HP 41CL; requirements and thoughts about donor shells


I think, with Monte's new release, that a review of appropriate donors would be helpful.

Physical characteristics:

* must be a fullnut; squared lcd corners; *NOT* rounded lcd corners. 
(figures 1 and 2)

* separate logic board.

* must have perfect or perfectly repaired screw posts.

* can have elastomere zebra strips but the wire wrapped are best.
(figure 3)

* display drivers come in two types with respect to the CL design.
- rounded square caps which require a capacitor modification.
(figures 3, 4 and 5)
- epoxy blobs which do not require modification.

* time module, although not required, it does activate the clock.
the functions are avialable without the module.

figure 1: square LCD panel (a donor).

figure 2: rounded LCD panel (not a donor).

figure 3: wire wrapped connector.

figure 4: example of square capped drivers.

figure 5: wiring diagram for capacitor modification.

figure 6: example soldered in place.

Personal preferences:

*CX's would be a waste of a good (albiet slow machine) although
the entire process is reversable from a collectors viewpoint
*CV's are a perfect choice depending on condition.
*C's with slanted keys are more readable.
*C's with tall keys are fun because they are the oldest and combined
with the newest make for a conversation piece.

I have a 41C from 79 with tall keys, square lcd drivers and the two gold ac connector balls. I chose the C out of sentimental reasons as it is my first HP 41C purchased way back when.

Hope this helps, and thanks Monte, still playing! and using!

Edited: 4 Dec 2011, 7:57 p.m. after one or more responses were posted


* must be a halfnut; squared lcd corners; *NOT* rounded lcd corners.
(figures 1 and 2)
Square corners would be the coconut. The halfnut has the rounded ones.

The "ALPHA" annunciator in the display in the picture can be gotten rid of by executing PROMPT to display the alpha register.

Now we need someone to make a time module to to build into the 41c memory modules (the ones that are useless for the cv & cx which already have the main memory capacity filled out) that are cheap on TAS. The time modules are so rare.

Edited: 4 Dec 2011, 4:14 p.m.



the halfnut display is a Display test routine found in PPC ROM or made with the bytejumper.



Just one more picture where you can see the front label:


yep, thats it!



Nice work!

Is there anyway do determine the type of donor by the serial number?

Obviously, the display type is obvious, but the other characteristeics require opening the machine.



Is there anyway do determine the type of donor by the serial number?

Yep. Check page 6 of the HP-41CL manual for the details which you can find here

You'll have to open it up to check the connector type, though.


Edited: 6 Dec 2011, 8:48 p.m.


Randy is correct and I should have mentioned it.

The repair shop replaced keyboards and LCD panels then reattached the original backs.

So you get a mishmash of fullnut serial numbers with halfnut guts. The EARLY HP41Cs have a different rearshell then the later 41's. These are recognized by the hex nuts used to attach the logic PCA to the keyboard.

Later models used a modified rear shell which pressed all subsequent logic PCA's into the keyboard PCA.

On some occasions, HP did not reassemble with the hex nuts as they are very on forgiving and the screw posts stripped. I have two early HP41Cs which HP converted the hex nuts into thick plastic ringed spacers which fit over the screw posts on the logic pca as it sits on the keyboard pca. This allowed for the reuse of the original rear shell which then pressed down on the spacere which inturn, pressed the logic pca into the keyboard pca.

Wlodek's book mentions that HP actually may have repaired some HP41Cs with the halfnut board creating a halfnut hp41C. They did however make from the production line halfnut 41CV and CX's.

The moral of the story is to pick a squared LCD panel and then open up the machine to find a removable fullnut board.

Cheers, Geoff


I have repaired several 41C's that had been "converted" by HP service to halfnut electronics. They were in fact 41CV's with 319 registers.

This brings into question, at least for me, the existence of a halfnut rom that was limited to the 64 registers of the original C. I suspect HP never went to the trouble of making a special version just for service spares but cannot confirm. Really not relevant to this thread and I don't mean to take it of topic but it is one of those things that I hear about but can never confirm...


The existance of a Halfnut 41C is hearsay at best. I remember having a conversation with Wlodek that it might exist as a 319 register machine. In other words, the keyboard shell side was replaced while keeping the rear shell.

I think he said, if you find a halfnut 41C you would have a rare bird indeed.



The existance of a Halfnut 41C is hearsay at best.
There are some pictures of one of these rare animals
here. One interesting comment is:
I have also pluged a quad module which has been recognized....

which may mean that prior to plugging the quad module there was only 63 registers available as on a standard 41C. But this is only speculative and would need to be confirmed.

In an ideal world, one were your 41 never went back to HP for service, the serial number could be used to determine suitability as a CL.

But, in the real world, HP41 serial numbers alone cannot and should not used to assume what hardware is inside the case. Geoff's list is far better and safer to determine suitability. The only safe way to check is to open the unit and look.

There are many early serial numbers out there that have had the top of the unit replaced with halfnut electronics, this occurred late in the service life of the 41 when HP ran out of coconut parts.

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