HP 9810A - current value?


I may have an opportunity to buy a 9810A and can't seem to find a recent value. The two prices listed here are $185 and $500. They must not come up that often on TAS. I haven't seen a photo of it yet, but assuming it's in good condition, what would a fair offer be? Shipping these things around can't be cheap either!



I paid about 260 Euros (plus 40 Euro shipping from Germany) for a very nice looking 9810 last year. It was in working condition electronically, but the printer roller was completely disintegrated and had turned into a black sticky 'tar' that had to be patiently removed from the machine's inside.

The printer roller, as well as the card reader rollers, needed to be replaced and now the machine works like a dream (thanks to Tony Duell's unique expertise and help!).

If you search on the web you should be able to find a few good articles related to repair for this type of machines.

Good luck with your purchase, they do indeed not come up very often.


Thank you for that. Looking back in the archives here, it looks like they can have quite a bit of trouble and are not easy to troubleshoot. They look more like a computer than a calculator inside. Of course, you don't hear about the ones that work fine.



These are beautiful machines, well worth some effort. For an account of what might await you with a broken machine, have a look at


I really like your library.



Thank you for documenting what you've done with yours. I have a feeling I'll be doing a lot more posting here if I end up getting it.



I bought an HP-9820A, the next step up, ten years ago for $10.

Of course, it doesn't work. But it was worth the $10 just to disassemble and inspect. The 9820A that I used in 1972 at Georgia Tech I was never allowed (for understandable reasons) to take apart!

Edited: 6 Nov 2009, 3:08 p.m.


That's a fine machine. You should give it a try! They're intimidating at first, but after all not that complicated, if you have the appropriate documentation. And there are many experts here on the forum.
The machine shares many parts with the 9810; I have a complete memory cage as spare parts. If I had a 9820/30, I knew what I'd do over the christmas holiday.


I've repaired the 9810, 9820 (very similar machines, actually) and 9830 (same processor boards). I am happy to help anyone else do the same...

The main difficulty in faultfinding is that they're bit-serial machines, and almost all the signals are (or at least should be) changing all the time as bits are clocked into the shift registers. A logic analyser or storage 'scope is very useful for serious debugging, but it is often possible to manage without one.


Tony, since you know the entire family of machines: Given the similarity of the 9810 and the 9820, how far would one get with boardswapping between an operational 9810 and a defective 9820?



[wait for it...]


With me you'll get nowhere!. I do not, and never have, approved of board-swapping as a way of finding faults. It's about the only thing I'll refuse to help you do.

The 98x0 series are well-made machines and deserve to be repaired properly! If you want to do that, I'll talk you through it.


A few thoughts...

Firstly, the sections that are different between the 9810 and 9820 are :

The memory box (the aluminium box at the left side of the machine) and all the boards in it

The display PCB

The keyboard and encoder PCB

The I/O backplane (the 9820 supports DMA IIRC, the 9810 does not)

The other sections are the same (processor, PSU, printer, card reader/controller, main backplane).

The machine will run with the I/O backplane removed and the keyboard disconnected (it plugs in on an edge connector under the front of the printer). So while I don't approve of board swapping, I will say that if you pull the I/O backplane and disconnect the keyboard, you could fit the 'wrong' memory box and display to either machine.

My diagnostic methods go roughly like this :

If I don't think the machine has been powered up recently, I remove all the boards apart from the 3 PSU ones. I disconnect the keyboard from the main backplane, but leave the power swich cable connected. I remove the printer and memory box (in case that's not covered by 'all the boards' :-)). I then power up and check the PSU voltages. There are 6 of them (+5V, +12V, -12V, +24V, +19V, +16V). All but the first have labelled testpoints on the PSU PCBs. For the +5V, I know where to find it on the main backplane (and will tell you too :-)).

Then I fit just the CPU clock PCB (09810-66512). I check the 8MHz master clock and the 2 main CPU clocks (bitclock and muclk). If they're wrong, I find out why.

Then I fit the other 3 CPU boards, the memory box, and the display. If it was working correctly, the machine would run like this and give the right power-on display. Most of the time, if it's faulty, the display remains blank. The first thing I check is the dispstb line on the display connector. It should be pulsing if the CPU is trying to drive the display. It nearly always isn't (pointing to a problem in the CPU or memory areas) but I'd feel a right idiot if I spent time tracing the microcode only for the fault to be in the display driver.

At this point I use the fact that it's a bit-serial machine and that all signals should be changing. I quickly look at the M and T register outputs on the memory box test connector. All bits should be changing. If only the top <n> are, it points to a likely problem with that register. So I look into that.

At this point, alas, it's time to grab the logic analyser and the microcode listings and see what the CPU is doing. You can pick up the microcode program counter on the test connector on the CPU control PCB (09810-66513). I generally find one of 3 things now :

1) The microcode is going through a crazy sequence of states, pointing to a problem with the microcode program counter and/or ROMs. This is rare

2) The microcode seems to be going round the fetch-execute cycle. Maybe a problem in the memory box, or a the data path. Time to look at that

3) The microcode is stuck, often in the I/O loop. So I look as to why.

Electronically-good machines with mechanical problems are easy to fix in theory, but harder in practice (you really need a good workshop including a lathe to do card reader or printer repairs). There are some photos in my flickr account (tony_duell) showing both main mechanical repairs.

Electronic faults are harder to track down (but I assure you it's always possible) but fixing them is much easier. What you want to take on is up to you!

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