HP-67 Card Reader became very touchy

[Alevin]

(09-26-2023 10:11 PM)teenix Wrote:  The HP-97 capacitor that matches the one that you changed in the HP-67 is marked on the attached image as C6 - 47uF. The image refers to later model 97's, earlier ones have the parts in different places.

C6 - 5V Battery bypass
47uF, 16-25V Tantalum

C4 - Vss (+6V filter)
22uF, 16-25V Tantalum

C2 - Vgg (-12V filter)
68uF, 25V Tantalum (Not sure what the original type was though)

Note the polarity of each as they are polarized and inserting new ones the incorrect way will damage them. (Lots of unpleasant smoke).

Also note that the HP-97 service manual has the +ve marking for C6 to the top of the capacitor, but it should be to the left side as in the image as the capacitor pins are horizontal not vertical.

cheers

Tony

Keeping the threat about the electronics of hp-67 hp-97 card reader, and in relation to the cited previous Teenix post showing the Hp-97 main caps in the transformer area...

My limited electronics knowledge is nevertheless enough to know that the capacitance value of caps measured when soldered on the PCB is affected my surrounding circuitry, and it can not be taken either as the real value, nor indicator of the cap condition.

Nevertheless, measuring on the board the capacitance of C2 (76 uF, ESR 0,036 Ohms) and C4 (26 uF, ESR 0,86 Ohms) these values are not very different the nominal values documented, 68 uF and 22 uF, respectively.

Surprisingly, the C6, the substituted one, gives me on the board a value of arounf 7 pF ?? ( on the contrary the ESR is the same on the PCB than individual, about 0.1 Ohms).

Could be possible the cap C6 capacitance measured, once soldered to the PCB, being so different from the nominal one? (Specially being in mind the C2 and C4 does not show so much differences).
Or may the cap has been destroyed while I was making soldering?

Checking the other discrete components I have noticed than the rectifier diodes from the card reader area (CR1, CR2, CR3, CR4) , and the diode CR5 (Zener?) located in between C6, C4 capacitors and the Toroidal transformer T1, give 700 Ohm resistance in direct polarization, but 1,4 KOhm in inverse polarization. Should they be replaced?


Regards