Restoring an HP-55, advice appreciated !


Dear All, in primis sorry for the bad quality of the pics ...

I'm restoring an HP55. I knew it was a difficult one, but I couldn't resist since it was a cheap one.

The good news (one...) : it works !!!

The bad news(list of...) :

The labels ... It is missing front and back label, which at least for the back label shouldn't be too difficult to get.

Does anyone know a good source for front labels ?

Then the keyboard ...
The top row has partially been transplanted from an HP45, and I think that the only way replace them is to find an HP-55 donor,

all of other classics are not suitable, I was thinking about the HP67
but I should be banned from the forum for having forgotten the obvious differences from the two keys shapes...

Then the display ... the second group of leds is much brighter than the rest of the digits, any hint on why this can happen ?

Could it be the controller ?

Then the switches ... the calc was without, and I had a spare set from an old HP67. The on/off works quite nicely, but the switch that toggles from run, timer and prgm does not ...

I may open my other HP55 to check how different is from an HP67 right switch, but I was wondering if any one knows the difference so that I can avoid to tamper a unit...

Than you very much for all the support !

take care Alberto



...all of other classics are not suitable, I was thinking about the HP67
but I should be banned from the forum for having forgotten the obvious differences from the two keys shapes...

The keys of the 67 are very different from these. They have the black labelling of the "h" shift key on their front face.

My honest opinion? Use this Hp-55 as a donor to restore a better one!



I'll second that.


The difference in display brightness is to do with the LED display modules themselves. The 15 digit LED display is made up with three 5 digit "modules", each module has a brightness indicated by a letter printed in the back of the display module. The letter is like "f", 'g" etc etc. To get displays that looks the same brightness you need to match or closely match the letter indicating brightness, but you don't need to be exact.

So, nothing to do with the anode or cathode driver chips.

I guess you're on to Ben Ong re back label ? Ben did help me with a front label for a HP80 at one stage, so ask him about the 55 front label, he maybe able to help.

Did you get the metal contacts of the switch the right way up ?



ciao John,

thanks for your reply,

I have a couple of HP45 that I can use as a donor and I will try to replace the led modules

Yes, it was Ben Ong I had in mind,I do have a replacement set for the back label.

I know him since at one point I wanted to have a spare label for every unit I was collecting.
That was mostly for ther 65s and the 67s, which often get opened because of the card reader.

The keys were loose because the plastic insert that goes between the keys and the contacts board was missing.
I had a spare and that has been replaced as well, including the thin plastic sheet that should prevent dust from reaching the inside of the unit.

Now the keys, neverless some are wrong, are firm and they click as they should.

The on/off switch is ok, it works and it is quite firm.
A thin layer of multipurpose grease has been applied and it works like a charm.

The other is not, and I wonder if being a three positions switch it has a different interior size.

I can't recall it from the other unit I have and I don't take picture all the time I open a calc (I should ... I know).

Do you know if these are of the same length ?

again, thanks for help, Alberto

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