HP-41CX Missing Clock

[Monte Dalrymple]

(10-24-2020 10:25 PM)rocket.scientist Wrote:  So I have been working on restoring a HP-41CX fullnut of unknown heritage. After a few repairs I did get it to power on, "MEMORY LOST", and then power cycle to what looked like normal operation. CAT 2 shows the usual complement of ROMS for a CX. Here is the weird part. The clock appears to be inoperative. I can do a SETIME to whatever, and TIME always returns 12:00:00. Executing CLOCK or Shift->ON results in a blank display.

I suspect either there is a ROM issue or a problem with the clock circuit itself. Doing some research, it appears the clock circuit consists of R4 (at 22MOhm) and Y2 (marked 0410-1381). I am replacing both of these, but I cannot find the specs of the 0410-1381. I am assuming it is a 32.768KHz jelly bean crystal, but who knows.

Does anyone have insight into this part? I am also curious if anyone else has run into the CLOCK blank display thing before. It sounds quite a bit like this post from 2006:

https://www.hpmuseum.org/cgi-sys/cgiwrap...read=85135

Thanks for any advice.

-Pat (KG5YPQ)

Either the counter isn't counting or the DATA line to the timer chip is open. Could be the 32768 Hz crystal isn't clocking the counter, but since you can't set the time (or date?) I would suspect the DATA connection before the oscillator. It is also possible that either PH1 or PH2 to the timer chip could be open. Either of those missing would also stop communication between the CPU and the timer chip.