Dying HP41 Displays

[Harald]

I have 4 HP41s. Three of which have a display problem now.

No. 1:
Has excessive current draw and will only start on almost depleted batteries. I got it off ebay and all was fine until I replaced the batteries. It wouldn't start anymore and I fiddled with it until it did. Only to find the batteries were almost dead again. Took me a while to figure out what was going on. Initially I thought it was the CPU board, but it tunrned out the problem is the display board.

No. 2:
I bought for parts (hoping the display was good) and for the very nice tall key keyboard. But after cleaning it of all its corrosion, the only thing that didn't work was - you guessed it - the display. It displays something that looks like @ signs all the time. Sometimes I can get it back to life by repeatedly resetting the machine and removing and inserting the batteries several times. Once it works it is fine. But removing and replacing the battery puts you back into the same trouble.

No. 3:
Has just lost a decimal point. On closer inspection there are also segments missing from the last two digits. So it will only display exponents partially - so not really usable at all.

No. 4:
Probably best not to touch this. I don't want any more display failures...

Any idea what the issue is? Especially with No.3. There is no way it could be a bad connection of the CPU board to keyboard, could it? If Segments are missing I guess it can only be the zebra connector, right? Maybe that is due to a climate change. I just took it from home to work, where it is much drier (we are at about 15% rel. humidity here now) than at home.

Chris Chung - Hurry up with your NP41! I need a backup plan

[Geoff Quickfall]

Many failure points:

The obvious to the less likely:

1 Contact between module/battery block and keyboard.

2 contact between PCA and keyboard.

1 & 2 depend on good screw posts; not cracked or stripped posts.

1 depends on a foam insert on the module/battery not being compressed over time. Similar to the foam inserts on the pioneers.

2 depends on a) zebra strip contact or b) wire wrap with foam insert which can also fail due to age.

----------------------------------

3 cold solder joints on the LCD strip.

4 zebra strips built into the LCD panel assembly.

5 LCD drivers.

I have seen 1 through 5 cause display problems. 1 through four are repairable, 5 needs a donor panel.

Cheers, Geoff

[Geoff Quickfall]

Further:

Using your example numbers:

#2. Sounds like module/battery block connection, see previous post.

#3. LCD internal zebra strip. Involves removal and disassembly of the LCD panel to access and clean zebra.

Geoff

[Harald]

(10-12-2015 06:40 PM)Geoff Quickfall Wrote:  Further:

Using your example numbers:

#2. Sounds like module/battery block connection, see previous post.

#3. LCD internal zebra strip. Involves removal and disassembly of the LCD panel to access and clean zebra.

Geoff

Hi Geoff,

Thanks for your reply!

I agree with you on #3 and I will attempt to clean the zebra.

On #2 I am not so sure, because I have swapped the main PCB and also soldered wires to the keyboard to power the calculator - still the same behaviour.
What I forgot to mention is that when I disconnect power lots of segments show up before the display shuts down. I think it is more likely to be the display PCB that is causing the problem

Harald

PS: sorry for bugging you - did you have a look for RAM chips?

[Geoff Quickfall]

Believe it or not, I am paying for taking time off to be at HHC2015. I have 25 ram in unknown condition but not 25c ram.

I will look further.

Sounds like display drivers. I replaced Roger Hills LCD panel on his early 41c from 79. It displayed all @ also and after extensive trouble shooting a donor panel was needed.

Cheers, geoff

[Harald]

(10-13-2015 05:55 AM)Geoff Quickfall Wrote:  Believe it or not, I am paying for taking time off to be at HHC2015. I have 25 ram in unknown condition but not 25c ram.

I will look further.

Sounds like display drivers. I replaced Roger Hills LCD panel on his early 41c from 79. It displayed all @ also and after extensive trouble shooting a donor panel was needed.

Cheers, geoff

Thanks for your effort - there is no rush, I just didn't want it to get lost.

I have several ROM and a few "non C" RAM chips in unknown condition as well. I really should prepare a calculator with sockets and test them all.

So I'll either hunt for more dead 41s to rescue LCDs or put some 41s on ebay so others can fix their 41s. I'll have to think about that one

Cheers,
Harald