HP-41C LCD Communication

[haiduc32]

Hi,

I got a HP-41C with LCD misbehaving. I took the logic board and put it into a 41CX and worked fine. But putting the 41CX logic board into the 41C still leads to random LCD characters.
Most of the time the LCD will just show @@@@@@@@@@, I managed to get to show some numbers very faintly with its original zebra connector by squeezing the logic board against the keyboard.. if that makes any sense. I replaced the zebra connector with a brand new one. Could not make it show numbers anymore, just @@@@@@@@@@.

Connecting it to a oscilloscope shows the digital signals on DATA/ISA/SYNC, but in between also some attempted pull-down signals, that get stuck somewhere in the middle. So I am assuming something is wrong with the LCD driver, which I believe is not the original one, as the person soldering it did a messy job.

Any ideas to why it would show only @@@@@@@@@ and why the digital signals can be broken by the LCD?

Also, anyone got any info on the LCD communication? I would attempt to write an Arduino sketch to test the LCD by itself but can't find any info on the subject.

[teenix]

I would have a close look at the solder connections and check for poor or shorted connections on the repaired part. I'm pretty sure the bus signals are similar to earlier calcs so something could be loading the offending bus line or by invalid signals from a faulty part. You might be able to remove the repaired circuitry or disconnect the offending bus and see if the signals now work as expected.

cheers

Tony

[AndiGer]

Maybe the image linked in the post helps (for me - I have to reply by quote to get the real link address displayed).
https://www.hpmuseum.org/forum/thread-36...l#pid34133

Please report what you find. I have an HP-41 with similar behaviour.
My guess: Faulty display driver (under the black "blobs")
Andi

[haiduc32]

Thank you both for the pointers. The link was very useful. Indeed OS1 does not behave correctly and OS2 does not generate a 7khz signal.

Here's my progress so far:

After rubbing all contacts with isopropilic alcohol and resoldering all connections on the LCD it... suddenly started working! only to start working randomly... now sometimes it works, OS1 has a nice triangular wave, OS2 square. Then without touching anything it just goes to Memory Lost, or some other garbage. Power on/off also works randomly.

Assumed it could be the OS1 making a poor connection and after I disconnected OS1 and soldered a 610pF smd capacitor to ground (don't have a 470pF, but assumed should be good enough) dirrectly on the LCd board, it doesn't work. OS1 just goes to HI, and does not oscilate.

Also tried to connect OS2 to a signal generator and put 7khz, as in the suggested post. That.. was a bad idea. When powering the lab power ssupply it went all 330mA and I noticed that too late.. Half the LCd doesn't work at all now.. So.. don't try that.

Will try the next days to play around a bit more, but... pretty sure the driver is toast.