How to repair my HP 9100A calculator ?


somebody gave me a HP 9100A calculator. It's very clean into but it doesn't work (there is only one green point on the screen).
What could be the problem ? What must I test and verify firstly ?
Where I can find a maintenance manual or informations about voltage?
Kind regards!

Edited: 10 Aug 2010, 7:37 a.m.


A 9100 calculator... you're the luckiest guy on earth!!!
You'll find a service manual on the Australian website:
The service manual will definitely help!

Joel Setton


Thanks a lot!!!!!!!
Now I can repair my calculator, I will need time but now I can repair my calculator :-)


I'm not so sure the service manual will help. It has schematics for the power supply and video driver boards but nothing else. It, like many of the HP calculator repair manuals, suggest swapping boards. Which might be an ok thing to do (I know that Tony will disagree) but only if you have a bunch of spares.



Perhaps... but I think there is a first problem with the power supply of the calculator (there is only a point on the screen) and with this manual I will be able to test voltage. Of course after perhaps there will have a second problem with logic system... but if I can just repair it in order the normal display appears, I will be very happy! :-)


Hi from Puerto Rico

These are very robust machines. In 1983 a service technician, that serviced the University of Puerto Rico Natural Sciences Faculty's computer centre HP-2000 Basic Access System give me a HP-9100A and he told me that he did not know if the machine worked or not (the only thing given was the power cord)
I went home, plugged it in and after waiting more than 5 minutes the image on the display started to show and after that it continuing working fine ever since. Once in a while I fire it up to make sure is working as I live in the Caribbean and heat and humidity make difficult for the calculator to boot sometimes. But it is still there with me. As a matter of fact is the oldest electronic device I own.
When this machine was launched in 1969 I was 15 and on high school, and I never dreamed of still having a technology of the sixties as revolutionary as this one is. On this year along with, UNIX, the Apollo Guidance Computer are the three concepts that were to change the next decade for portable operating system written in a high level language, first navigational portable computer, and the personal computer (for engineers and scientists only at least) that made me part of the history of achievements to dedicate the rest of my life to love HP classic calculators, UNIX/Linux and space exploration.

I hope Jonathan that you will able to repair your HP-9100 soon.

It represents to me the eternal truth of an ethical corporation as HP was in it's golden years and a pleasure to me to share and be here with everyone of you.


Thanks for your story!
It's always very interesting to listen this! This period of the beginning of the computers should have been very exciting!
Nobody have invented yet the time machine ??? :-)


Israel Otero from Puerto Rico.

Thanks Jonathan for your kind response.

One thing that I learned is if you want to know something very well, you need to study from its earliest beginnings so you grasp, value and make you understand it and in some way love it better. That's why history is so interesting to me.

Talking of that special year of 1969 Linus Torvalds the creator of Linux born in that year too (28.12.1969) I just discovered it today.

I see things in life linked because of the relation of dates and facts and that eases understanding.

I wished so many times that someone could invent a time machine too.
Let propose it to HP to do it. :-).

I love to know about those beautiful people who love these devices, they make feel happiness and my life has a purpose.

Let wish your HP 9100 will be alright soon.


Ok for the inventor who will create the time machine: Don't forget to come back with many HP 9100A! :-)
You can see my HP 9100A here:
Have you got other calculators or just only this ?
Kind regards


Israel Otero from Puerto Rico

Hi Jonathan,

Unfortunately this only one I have.

I have some other HP calculators about a couple of them because of sentimental value. But a collection as such no. The ones I have were given to me when I was a student or purchased by me when they were at sale.

I feel that those machines are to be used and enjoyed because there is always persons like you are that are able and lucky enough to start and maintain a proper collection large enough to be relevant historically.

I wish you that you are able to keep growing your collection and in the future land in the hands of a person or group that value it and take proper care.


The display is entirely driven by the processor section. If the digital side is malfunctioning, you will get no display (or maybe just a dot). And while (obviously), 'no display' can be caused by a problem in the CRT driver cirucits or PSU, most of the time it is a logic problem.

From the Australian museum site you can get the official HP service manual ('boardswapper guide' :-)), the interfacing manual (you may wonder why you need this -- it's actually the most technical of all the 9100 manuals and is well worth reading), but no full schematics. I don't own a 9100A, you see... But there are 9100B scehamtics that might be a starting point.

The 9100A and 9100B are similar in some areas and quite different in others. From what I remember :

The flip-flop boards are the same

The control logic (clock and core-on-a-rope microcode) board is similar, different ROM programming, but otherwise much the same

The core memory system is very different (but similar is design concepts?)

The main ROM/gate board assembly is different (again similar concepts?)

The keyboard is pretty similar.

The card reader is different

The entire top case, PSU and CRT circuitry is the same.

Now, getting started...

Firstly, I would take the whole machine apart (this includes removing the 'sideboards' that connect the ROM to the gating board) and clean all the edge contacts. Look for silly faults too at the same time.

Check the 3 fuses (mains, -15V supply, EHT supply) on the PSU chassis in the top case. Also check the 3 little filament lamps up there, you need them for the tests.

Reassemble, but leave the card reader out for the moment. You must connect the keyboard (the machine will not run if there keyboard is not present). Power up.

Check the -15V supply to the logic. Normally, if the 2 lamps that illuminate the register label to the right of the CRT are lit then the -15V supply is OK, but it's worth checking.

Press STOP twice (in case the thing is stuck in a running program), the do something that will cause an error -- 1, CHS, SQRT is my normal sequence. If the error lamp (left of the CRT) comes on, then the processor is basically working, so troubleshoot the CRT driver circuits

Check the CPU clock is running (test connector on the control logic board, leftmost in the cardcage). Check the various bias supplies on that connector (and on the flip-flop board connectors).

Now, some of the flip-flops are the program counters for the microcode and firmware ROMs. I can look up which ones they are for the 9100B, but the -A may well be different. If you can find them (I can give you hints...) then check the outputs (E and F signals) on the test connectors on the flip-flop boards. All should be toggling. If not, either the flip-flop is defective, or the logic driving it, or...

A 'silly' that will remove the display is a lost of the mains sync signal (the display routine is synchronised to the mains to prevent flicker). In the 9100B it's a transsitor on the right sideboard -- no idea where it is on the 9100A, but tracing the AC signal in the PSU cableform should find it.


thanks a lot for your knowledge about this computer. My father works in electronics and he will help me to try to repair it.
Of course I will tell on this place my progress on the problem.
Kind regards


In case you have not already done so, I suggest you remove all the boards (one at a time), clean the edges, and securely re-seat the board. Then try it again.


Thanks, but like I said, into it's practically new. But ok I will clean all the connectors.
Just in case, do you know if somebody had a same problem with his HP9100 ?
Kind regards


I've got a couple of 9100Bs that have had similar problems.

One time it was a defective address driver transistor on the main ROM PCB. The other time it was a defective transistor on the microcode branch logic on the left sideboard.

Basically, a 'no display' fault can be almost anywhere in the machine.

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