HP 97 card reader - tantalum capacitors

[teenix]

(11-23-2021 02:58 PM)AndiGer Wrote:  Flogging a dead horse, I know ... old thread.

These days had another HP-97 on the repair desk. Didn't read any card but today I tried and it wrote the card. Successfully read in another machine.

So I found this thread and did what I had done before:
Unsoldered C4 and changed to another one with good values. C6 was ok as is the other one I still have. So resoldered original C6. The cards are read now :-).

I checked Vss and Vgg. IIRC Vgg before changing C4 was -11.25V, now is -11.50V. Maybe that was the culprit? Design voltage according to the service manual is -12.00V.
Vss before and after change of C4 = +6.08V (design +6.25V).

It is still good information :-)

This would make sense as C4 and C6 filter the Vgg and Vcc power rails from the switching power circuit. The sense chip is powered from the Vcc rail, so if it is overly noisy maybe the amplifier and level detector circuits won't work properly giving card errors.

From what I've seen, the sense chip is also finnicky about the supply voltage being close to 6V.

Maybe for HP-65's and HP-67's with card reader problems, it might be worth changing all the capacitors in their power supplies - 4 used in 65 and 3 for 67.

cheers

Tony