HP41 Coconut: LCD blanks in Standby mode


Hi, all;

as you may have noticed, I am struggling to get back as a regular contributor, but I'm still trying to find balance while remembering how to ride the bike... Takes some time, though.

Well, now for the question: does anyone remember (or know) which capacitor in the HP41 coconut (fullnut) acts as timer for the LCD module while the calculator is in standby mode? I have an HP41C that suddenly decided to behave like this: LCD contents flashes briefly when contents change, otherwise it is blank. I remember reading about this some years ago, but I did not find the posts about this (it´s been a long time not riding the bike...). The mainboard can be seen here


Luiz (Brazil)


It's the small red 470 pf cap on the lower left side of the board.

Older display drivers with the square caps, not the later epoxy blob covers, use a 0.01 uf capacitor in this position. Using the wrong cap will cause problems. There is more info about this in 41C service manual available from TAS. Look for page 41C-13 dated 11/21/79.


Thanks a mil, Randy!

I´ll soon grab the soldering iron and transplant a new one ;-)


Luiz (Brazil)


Hi Randy, guys;

before going ahead with the replacement, I decided to inspect the mainboard and found some remaining varnish covering part of the contact related to the 470pF capacitor. It was probably OK but I consider that the PCB shifted sideways enough to isolate the contact. After cleaning and polishing, I rebuilt it and it is working fine.

Thanks, Randy!


Luiz (Brazil)

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