HP9100A failure after years in storage

[peter scott]

The 9100 was working when put into my loft but is now faulty. I have been receiving some very good suggestions from Trevor Marshall and Tony Duell but I am really struggling to locate the fault because I don't have schematics/layouts for the arithmetic unit.

I have the 9100A field service manual but this only covers the power supply, keyboard and display and these functions appear to operate correctly in my machine.

I do also have good schematics for the 9100B but these very different in many ways. If anyone could point me to where I might find the 9100A schematics I would be very grateful.

So far with lots of help from Tony and Trevor I have diode checked every transistor and diode on all the assemblies and carefully eye-balled all solder joints and cleaned all edge connectors. I have confirmed the -15 volt, -9 volt and -2.4 volt rails and generally cleaned the assemblies with compressed air.

The fault is that it powers up and displays the three registers and although all the keyboard keys are active, the numeric entry is not entered except for the Pi key. The arithmetic operators do work as do the register operators but pressing any of the trig keys causes the display to blank and no key or switch operation will return it except by power cycle.

Having cycled the power the display returns but it has two modes. It either operates as in the paragraph above or by pressing absolutely any key the display blanks again.

It is possible to get out of this mode by entering a program from the magnetic card reader.

Thanks for any suggestions,

Peter