Help with HP-67

[twoweims]

I recently acquired a non-working HP-67.
It appears that there is a short circuit between Vbsw and ground.
When I power it up from the battery terminals with an external power supply my power supply voltage just collapses. The low battery LED blinks momentarily as the voltage drops, which I would expect.

I have isolated the card reader and the logic board and tried to power the keyboard/display board alone and the fault persists.

Vbsw does not seem to do much on this board other than provide Vcc to the cathode driver and the low battery LED. I may be missing something because I don't have a good schematic. I am suspecting a problem with the cathode driver.

Is there a service manual for this calculator. I have the MOHPC Thumb drive and I don't see one on there.

If anyone has worked through this particular problem before, I would appreciate any advice.

Thanks!

[Massimo Gnerucci]

(06-06-2019 01:34 PM)twoweims Wrote:  I recently acquired a non-working HP-67.
It appears that there is a short circuit between Vbsw and ground.
When I power it up from the battery terminals with an external power supply my power supply voltage just collapses. The low battery LED blinks momentarily as the voltage drops, which I would expect.

I have isolated the card reader and the logic board and tried to power the keyboard/display board alone and the fault persists.

Vbsw does not seem to do much on this board other than provide Vcc to the cathode driver and the low battery LED. I may be missing something because I don't have a good schematic. I am suspecting a problem with the cathode driver.

Is there a service manual for this calculator. I have the MOHPC Thumb drive and I don't see one on there.

If anyone has worked through this particular problem before, I would appreciate any advice.

Thanks!

Maybe Tony's schematics can be of help.

[teenix]

(06-06-2019 01:34 PM)twoweims Wrote:  I recently acquired a non-working HP-67.
It appears that there is a short circuit between Vbsw and ground.

I have isolated the card reader and the logic board and tried to power the keyboard/display board alone and the fault persists.

Check for obvious shorts on the key/display circuit board.

Look closely at the wiring that is soldered to the top of the board. These are mounted really close together and can easily short if the wires are frayed.

Same for the card reader board.

Try isolating the power wires to the card reader. The problem could also be the sense chip as it has direct connections to the battery.

cheers

Tony

[Zaphod]

It could well be as simple as a cap

[twoweims]

Success!
It powers on and calculates.

I'm not sure how I fixed it. I cleaned some corrosion on the display board with a pencil eraser and then it started working...

Next...

I need a battery door and a battery pack. I'll start watching eBay for these.
One of the battery spring clips is broken but I have a good idea how to fabricate a replacement.

The card reader is yet untested. The roller is gone so I will need to get a replacement

Thanks to all that replied.