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isanchez · (This post was last modified: 07-22-2015 12:22 PM by isanchez.)

Hi,
Today, I have recieved an HP91. It doesn't work propertly: when turned on, the display shows ramdom segments on without sense (not so many, just concentrated in two or three digits, the minus always is on and the low battery led also always on). These segments change when any or many keys are pressed (the full display on is never shown). Also the printer prints randomly from time to time, without sense....what do you think this can be? Could be the famous ACT? I have some Woodstock that can be used as spare parts.... but the diagnostic looks very pesimistic: it seems fully "fried" ;-C.

No signs of corrosion anywhere. It looks externally pretty good.
The printer tracking rolls are in bad state, but this does not worries me...

thanks in advance for your help.

Kind regards

Ignacio

Paul Berger (Canada) · 07-22-2015, 02:42 PM

The first thing I would check is make sure the power supply voltages are correct. You can use the service guide for the 97 ( available on the museum documentation collection) to guide you, the hardware is the same minus the card reader. From the symptoms you describe, if it is not a power issue I would be inclined to think it may be a ROM issue.

isanchez · (This post was last modified: 07-22-2015 02:59 PM by isanchez.)

Hi Paul,
I am going to start with your recommendation. If it is the ROM, there is not so much to do....maybe panamatik integrated ACT...
THanks and kind regards
Ignacio

PANAMATIK · (This post was last modified: 07-22-2015 05:47 PM by PANAMATIK.)

(07-22-2015 02:58 PM)isanchez Wrote: Hi Paul,
I am going to start with your recommendation. If it is the ROM, there is not so much to do....maybe panamatik integrated ACT...
THanks and kind regards
Ignacio

As for now I don't have a direct replacement for HP91/97 ACT. But there is an easy test you can do. You can insert any HP-21/25/25C ACT into your HP-91. If the display runs and shows normal numbers, then the ACT is defective. If not there is more damage somewhere. Unfortunately the normal HP-25 ACT 1820-1523 does not reanimate your HP-91 completely because it cannot do printer and card reader functions, as far as I know. If you have an 1820-1596 ACT it does work and you can find it in HP-29C and some later HP-21/25s and of course in HP-97 machines.

Good luck.

Bernhard

isanchez · (This post was last modified: 07-22-2015 06:43 PM by isanchez.)

Hi Bernhard,
Thanks for your help. The 91 does not implement the card reader, only the printer. I am going to see which is the best option for the Act test...and the possible definitive solution.
I understand that using the second referenced Act you mention, it will manage the printer, but not with the first named Act.
By the way, I realised today that I had a PM from you from many weeks ago., when I was working in a 25....sorry, I noticed that very late. I have sent one PM to you today.
Steps could be:
1. Check input voltages
2. Check the ACT with any working ACT from an old 21/25...it will manage only the display.
3. The definitive ACT could come from a 29c, a late 25 or wait to your ACT progresses.:-)
4. If the Act is fine...then the ROM is the candidate...dead end....wait to your progresses :-)

Thank to all

Kind regards

isanchez · 07-23-2015, 02:23 PM

Hi again,
The thing is that I have an hp97 with the same symthoms and I have done the following: I have taken the supposed faulty board and moved into a working 97....and it has worked fine....this means that the problem is in the display...more probably in cathode or anodes?
I am considering that the problem in the 91 could be the same....is it possible to find these anodes or cathode? I have replaced one in an hp67 in the past, and the chip already exists...
Thanks and kind regards

Ignacio

isanchez · 07-24-2015, 04:40 PM

Hi,
I have checked the 91 with an ACT from a working 97... And the problem is still there.
So, the act is not the problem. As I mentioned I have a 97 with a similar behaviour...and the problem was not in the main PCB ( I have switched the PCB with a working 97)....this means the problem is in the display control area/ board.
Is it feasible to find equivalent elements to replace the anode/ cathodes for the97/91? For the hp67 it is. Thanks for your help!
Kind regards

Paul Berger (Canada) · 07-24-2015, 05:21 PM

The segment or anode drives are reputed to be a 1858,0044,MPQ3904 Quad NPN transistor array in pinch you could replace with discrete transistors. The cathode driver however appears to be a custom chip, but would be the same as used in a 97, in fact when I repaired a 91 it seemed the display circuit was identical.