Dave Ramsey et al with early HP 41C conversions to CL


I thought I would start a separate thread for this as a few people have converted the old hex nut version 41C's to the CL.

The hex nuts were not meant to be removed and replaced more then a few times at best. Maybe it just my clumsy fingers but read on!


The hex nuts indicate an early 41C. Trouble sometimes is the posts are not threaded initially and reinstalling them can result in a cross threading leading to a stripped and thinner post.

You are correct (from Daves posting below) as the back shell is different from later versions due to the hex nut installation. HP actually, on repairs, removed the hex nuts and replaced them with plastic spacers. These were slightly thicker then the height of the posts protruding from the logic pca. This resulted in the rear shell pressing the board onto the strips as opposed to the hex nuts doing the pressing.

A simple replacement for the hex nuts would involve similar spacers or silicon tubing cut to a slightly higher height then the protruding portion of the screw posts. This would have the same affect of the rear shell compressing the pca into the zebra strips.

Silicon tubing replacements for stripped hex nuts:

Spacers precut; #1, #2

#1 top post reinforcements cut to length (flush with top of screw posts.

#2 bottom spacers/reinforcements cut to a silly millimeter more then flush.
#3 original HP spacers, which do not compress the screw posts, hence no reinforcement ability.

One can also purchase styrene tubing for the top posts but this requires shaving the very tips of the battery/module block. You probably remember this from the HCC2011 presentation and the youtube copy.

Shaving a tiny portion of the indicated vertical plastic from both sides of the battery/module block:

Module in place showing the new reinforcements and indicating the area of the battery/module block that needs shaving (immediately adjacent to the reinforcements on the screw posts (white):

The logic PCA plus spacers and upper post reinforcements in place:

The tubing is 7/32” or 5.5mm and also comes in brass (overkill?).

As Monte states, the posts and contacts must be good, or, as I found out during a flash write, BOOOM, no more CL. Fortunately Monte re-flashed the machine successfully. But as a simple google of "flash drives power interupts during write" this can be fatal to the drive.

So insure good connections. Non stripped posts, internally as well as the external hex nuts. Good elastomer (as Angel states below, how does one tell?) or wire wrapped zebras, clean pca and battery/module block contacts.

Cheers, Geoff

Edited: 30 Dec 2011, 3:40 p.m. after one or more responses were posted


Many thanks for posting this again Geoff, it´s always valuable and I keep misplacing the links to previous postings.

Good elastomer or wire wrapped zebras, clean pca and battery/module block contact

In your last sentence you imply that there are ¨good¨ elastomer stripes - how can we tell them from the ¨bad¨ ones??



Well, that is a good question? Other then obvious defects I can only state that a working elastomer on a CL in normal operations would be and indication of a good elastomer.

Of course that does not involve the increased load on the elastomer during a flash write operation. But then, how does one now if the wire wrapped connector is any better in each individual case.

For me, the elastomer connector would, when placed in its brackets, be proud, that is extend above the retaining bracket to ensure a positive contact.

I guess a caveat on the elastomer statement is required.

Cheers, Angel!


I also saw one machine (my original dead 41) with the plastic spacers. The nuts are a way better solution: the only way to improve them would, of course, be to have the posts they screw onto made of metal, or at least pre-threaded.

I was very careful putting the nuts back on. HP appears to have used some sort of mild thread lock compound originally. None of my nut drivers would fit so I started the nuts with my fingers (one of them took quite a while to get lined up right) and tightened them with-- take a deep breath-- needle nose pliers!

But it all works. Just wish I had a suitable calc with a better keyboard. My Blanknuts were the only other candidates I had and I didn't want to potentially screw them up...


Hey, Geoff, would acetone work as an adhesive to re-attach the broken post?

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