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HP 97 card reader - tantalum capacitors
11-25-2021, 06:39 PM
Post: #12
RE: HP 97 card reader - tantalum capacitors
HP-67 and HP-97 both have the same CRC chip 1820-1751.
According to the HP-97 service manual I could locate Vss at pin 2 of that chip, Vgg at pin 15. That were the points I measured with the scope @50µs and saw the ripple of Vss.
If you find ripple as Vss I think you can track down which one is the appropriate cap for smoothing Vss as a direct connection to the mentioned pin 2 of the chip, opposite side is ground.
Maybe this link to Kees's page helps as well as it explains schematically how the HP-97 power supply works:
IMHO the HP-67 could be similar. The power supply for the HP-97 shown there is also similar to that of the shown classics HP-35 and HP-45. And as we learned the HP-67 is more like a woodstock. So I guess it is a 22µF cap as well like C5 in the shown HP-25 power supply. With that guess I think it is the red cap with blue foot left to the torroid.

Be sure you have a replacement cap with low ESR. The one I re-removed had more than 5 ohms as had the original C4 tantalum cap (with more than 3.5 ohms read failed). The working replacement has around 1 ohm. I used a (cheap) LCR-T4 to test the caps.
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RE: HP 97 card reader - tantalum capacitors - AndiGer - 11-25-2021 06:39 PM

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