Please help! HP-55 odd behavior


Thanks to Eric for responding, but I need more input from the Classics experts in the group.

Testing a newly-acquired HP-55. Timer mode seems to work fine. In program mode, I can enter keystrokes fine. But in run mode, no numeric input registers, and trying to run a simple program results in a flashing display.

Other keyboard functions (at least what I can try) seem to work. For example, although I cannot enter any numbers (the number keys just don't register on the display), I can, for instance press Sigma+ a few times, which leaves an integer in the display, then act on that integer. Correct results.

Any ideas?

Edited: 7 June 2011, 1:30 p.m. after one or more responses were posted


Sounds to me like one of the ROMs has gone bad, though there are probably other possible causes.



Is the calculator architecture such that different ROMs control registering numeric input and functions assigned to the same keys?
And if one ROM were bad, everything else would work fine? Thanks.


The stuff is distributed somewhat haphazardly between the two ROM chips. However, usually when a ROM fails, it isn't the whole ROM that fails, just some locations.


More observations:

1. Start in run mode. Press either STO or RCL X (where X is an integer 0-9), get 1X.000000 in display. Press CLX, get all zeros flashing.

2. In spite of the above, in timer mode the lap times are faithfully recorded in registers 0-9, and can be recalled without error.


It couldn't hurt to check the mode switch - make sure that the "RUN" position is actually doing what it's supposed to (i.e. that the proper connections are being made in this position).


Bob, it does do keyboard functions correctly if I can get a number in the X-register, by a slight of hand, such as using Sigma+ as described above, or by letting the timer run, then switching to run mode, which puts the timer result in the X-register.

For example, the trig functions and unit conversions return correct results after the trick above. So my question to you is, could these things happen if the mode switch wasn't working correctly? I don't want to open the calc unless I have to.



I don't want to open the calc unless I have to.


May I ask why you hesitate to open it? Unless you are worried about the decision to try to remove and re-apply the back label vs. poking holes in it to get at the screws, opening and working on any classic is very easy. Enables a thorough cleaning of keys, keyboard, key contacts, and switch contacts.

Sorry if you are well aware of the above, just wondering.



Thanks, Jeff.

I am reluctant because I may not then be able to return it to the seller if it is non-functional.

Anyway, the calculator is immaculate on the outside; seems it should not need any cleaning on the inside!

Edited: 8 June 2011, 5:15 p.m.


Hi Martin:

Unless someone has seen exactly the problem you've got (ideally a couple of times) it's going to be pretty hard for you to place a bet on whether it's going to be fixable.

If you got it for a good price, it's probably worth keeping for parts. If you paid a lot, that may be another matter.

Hopefully somebody will chip in with "oh yeah, seen that" and help you sort this out.



There are only three possibilities that occur to me:

1) A rom such as the keyboard controller is bad on the logic board

2) A connection between the logic board and keyboard is bad

3) A circuit trace on the keyboard is broken (open circuit)

If the calc has never been opened and there is no evidence of corrosion, then I'm inclined to believe case 1 is most likely.

I agree that if the purchase price was high and the seller advertised it as fully functional, then I'd return it.

Edited: 8 June 2011, 7:38 p.m.


Sorry, I did not realize or recall the situation that you just purchased and may return it.

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