Trying to revive a HP67 (Dead on arrival)

[teenix]

It is possible that corrosion has affected the sense board (long one you mention). This board has a direct connection to the battery, so if it is faulty, it might explain the switched off current, which should be zero, unless batteries are installed in which case there might be some charging current visible on the lab supply.

Leave the sense board unplugged from the CPU board. If the sense board is faulty, it could damage the CRC chip on the CPU board.

There are two red wires connected to the board. One to the motor, the other to the battery.

Disconnect the one to the battery, you might be lucky, as sometimes these wires can be unplugged, otherwise unsoldering is required. See if the switched off current is now zero. If so, the sense board has a short (corrosion) or possibly the sense IC is faulty.

If the switched off current is still there, then you need to check under the keys circuit board around the circuit traces for the power switch. This will require further disassembly of the calculator.

It is also possible that the card reader has a "gummy" wheel problem and will need some repairs. You can check this by looking ate the gear and worm drive connected to the motor. You might see some "mess" where the drive roller contacted the magnetics cards to pull them through the reader.

If you manage to fix this fault, then it might be wise to replace the 47uF 16V tantalum capacitor as a faulty one can sometimes interfere with the reading of cards. It is located at the 1 o'clock position from the power supply coil. (Polarity sensitive - be careful)

If you would like to make a new sense board, I have a zip file here. Inside is another zip file that you can send to a PCB manufacturer (like PCBway) and get some boards made quite cheaply.

cheers

Tony