HP 97 card reader - tantalum capacitors

[AndiGer]

(11-23-2021 10:18 PM)teenix Wrote:  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.

I had a second 97 on the bench for quite a long time that didn't read and write although I changed the mentioned caps to new electrolytic ones. After Tony explained the task of these caps I hooked both calculators to my oscilloscope and found ripple on Vss. Put in another one of the same charge and now ripple is gone - and cards are read.
(Now after some reads the coupler/damper is gone but that's another story.)

So I'm glad I posted here these days.