Early HP-41C PCA picture as well as memory problem...


HP-41C 7931Axxxxx tall keys with ac contact balls and all the bugs! Just one problem:

Setting size equals zero results in program memory totalling 47 instead of 63. Obviously one of the D/S chips is shot as each chip is 16 56-bit registers. Which one though? All programs function in the allotted memory. The machine acts normally except 47 registers instead of 63. Since the curtain moves between program memory-registers and machine memory could it be the last D/S chip that is unserviceable?

Here is a shot of the PCA with the defunct D/S chip. Since I do not have a service module (anyone out there willing to throw one at me for free ;-) am I correct in my assumptions that:

a.     it is the last 16 bit D/S chip (63-47=16)that is shot and
b. it is chip marked E in the figure below.

Note the hex nuts that hold this in place!

Here are some off topic pictures of the card reader internals going into the manual. Just trying to pique your interest.

By the way, what is the correct term for the wire connectors pictured in the second and third pictures below, I think the term starts with a Z (zygat)?

Edited: 3 Aug 2009, 8:13 p.m.


Nice pictures of gummy wheel. looks like crusted cereal to me :-)


Mmmmmmmm ... tres delish ... like to have some for breakfast!!!

: oo
: oo ==
: |
: |
: oo |
: oo ==


Edited: 3 Aug 2009, 11:22 p.m.



I dropped the drive roller in some Kentucky fried chicken batter!

Cheers, Geoff

I'll bring some to Ft. Collins!

Edited: 4 Aug 2009, 12:00 a.m.


I dropped the drive roller in some Kentucky fried chicken batter!

Is this a brand new repair method that you're experimenting? ;-)





The bad chip should be the one marked 1LA7-0005 (or 1LA7-05). I can't read the markings on the chip you labeled "E", but that's probably it.

If you do a "0 SIZE" and plug in one or more memory modules (starting with port 1) or a single quad memory module, you should be able to address their registers starting at register number 16. There will be a gap from 0-15 for the bad chip.

You could replace the complete set of 1LA7-0002 through 1LA7-0005 with a single 1LE7-0001 or 1LH5-0001 (but keep the 1LA7-0001). If you also added the 1LE7-0002 through -0005 (or 1LH5), the result would be a CV.


Thanks Eric, still not clear but yes the one labelled E is 1LA7-005.

So the repair has now entered a new realm. To create an early HP-41C and creat a hybrid CV board.

That would be Gene's call as it is his machine! What to do eh Gene?

Now to source some spare D/S chips so labelled!

Cheers, Geoff


Hi all,

Geoff, although the faulty IC has already been identified as 1LA7 005, you can always configure your Clonix as any of the service modules available:

ET-11966: for HP-41C and Card Reader test

5061-7221: for HP-41C, CV and CX test

ET-11968: for 82143-A printer test

The required files are available at my web, under "Download" on the left.

Best wishes.



don't want to mess with the clonix that I have as it set up with my own ROM images as well as CCD and PPC.

So, I guess I should order another Clonix with the ROM images of the test modules.

Another email to follow!



I read 1LA3-0015 and 1LA3-0016 for the top two ROMs. Can you make out the designation on the bottom one? Is it 1LA3-0017?

Does this unit have bugs 1-4? I'm guessing that without the service module you don't know the firmware version, but I'd like to try to correlate ROM versions to part numbers.


Hi Eric,

Let me take a look under good light and a magnifying lens, but it does have ALL the bugs so version one would be a good guess.

Yep 1LA7 005 production date 7924

Edited: 5 Aug 2009, 12:23 a.m. after one or more responses were posted


Sorry, I don't think my request was as clear as it should have been. I'm interested in the markings on the ROM chip at the bottom left of the photo. I think it's probably 1LA3-0022, the earliest rev listed in the service manual.



ILA322: date code 7917.

Wasn't you Eric, I have been running a limo service here all day for family, and catch the museum in between runs!

Missed the nuance of the question and just assumed it was about the D/S chips.

Gene, I am game, just have to find some donor D/S as Eric has described!

Also one could hard wire a Quad directly into the board if we can only find a replacement for the bad chip.

Cheers, Geoff


Well, it will already be a repaired machine, so it isn't vintage, is it? :-)

So, sure... why bother having to have a QUAD module in this machine if it can already have the full memory? I'll drop the QUAD into the other all bugs machine I have. :-)

So, if we can bump it to the full memory, by all means... if it works and if it still has all my bugs! ha!

(Would the other all bugs machine be of interest to you two to be disassembled to check on stuff? It is a 1932A S/N while the one pictured here is a 1931A S/N)

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