HP-97 capacitor


David Smith suggest that i should change a blue capacitor on my HP-97 card reader. Where is locate this capacitor - is is the one soldered on the motor terminals - It's not blue but green?



No, it's not the one on the motor. It is on the card reader circuit board. One end connects to ground an the other end to one of the power terminals on the 20 pin reader chip. I don't have a '97 reader handy to pinpoint it. It is not one of the pair of larger 22uF caps on the read head circuit.


Which parts is the capacitor i should change; the one tha looks like a resistor?




It looks like the picture is of a very early HP65 or HP67 reader, not the HP97 reader. The cap that normally goes bad on these machines is usually found where that resistor is along side the top end of the chip. (the resistor sets the motor speed, on later readers there is a pot that does this).

I think the cap that is causing problems is the silver one adjacent to the pins of the chip... but I have not seen one of that style go bad. The other two capacitors are in the head circuit. I think one end of each of them connects directly to each of the head windings. I recommend using tantalum electrolytics, although modern aluminum ones may be OK. (BTW, since the price of tantalum has gone wild lately, capacitor companies are now pushing niobium electrolytics)


Well... It's a 67, not a 97... sorry...

I tried something and now i can read 50% of tries but i was able to write only one time on A LOT lot of tries...


It could be the adjustment on one or more of the reader switch contacts. These are the small gold leaf springs in the reader mechanism. They are activated by the card pressing on little nylon roller balls that then press the leaf springs against contacts on the back of the keyboard circuit board. There are very small screws on each spring contact that adjust the contacts. Setting them properly on an HP67 is a very tedious and touch process since you can't get to the adjustments while the reader is put together.

I adjust mine by tracing the switch contacts to the CPU board contacts and monitoring them there with an ohm meter. While insering a card, the three switches make contact in sequence. The three switches are the motor start, head enable, and write protect. If the head enable switch is wrong, you may not be able to read cards and they won't write properly. If the write protect is wrong, attempts to write to any card will show error. If the motor never starts or always runs the motor switch is wrong. The switches may never be closing or stuck always closed. Sometimes they are real flakey. Also the adjustment of one switch can affect some of the others.

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