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

[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

[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

[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