41C with LCD problems


I finally got an older 41C with the steep faced keys. It's in great shape, except for the LCD. It will only display characters while you hold a key down. Release the key and the LCD goes blank. I checked and resoldered the LCD connections with no change in operation. I found there is no sawtooth waveform on the OS1 pin of the display. I would guess this is problem and since the signal appears to be generated inside the LCD module, is it safe to assume the module is the problem? I just wanted to check with the experts before I cannibalized another 41 just to prove my hunch.


Hello, Randy;

can you check if the OS1 pin has the signal when you have a key pressed? Just to suggest a starting point.

I do not remember what is this LCD's OS1 signal for, but if you have a reaction when a key is presed, than something in the mainboard wakes the LCD up and bypasses this condition. Have you chaecked for the mainboard connections?

Anyway, if you have another HP41C, you may test this steep faced's mainboard in the other one, instead. I think it's safer and you'll get to some conlcusion about the sleepy LCD.

Hope this helps.

Best regards.

Luiz C. Vieira - Brazil

Just another hunch


Hi Luiz,

I did swap out the cpu, it works fine in another unit.


This is a fairly common HP41 problem. When the machine is idle the system is clocked by the display. When it is calculating or a key is pressed it is clocked by the CPU. The problem can be caused by a bad CPU or a bad display module not doing the clock handoff properly.

But, I have found that the most likely cause seems to be some solder flux or invisible cooties on the back of the LCD module or among the module pins. Clean the back of the LCD and around the pins with 91% isopropyl alcohol. Be VERY careful not to get any alcohol on the display window or you will then have to unsolder it and polish off the white stains. After cleaning the module you may have to wait overnight for the fix to work.


Thanks David.

I had cleaned things up last night with Chemtronics Flux-off. Everything was spotless. It made no difference then or now, 24hrs later. I was hoping for a miracle, looks like I better keep praying, as it is beginning to look like a bad lcd.

I'll re-clean everything again and give a day. If I have no joy, it's time to take a good CV and make it a parts machine... Anybody need a battery/port connector?


Hello, Randy;

A friend of mine (ours, of course) needs a battery door, as many others that I know and would like to ask not to be mad at me. But this one - Daniel Sancho - asked me about it and I told him I had none. In fact, now I have two HP41 in the box - not new, not mint, working and stored in a box - without the battery door/compartment. And one of them has no I/O connector, too.

But I have three other HP41's working fine and complete. I believe others need these parts most.

If you think you can, contact Daniel Sancho. This is one of his posts.

Just a way to help someone, I think.

Best regards.

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