Hp-67 card reader: tantalum capacitor identification

Hi everybody,

I have been following all the posts regarding a typical problem in HP-67 card readers; that do not read, but do write.
Seems that the problem is to replace a tantalum capacitor in the main board, but I am having problems to identify it:
-First seems that the capacity for the replacement should be 47mF...is it OK?

-I can see two "ball" tantalum capacitors in the top part of the main board. Both look like a small ball. One (full yellow with a red spot at the top) is down BETWEEN the two transistors, more to the LEFT-TOP CORNER; and the other (3-coloured strips-- blue, then, violet, and yellow at the top, with a small black spot at the very top) is at the right side of the more right transistor...
Can you help me with this?

Many thanks in advance.

Kind regards,


Edited: 23 May 2013, 7:11 a.m.


I went down that road and tried replacing the capacitors, but that wasn't the problem. From what I was told on this forum, the capacitors rarely go out and it is usually a mechanical failure. They were right, but I had multiple problems:

Starting from easiest (and most likely) to hardest:

1. Check the battery, it won't run without a good battery. I've heard it's bad to run it without a good battery. I ordered nicad cells with solder tabs for about $1.50 each on Amazon. Don't try soldering normal batteries, they could explode, I'm told. I gently opened my battery pack with a utility knife along the edges. I used a sharp chisel to scrape the connectors off.

2. Check the wires that go from the r/w head to the daughterboard, they can break easily. Mine broke from all of the messing around I was doing with it. Soldering them back is easy, but the wires are not very long and they are very small.

3. Does the motor run? Can you hear it run, but not see the gear turn? The worm gear can sometimes slip on the motor shaft. This also happened to me. I used a very very small amount of super glue on the shaft and that fixed that problem. You can test the motor with the back off of the calculator while holding the battery in.

4. Does the motor not run at all?
Probable Cause: There is a rubber roller that "melts" with age. It gums up everything and makes a mess. See this link for instructions. This requires a more involved dissassembly. You have to remove the plastic gear and clean the goo off. Then, you can use silicon tubing (available from hobby shops and some hardware stores) to replace the roller. Don't try the o-rings, I tried that and it didn't work as well as the silicon tubing. The tubing was about 3mm or 4mm, I can't remember. It's pretty much standard tubing for a model airplane fuel line. The thickness should make it even with the ridge on the gear shaft.

There is also a cam adjustment that is used to set the pressure between the roller and the card (white plastic flathead screw that is pressed in and holds the gear in). After you get the calculator mostly reassembled, Turn the screw back an forth until the card runs through smoothly without slipping and without a stressful whining sound coming from the motor. I used needle nose pliers to turn it because the screw is not in a convenient place to use a screwdriver.

It's black in this picture, because this is an HP-97:

Possible Cause: There are reed switches with small balls (and one wheel) inside the card reader. This requires even more dissassembly. The reeds should be just barely even or a little lower than the plastic housing. The silver flat head screws are the adjustment screws. If you take it apart and look from the side, you will see what I mean. I cleaned the contacts, but they probably didn't need cleaning because they look like gold plated contacts and reeds.

5. If you do all of this and it still fails, then look for the capacitor (Yellow in this picture).

Edited: 23 May 2013, 12:22 p.m. after one or more responses were posted


Many thanks for your answer:
All your checkings have been already done: I have a lot of experience in repairing the gummy wheels. The motor runs and sounds perfect, there is no dust inside, the card passes perfectly, the speed has been adjunted to the optimum. the cards have been checked to be ok....I believe it is the capacitor....but which one??

the 67 is new in the box, never sold/used, coming from an old stock never sold...

Kind regards


I'm not an expert, but this was a picture I used when I replaced it:

He claims it's the yellow capacitor. Mine was a different color.


Many, many thanks for your answer!!!
your photos are impresive, good work!!

In this Dropbox folder you can see a couple of photos of my board.


I understand that the capacitor I must replace is the more right side one, yellow, with the black spot at the top...

The capacitors look with different colors. Can you confirm the capacity?? I have also many doubts, as according with the color code it should be 4.7mF, but in some posts I have read it works 47mF...

Thanks in advance for your help

Kind regards,



Hi again,

compairing the photos of both boards, seems that the capacitors have exchanged their positions...this is really weird! I am really lost with this.

Kind regards,


Yes, another Ignazio ;)
IMH that you are thinking to replace the wrong capacitor. This component is part of the converter power unit, no relation with the reader.
You must look at the top of the daughter PCB, near the sense amplifier marked 1826-0322. There are three cilindrical tantalum condenser that you must test, starting with the unit near the chip.
Hope in your success.



Problem solved: finally I changed the capacitors (without any change in the behaviour, but it is a good measure anyway).
Then I realised that the motor axis was not working properly: at the end of the card passing it slipped a little bit,causing the malfunction. I have glued it carefully and problem solved.
Additional recomendation: I you want to reduce subsantially the card reader noise: put a small amount of vaseline between the axis and the plastic wheel...do not put on the o-rings!!!

Thanks to all for your time and efforts.
Kind regards


I will add yet another trouble shooting tip.

Make sure ALL the contacts are good between the daughter board and motherboard and motherboard and keypad board.

Two units I've repaired had everything else seemingly working fine and it wasn't until I applied some DeOxit to these contacts that the signals were cleaned up sufficiently to allow for reliable reading.

Buy some DeOxit. It works great restoring old HPs.

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