HP-97 card reader repair question



#14

OK, almost done with Easterling's 97. The keyboard problem was due to the fact that someone had worked on the calc previously and re-installed the white plastic keyboard spacers incorrectly: instead of spacer-gold foil-spacer it had been put back together as gold foil-spacer-spacer. This lifted the contacts another millimeter or so and prevented the "0" key from making good contact when pressed.

The broken battery contact was replaced with a good contact from an HP-82162A printer with a bad print head.

The card reader wouldn't pull cards, so I took it apart. The rubber roller was in the "rock hard stage just before gummy", so I scraped it off and replaced it with standard O-rings. The inside of the reader, including the read/write head, was kinda dirty so I cleaned that up as well (btw, ultrasonic jewely cleaners work great for cleaning those nylon balls).

The reader now pulls cards very well and will write cards my other calcs will read, but reading a card always produces an "ERROR" display. Any ideas? I have another 97 card reader I can use if I have to...


#15

Since it writes okay I would start with the 3.3uf bypass cap across the motor. No joy? Next up are the two 22uf caps in series with the head. That usually takes care of it.


#16

Thanks Randy. Presumably these caps are readily available at my local Radio Shack...

#17

Foil, Spacer, Spacer IS THE CORRECT ORDER!!!! I've worked on a couple hundred of the machines and all the ones that worked and had not been futzed with were in that order. The only ones that were not in that order had been obviously klutzed with.

To fix your read problem replace the 6.8 uF (or so) blue dipped tantalum electrolytic cap on the chip power lines.


#18

Quote:
Foil, Spacer, Spacer IS THE CORRECT ORDER!!!! I've worked on a couple hundred of the machines and all the ones that worked and had not been futzed with were in that order. The only ones that were not in that order had been obviously klutzed with.

Weird. It's not that way on my other 97, and redoing it this way sure seemed to fix the keyboard problem. Do you see any disadvantage to leaving it this way?

Quote:
To fix your read problem replace the 6.8 uF (or so) blue dipped tantalum electrolytic cap on the chip power lines.

Will do...

#19

David Smith on 13 Jan 2006, 12:13 p.m. wrote
> Foil, Spacer, Spacer IS THE CORRECT ORDER!!!!

This is what the book [1] says:

**vp

[1] p. 3-15 of the HP-97 Service Manual (file 97serv.pdf in the MoHPC DVD collection)


#20

Apparently somewhere along the way HP changed the stamped sheet. I think they did it to get more bend (and spring) to the contacts. They probably changed the order of the spacers to get keep the contact clearance consistent. If the spacers are in the wrong order you either get swicthes that are shorted closed or never make contact. I know what the service manual says. I also know that most of the machines are actually configured differently. When I first noticed that the spacers were in a different order, I tried fixing them like the manual said. I then started having problems with the keyboards. I just got in two machines last week and both had the spacers together.

#21

Quote:
To fix your read problem replace the 6.8 uF (or so) blue dipped tantalum electrolytic cap on the chip power lines.

You mean this one, right? I read that as 6.9uF. Would it be prudent to replace any other while I'm at it?


#22

Bad Booze Rots Our Young Guts But Vodka Goes Well -> Blue, Gray = 6.8

That is probably the one, but the ones that go bad are usually all blue. It is on the power lines to the chip. The exact value is not critical, better too large than small.

The other two caps are in series with the head windings. I have seen some readers with a small extra cap... don't remember where it goes. (looks like yours has it)


#23

There are only four caps on the board, all visible in that photo.

Whether the one I pointed out is on the power lines to the chip or not, I can't say...no schematics.

My last question regards polarity: there are no obvious polarity markings on the cap or the circuit board. How to I discern which way to orient the replacement part?


#24

HP convention for polarized 'thangs is that (+) has a square PC pad, the (-) is a standard round pad. Same goes for pin 1 of DIP's.

The 6.8 uf cap is in a motor timing circuit, the 3.3 uf next to it is the Vss bypass cap. Whenever I have a dodgy 97 and the switches have been adjusted, I just replace all four caps and move on. The 22uf's in series with the head tend to give me the most grief. FWIW, never had a problem with the older hermetics in the 67, only the dipped red lollypops that are on your 97.

#25

HP97 service manual is on the museum CDROMS...


#26

Yes, and I will order the Museum manuals any day now, honest!

In the meantime I only need one last check of my profound Capacitor Ignorance. Here's one of the 22uf caps I picked up at Radio Shack:


Will this be a functional replacement? I don't know the significants of the "35v" marking, although obviously that's much higher than the voltage in the card reader.

Radio Shack did not have separate 6.8uf and 3.3uf caps. The closest I found were in a "Pack 'o 100 capacitors". I don't know if the caps shown below are 6.8uf or 68uf, nor do I know how to tell the polarity:

Advice solicited. Surprisingly, I CAN solder.


#27

No, that Radio Schack cap is an aluminium electrolytic (BTW, most RS stores are getting out of the parts business... and would you like a cell phone with that blank stare). It has a much poorer performance in this application than a tantalum capacitor. I have never had any problems with the head winding caps or the smaller electrolytic. The one that fails is the one on the power line to the chip. The blue ones are particularly bad. The cap will test good on most cap checkers, but it develops a high ESR (equivalent series resistance).


#28

Thanks for the info, David. Another forum member has offered to send me all four caps, which (as I noted) beats ordering them from DigiKey.

Yeah, Radio Shack's component selection wasn't too impressive. I remember when they had tube testers in all the stores.


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