HP-41CX Missing Clock

[rocket.scientist]

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)

[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.

[rocket.scientist]

Thanks for the input, Monte. The resistor has drifted up from 22M to 27M, so I am going to replace that. I did find a reference to a 32.768KHz crystal in the 00041-90472 Service Manual, so that assumption was good. I scoped the oscillator circuit and it is not running. I am hoping replacing either the resistor or crystal will be the thing. Past that, it looks like replacing the timer IC 1LF6-0001 is next.

-Pat (KG5YPQ)

[rocket.scientist]

Got it going! The issue was a broken lead on the HP factory installed crystal at Y2. I replaced both R4 and Y2, and CLOCK fired right up. :-) The scarce timer IC 1LF6-0001 is just fine. Now lets see what the drift on that new crystal is like.

Whew! That was a fun little project, and now I have nice little CX to add to my collection.

R/

-Pat (KG5YPQ)

[cruff]

Good news indeed! The timer module is really a nice piece of design. I used mine once to gather data for a statistics class project in college. I timed the arrival times of vehicles at an intersection to gather the base data set. The timer module in my 41C made it very easy to do that. Then I explored what would happen if the timing of the light or traffic density changed.