HP-21: Odd intermittent operation

[PANAMATIK]

(12-22-2014 05:26 AM)BobVA Wrote:  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! )

The frequency of Pin 20 (Sync) depends on the CPU instructions, that are performed. When you are measuring without doing calculations, the CPU is within an small idle loop, waiting for a key pressed and executes many jump instructions, that outputs a sync every two instruction cycles. Other single cycle instructions do the sync signal every cycle and you will measure 3,2 kHz. Obviously the ACT is not executing its normal idle loop when you measure 3,2 kHz. So we can also conclude, its not an RC oscillator, that is running with double frequency, but some defect inside the ACT.

Bernhard