HP-21: Odd intermittent operation

[BobVA]

Hi:

I've been tinkering with an HP-21 which exhibits the classic symptoms of a failed ACT chip: single zero on the right of the display, display filled with zeros or decimal points, single digit counting up, etc when powered on. Current draw is about 120 - 150mA from a stable 2.5 VDC bench supply.

But, about one in ten times it will display "0.00" and work fine as long as it's powered on. Flexing the PC board, wiggling connectors, etc. won't cause it to fail. Turning it off then back on takes it back into psychosis.

Just another act in the Bad ACT drama? Or perhaps something else? Is there some kind of power-on reset circuit that may be faulty?

Thanks!
Bob