HP-67 Card Reader became very touchy

[Alevin]

I bring up the topic of the old forum “A weird HP-67 card reader problem” `posted in June 2001 where Viktor Toth documented the case (and Tony Duell proposed the solution) were the card reader reads/writes with low batt, but not a high or normal level. After going through the entire card reader mechanical settings protocol, I believe that my machine shows similar symptoms, that is: it reads prerecorded cards, and reads and writes cards, but only when the battery voltage is low.

Similar behavior is observed in one Hp-97 I got. Card reader, low bat reads and writes, but not at normal voltage.

The thread concluded to solve the problem replacing a capacitor (15 uF or 47 uF not clear) that communicates the battery with the SENSE Vbatt ( I guess Vbatt it is pin 20 noted VB on page 36 schematics, in Tony ClasicNotes).

Please, could someone clarify me where this capacitor is located? Is the one connecting the SENSE chip pins 17-18 to ground in the same pag 36 schematics?. (small blue one in the below linked video). Or it is another one going thought the main logic board or elsewhere?
Also a confirmation of its type (tantalum?) and characteristics (cap value, voltage) is welcome

The following short video shows the calculator and the main PCB and the card reader PCB.

https://youtube.com/shorts/0pGET7dNivI

The next one shows the error when reading with full or medium battery level

https://youtube.com/shorts/v-pWB6EqYsw

A this last, with a low battery light on, successful reading (not always, thought)

https://youtube.com/shorts/H29nE7X0NYY

I have also noticed, that in the Error readings, the card travels further (with no effort to extract) than when the reading is OK (it does not advance so much, at it takes a little effort to extract), as Tony as pointed out.

Equipment description, and settings tried

The serial number is 1710A1266, with PCB Korean chips seems dated at 1977. a relatively early model. However, the PCB of the card reader is the type of the later series (Perhaps the calculator was repaired and the card reader changed in 1986?).

On the card reader PCB you can see a blue potentiometer (variable resistor for the motor speed control, as I have tested) and two yellow capacitors and the afore mentioned small blue on, in different configuration from the three parallel cylindrical capacitors that appear on the HP65 and early HP-67 models.

Its cosmetic condition is quite good with the electronics completely clean, without a trace of corrosion.

After cleaning the goo of the gear wheel, the nylon piece was substituted for two O-rings. The cards pass smoothly and without strange or squeak noises. But I get a read error permanently.

I have worked on all the usual mechanical aspects:

Clutch fix (it was not necessary, but I added a little super glue just in case, since I previously tested other recommended procedures; insulation wires and silicone,. on a HP-65 ,and I could not make them work).

Lubricating the worm and fulcrum with silicone grease.

Playing with the gear the eccentric cam.

Playing the blue potentiometer.

Applied Deoxit to card reader connections.

I have not touched the reed switches,. since they seem to work fine, as the card passes and stops without problem.

But I can't read or write cards except with low battery, namely:

I have been able to read the pre-recorded Standard PAC cards, and and write and read blank cards when the battery is low and the dot on the bottom left of the screen, that indicates low battery starts to light up.