41c vs 41cx display compatibility

[3p141592654]

I have a fullnut 41cx that the top rocker switches seized. I have no idea why and it looks irreparable.

I bought a fullnut 41c thinking I could swap display board over. But the 41cx motherboard does not turn on with the 41c display despite the interface looking mechanically identical.

41c display board is 00041-60004

41cx display board 00041-80004

Are they just fundamentally incompatible?

Photo of boards can be found here.

https://photos.app.goo.gl/ekSLkd67ZEWdazA66

[GreyUser]

You’ve got a bit of a mix up there.

The original 41C lcd driver was pn 00041-80003.

The later board used on all models was pn 00041-60146.

Both driver boards appear to me to be the -80003 variant in the attached photo. That must be a very early CX model, one of the first if those keyboards are all original.

The later -60146 is approximately 2 mm higher than the original -80003 so the lcd’s are not (easily) interchangeable between the two boards but as an assembly (driver board and lcd clipped together), they are interchangeable. That said, the later boards are a very tight squeeze in tall key C’s.

Any 41 that has one or both immovable rocker switches is probably stuck together with crystalized sugar from a pop or other high sugar beverage spill. These are always cleanable in warm water, once the lcd and driver board assembly has been removed. Crystalline sugar can have surprising strength when lodged in key hinges.

To complete the display driver discussion, early tall key C display drivers with the square, molded chip covers require a different value clock timing capacitor. IIRC, it’s discussed in the 41 service manual under revision histories.

Be careful swapping fullnut CX boards, the majority of CX processor boards were built with .050 thick material, not the original .062 thick FR-4. The reason for this change is unknown but I suspect it was a cost savings or wave soldering issue. So, don’t lose or misplace those .015 Teflon washers placed on the lower posts, they are absolutely required. If omitted, you’ll have what you describe, an apparently dead processor board.

[3p141592654]

Wow, thank you for that info. I went back and double checked. The seized 41cx is on 00041-80004 and the 41c is 00041-60004 as the photos show. I bought the cx when it first was released probably in 1985 so it would certainly be an early model. Its never been apart until now so its guaranteed to be as it was from the factory.

So I measured the motherboards and indeed, the 41cx is ~50 mil and the 41c is ~60 mil as you note. Excellent, that clears that up. Except I see no trace of 15 mil teflon washers. I must have overlooked them and now they are lost. Mcmaster has these which seem like they will work

Chemical-Resistant PTFE Plastic Washer, Number 8 Screw Size, 0.197" ID, 0.28" OD, 0.013"-0.017" Thickness

I'm not sure how to spot a tall key 41c but the measured key height is ~100mils for both.

So I did try flushing the rockers with IPA, but that did nothing. However, if its sugar then I need water not IPA. Do I literally dunk the keypad in a jar of DI water, or should I spray it on only the affected keys?

[GreyUser]

Excellent find on the washers, I was not aware that McMaster Carr stocked them. Part number is 95630A239 for others reading the thread.

As for the keyboard cleaning, once the lcd display is out and the keyboard connector has been lifted off (VERY important!), immersion is the way to go. An ultrasonic cleaner is very helpful but not essential. Let it soak for 5 minutes or more, press all the keys while immersed, this will loosen any crud in the hinges. Rinse and repeat the key pressing while submerged. Once you’re okay with the cleaning level, shake as much of the water out as possible, press all the keys several times to force the water out of the domes and place in a warm area with small fan blowing over the pc board side for 24 hours minimum before re-assembly.

Really filthy 41’s clean up like new with a mild cleaner like dilute Simple Green and a toothbrush then the dunk treatment to flush it all away. Restoration hint: Polish any scratches out of the lcd window before immersion cleaning.

[GreyUser]

Ah, now I see what you’re saying about part numbers. You’re referring to the keyboard pc board copper pattern, I was referring to the display driver boards. The keyboards are 100% electrically and physically interchangeable.

You’ll only see the part numbers of the display drivers if you have the lcd’s separated from the driver board. The entire lcd assembly is called the A2U2.

The “tall key” reference is to early 41C’s that had keys the same height as the 30 series model that preceded it. It looks like a few CV’s were made in this variant as well, based on this thread (post 13):

https://www.hpmuseum.org/forum/thread-12012.html

[3p141592654]

Thanks for the detailed cleaning steps. I assume the keyboard connector pops off the board. I tried prying it gently and it seems well stuck. A little terrified to pry harder! The teflon washers are a bit of a miracle. They have very few that thin but that one is pretty near the ideal size.

[GreyUser]

You should be able to lift the keyboard connector off with nothing more than fingers. I don’t think I’ve ever had to resort to anything more drastic than a squeeze and pull with the version you have.

A quick, low-res measurement of the washers yields .015 thick x 3/16 id x 5/16 od so the McMaster part is likely spot on.

[3p141592654]

Strange, both of the connectors are a tight fit on the posts. I'm using plastic cell phone tools to remove it gently.

My washers should come in tomorrow, at which point I will see if the CX motherboard works on the 41v keypad. Good news that the dimensions seem correct.

[3p141592654]

The teflon washers work. They measure about 16mil. The thinner 41cx motherboard does indeed function with the 41c display with the washers added.

Using an ultrasonic cleaner and DI water I was able to free up the sticky rocker buttons. Thanks for the advice to try that.


https://photos.app.goo.gl/xLmR1KBvuEL3rzcWA

https://photos.app.goo.gl/5naERHitdEGkizHb6

[GreyUser]

Thank you for posting your success. It’s always nice to see another 41 returned to service.

They’re pretty robust devices as HP had the design pretty much perfected before switching construction methods to the later halfnut which ironically was more reliable but far less repairable.