HP-65 short circuit


Dear friends,

last week I bought a working HP-65. The card reader wheel was obviously in bad state. This week-end I proceeded to remove the gummy wheel and replace it. I have done this more than fifteen times in 67 & 41c card readers. I was very surprised when, (after replacing the wheel and ensambling the calculator back) I tested the calculator and it was short circuited (continuity between + and -). I tried to identify the origin of the short circuit without success.

-I checked the wires/connections, etc...all seems fine.

-I removed the main board: the short circuit was still there.

-I disconneceted the card reader motor (by unsoldering the red wire): the short circuit was still there. I was a bit surprised also because I realised that there is continuity in the capacitor installed in the motor (I firstly thought this was was the problem, is this normal?)

I would appreciate very much your ideas or comments to try to repair this jewel. Is it possible to get a copy of the service manual? I know there are some reverse engineering diagrams from Tony Duell, where can I get them?

Thank you in advance.
Kind regards


Edited: 21 Oct 2013, 3:34 a.m.


There would have been a service manual, but AFAIK it was never sold, and I have never seen it. You can get my diagrams for the machine on the HPCC CD-ROM, please contact Dave Colver (secretary@hpcc.org) for details. He _may_ be prepared to send you just the HP65 diagram if you ask nicely.

As to the problem...

Does the short still appear if the on/off switch is 'off'?

There is a gold contact in the the charger connector which connected the outside 2 pins together if the charger is not plugged in. I would start by using a bit of card or another insulator to open this 'switch'. Then see which of the pins is shorted to ground (the middle pin). Basically, there is very little connected to the battery when the power switch is off, if, as you say, disconnecting the power from the card reader PCB doesn't cure the short, the problem is either the battery holder wiring (is a wire trapped somewhere), the on/off switch (is the contact postitioned properly) or a short between traces on the keyboard PCB. I don't remember a capacitor across the battery on the keyboard PCB (check the schematic!), but if there is one, that could be the problem.


Dear Mr. Duell,

you were ABSOLUTELY right! I followed your recommendations and I realised that one wire was smashed by the card reader (the pair that feed the main board). Now the caculator works and passes all the tests.
I would like to thank you very much not only for this concrete issue but for all your knowledge put in common and avaliable to all vintage HP calculator fans (I am modestly one). I have used your documents and forum contributions for many calculators repairs; mainly for a 9100B, an hp-67 and a HP46. Again: thank you

Kind regards


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