Dead HP 35

[teenix]

The logic diagrams for the various ICs used in the HP-35 would boggle the average mind :-)

You can check out some basic operation with scope traces in my Classic Notes.

Have you measured the circuit current when operating, with just 0. on the display. (Post #3 180mA or so).

It may be that a component or two are "on the way out", after all the years since manufacture. You might like to replace the capacitors in the power supply section. If the original solder appears black at the joints, they might be a bit difficult to get new solder to "bite".

I attached a diagram locating the caps, these should be available from Altronics or Jaycar(maybe). Through hole tantalums of 16-25V rating should do.

Also, the transistors in the top right corner might be failing. Jaques Laporte mentioned that 2N3904 types should be a suitable replacement. Altronics Part No: Z1200, although you will have to check the pinouts to see if they match.

If the circuit board is non useable, I have a replacement design here that you could get made up. Lot of work though.

cheers

Tony