HP-21: Odd intermittent operation

[BobVA]

Thank you Bernhard! Your information let me cross the POR off my diagnosis list. The reset pulse was quite good, and manually resetting the ACT didn't help.

Interestingly enough, this repair article by Katie turned out to describe the problem I was having exactly:

Quote:On the 27 what I found was that when the RAM/ROM worked, the 1.6KHz line was really 1.6KHz. But when it was not working it was exactly twice this frequency (i.e., 3.2Khz)! I isolated the clock line and found that the ACT chip is the one that generates this clock and outputs it on pin 20.

On the rare restarts when my 21 began operating properly, pin 20 had a 1.6 kHz clock output, and a 3.2kHz signal otherwise. Unfortunately the described fix didn't work for me - even with Pin 20 disconnected the ACT would continue to output a 3.2 kHz signal (except after the occasional "lucky" resets). So I'm concluding the ACT chip is defective. (Looking forward to your replacements! )

Thanks again for your helpful information!

Bob