fix the handheld HP's with 2 to 12 chips blindly, but the 9810 is impossible. I tried repairing a dead Wang 520 without a diagram and got nowhere. Luckily I found someone with a full set of schematics and it was a relative breeze with them. [The Wang 500 design seems a lot more straightforward to me than the 9810, but the Wang is a simpler machine.]

BTW, I forgot to mention that the 5V supply looks pretty clean in my 9810 so I figured that wasn't the problem.


there _is_ a full set of 9810 schematics. You find it in the original patent 3859635 available at USPTO. I downloaded the about 400 TIF pages and made a 35MB PDF of them for convenient usage. At the beginning it's a bit difficult to identify which schematics belongs to which board. If you like, I can tell you which figure corresponds to which board.



Thanks for that pointer! I'm printing it out now. It'll make for some wonderful bedtime reading :)



Don't you already have a set of 9810 schematics... your friend in Texas...


Oh my, I completely forgot about those! Thank you, thank you. They are much more readable than the fragments in the patent. However, the patent makes a nice complement to the schematic in that it explains the detail that's not noted on the drawings.


The HP Patents for the 98x0 series are a wonderful resource, but do be careful. I've found differences between the schematics in the patent and production machines (the memory system in my 9830 is _totally_ different to that in patent GB144141).
IMHO, use the patents for clues, but don't follow them blindly.


Actually, I generally have a pretty good idea as to the problem area from testing on pins I can reach with the machine fully assembled. I then pull the probably bad card and solder up to 10 wires on it (I can't find more than 10 different colours of wire easily :-)). I then probe those. It rarely takes more than 2 attempts at soldering on 10 ways to find the bad chip.
As regards time, alas I don't have much else to do (like a job), so I figure I might as well work out how to repair HP calculators. And then share any tips here.
FInally, you didn't mention you'd checked the 5V line, so that's why I asked about it. OK, it's not the problem...

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