HP-41C partially repaired, help with "ROM", alpha glitches

About 4 years ago, I discovered my HP-41C had a battery leak. Honestly, the leak could have happened any time in the 4 years before that, I had not used it. I cleaned out the leak, by now all dry and crystalline, tested the 41 perhaps not thoroughly, and it seemed to work.

I went back to it a couple of times a year in the last few years. Some times it lit up no problem, sometimes I had to do a hard reset, take out batteries, short battery terminals, whatever, it eventually came up. This last time, nothing. So I found the screws, opened it up and found more battery leakage inside. I carefully cleaned it up. I have reassembled the calculator and it comes up. I was trying more extensive tests, and to use it, but there are glitches. When I hit [program] the first time, I get "00 REG 46" like I expect, but from there it has problems.

When I press XEQ [alpha] (in program, user or normal mode) preparing to enter a command name, it comes up with "IQ -" or something similar on the screen, and I cannot type keys to enter a command name. The message was the same in dozens of experiments. (After a second cleaning, the message changed slightly, still starting with "IQ" and from that point on was again consistent.)

Soon after, regardless of what keys I press, I get the "ROM" message. Some sequences lead to the calculator shutting itself off.

Once there has been any glitch, Goto.. does not get it back to "00 REG 46" or ".end." or whatever is normal. It typically comes up with "ROM" again after a few keystrokes.

Same thing happens when I try to enter an alpha after any command, including LBL.

When I enter program mode, after a few keystrokes, it says "ROM".

Once it has thrown this error once, only a "memory lost" reset gets it back to "00 REG 46" when I hit [program] the first time.

Any ideas on where I should clean next to get this to work ?

(I have used no solvents, only a clean cloth with DI water-fully dried after, and gentle mechanical methods were used to clean)


Could be a poor electrical connection between the I/O Battery contact block & the mother board. Did you find any corrosion in this area of the mother board & is this where you cleaned?

If you did it may be worthwhile pulling the calculator apart again to ensure there is no lint or other matter from the cloth present in this contact area. It really needs to be perfectly clean for the calculator to work.

The other thing you can try is to gently push the I/O block with say a cotton bud via the battry compartment to ensure it has settled on the mother board correctly.

Could also be an issue between CPU board to main board connector (if a full nut type 41) but only if you removed this as well.




Hi Matt,

Corrosion is well known to be the worst enemy of our beloved HP-41's.

Although it causes most of the damage to the I/O Flex PCB connector, it may also find its path to the internal circuitry; eventually rising the keyboard PCB as well as the CPU board and the "zebra" connector between them.

My advice considering the symptoms you've mentioned (and assuming it's a coconut type) is to take it apart again.

Use Q-tips dampered in diluted lemon juice to throughout clean all and every single circuit trace (regardless you "see" corrosion signs or not) then "rinse" (also using Q-tips) with isopropyl alcohol and dry.

Pay extra attention to the "zebra" connector and the contact pads it matches to (where the described type of problems are more likely to be originated) and make sure it is as clean as possible.

This should have fixed any electrical issue.

Now to the mechanical troubleshooting, double check the plastic post for cracks or damaged screw treads, as these can cause your CPU <> Zebra <> Keyboard connection to become unreliable.

Reassemble and post again with a follow-up... or with any other question or request you may have.

All the best from Caribbean Sea.


Edited: 26 Sept 2012, 2:27 a.m.

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