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I just got an old 41C #1936A..... with all the bugs! I know that these bugs are intersting but do you know why? Furthermore, some keys are rather hard compared to the others. Do you know whether it is common or not or if it is possible to fix? Thanks in advance, Thib.

The bugs are of interest because:

a) they're there b) they allowed the internal structure of the calculator to be probed c) they allowed new "synthetic" instructions to be coded d) only the earliest calculators have the really interesting bugs (that HP fixed really quickly) and are thus somewhat rarer.

I believe that if you've got bug 3 you've got the lot, as HP rushed to fix it.

Quick battery discharge technique :-)

49 STO 01 SF IND 01

As to the "hard" keys, these are basically worn out. I have never attempted to fix them, and from what I have read it's (at least) a VERY difficult thing to do.

My very early 41C 1937A... has the 1 and 2 keys a bit spongy, but my newer 41C 1944A... has almost completly dead 1 and 2 keys (rather hard is a good description)

In my case the keys no longer click at all (on 1944A), and barely move, but require only about twice the normal effort to actuate. Looking along the face of the keys, these two keys appear to have "sunk" slightly (about 1/2 mm) and are at a bit of an angle. The effect is not visible until you look along the face of the keys, so is cosmetically of little importance.

It's not your 1 and 2 keys too is is? What's so special about these keys?

You're right Steve, My "1" key WAS a bit hard and did not click, same for the "cos" key! I wrote "was" because I somehow cleaned the keybord with Isoamyl alcohol and, aside a terrible smell, it now works much better...It even clicks and does not require any additional pressure to work. Of course they do not click like new but it's ok now. I think you should try that. I have the battery bug! known as bug 3 so it is really an "allbug". Thib.

Having gone back to the original reference, it appears that although HP seemed (?) to have moved on BUG 3 first, the way that the calculators were assembled means that some may have BUG 3 and not BUG 2 (and vice versa)

BUG 2 is confirmed by trying

999 STO 01 RCL IND 01

WARNING!!! this will normalise the contents of a register 25 registers below the "curtain" and may thus cause horrible problems

for reference:

BUG 1: Sigma+ and sigma- do not store X into Last X BUG 2: STO IND xx and RCL IND xx work where xx contains a number between 704 and 999 - they address data outside of normal register space. BUG 3: SF IND xx and CF IND xx work for all flags (as long as xx is not a stack register) BUG 4: SIN of small angles is wrong (in deg mode get sin of 1e-99) BUG 5: problems with incomplete CLP with printer attached BUG 6: (uncorrected?) problems converting HP67 cards to HP41 code BUG 7: (uncorrected?) problems with ASTO copying additional data in a nybble it shouldn't BUG 8: Programs not decompiled in 41 turned off while in PRGM mode. BUG 9: (uncorrected) CAT 1 in PRGM mode stopped with R/S can cause unusual behaviour.

Historically BUG 2 was the most important for the early discovery of the innards of the HP41, but without BUG 9, synthetic programming would have been tremendously harder as it provided the "modern" way to bootstrap synthetic codes into a 41.