HP-65 - 1605A02841 - Weird display behavior

[albertofenini]

Hi Tony, thank you very much for the tips.

The problem does not occur with another CPU connected,
on the other side, the problem occurs in other HP65s when we connect this CPU.

I think we can almost certainly say that the problem is in this CPU logic board.

We would like to take the values that you suggest, just to know better what happens,
but it is not clear to us where we should connect the scope, did you mean the pins
Disp0, Disp1, Disp2, Disp3 and Disp4 from the schematics ?

It appears that this unit was heavily tampered, the CPU and some of the RAM have been
de-soldered and re-soldered, this can be seen from the close-up we have attached.
Also the card reader has a gummy wheel rebuilt although is dirty and with some cables
disconnected.

However, tonight we worked on the card reader but with no success.
We don't know if it was working or not before our intervention.
We took all the components off, removed the oxidation and rebuilt from scratch the board.
We cleaned the engine, rebuilt the clutch, and remounted everything.
We also checked the that switches were closing correctly.

BTW what should be the correct sequence for a prerecorded card, MTR first anf HDS after ?

Anyway, if we power the motor directly the card reader can pull a card nicely, which
we know is not the proper way but it tell us that the thickness of the gummy wheel is
correct and at least mechanically speaking everything is quite ok.

However, when the power comes from the unit the motor is going very slow ...
More or less like the HP67 we have described in another thread.

To speed the motor we replaced the fixed resistor on the card reader with trimmer however without much improvement.
If you look at the link with the pictures you will see that we have added a short clip where the behavior can be seen.
Consider that the clip was recorded with an open circuit rather than the trimmer or the resistor,
so we were expecting a card to go at maximum speed although without reading the contents.

At the first try the card went thru
At the second try it went thru but stopped a little earlier
At the third try it went thru but stopped even a little earlier than before
From then on, you could hear the switch closing, but the card was not pulled nor the motor started.
Disconnecting the battery or disconnecting the card reader, or leaving the unit off for a while allowed a new cicle of operations but with the same result.

Card Reader Behavior

What could it be that goes so wrong ?

Thanks for helping us out !!!
Take care !!