HP41 Coconut: LCD blanks in Standby mode



#5

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

Thanks!

Luiz (Brazil)


#6

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.


#7

Thanks a mil, Randy!

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

Cheers!

Luiz (Brazil)

#8

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!

Cheers.

Luiz (Brazil)


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