HP-21: Odd intermittent operation

[PANAMATIK]

(12-21-2014 05:43 AM)BobVA Wrote:  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

I had the same behaviour on my HP-25. Measuring Power supply with oszilloscope showed stable voltages.

You can perform a manual ACT reset, by connecting POR Power On Reset Pin 18 with VSS 6,4 Volt Pin1 of the ACT chip for half a second. If it works, then possibly only the condensator (2.2uF) connected to the POR Pin is defect.

But in my case I had to replace it by a new ACT chip. They are fitting also in HP-21's.

Bernhard