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HP41CX, not working. - Printable Version

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HP41CX, not working. - isanchez - 06-04-2020 02:57 PM


I am trying to repair an HP41cx fullnut version.
Looks like new. It has the typical problem in the battery contacts. I have replaced this part with the model mounted by Jose (thecalculatorstore)

The thing is that it only shows the upper third of the display with an identical pattern (please look at the attached photo). No response at all to the keyboard (none of the keys, even the on/off).

I can tell you what it does NOT seem to be the cause:

-I have replaced the board with a 100% working CV.
-I have used all the zebra models exisiting (the golden old type, two parts golden new type, the one provided by thecalculatorstore).
-The display contacts look absolutely clean, well connected).
-No traces of oxid at all. It looks like new.

The only issue I have in mind could be the display driver, could this behaviour be caused by the failure of the display driver?

Thanks and kind regards


RE: HP41CX, not working. - Didier Lachieze - 06-04-2020 04:50 PM

Hi, when comparing your display picture with the display character structure in the 41C/CV/CX service manual it appears that the segments lit are all segments in row 0 (segments 00, 01, 02, 03, 04 and 05). There are 3 rows and 72 columns signals driven by the display drivers to control the 12 characters (the row signals are common and each character has 6 columns, so for 12 characters this represents 72 columns signals).

So the display drivers state is: row 0 set to on, row 2 & 3 to off, all columns signals to on. But I don't know the root cause. You say that there is no response from the keyboard, so I suppose that even BEEP is not working, which means that the calculator is locked. As you changed the CPU board this locked state may come from the display driver, but it's just a guess.

RE: HP41CX, not working. - isanchez - 06-05-2020 06:16 AM

Hi Didier,

thanks for your accurate information.
From my side, the Display Driver is a diagnostic by exclusion.

-Many zebras tested.
-Many flex (including an original), tested
-Board switched.
-All connections fine and tracks free of oxide.

It is curious because you usually get a flash or a couple of seconds of "good" connection that gives you a clue of the problem. This has never happened, which means that is something that is present all time. Hence the display driver guess...no more science behind.

thanks again and kind regards,