hp41cv repair question



#5

Hi,

I am reparing an hp41cv. The "2" key was missing strokes. I took the 41cv apart and everything inside looked fine (its a fullnut but its posts were not cracked). Next I removed the lcd (de-soldered) and sonicated the keypad for about 1 hour in 39C deionized water. Then I let the keypad dry by placing it on the top of my computer monitor for 4 days.

Upon complete re-assembly the 41cv does not respond. So I replaced the keypad/display with a spare keypad/display from another 41. With the spare keypad display everything works so the problem is in the original keypad/display.

Any thoughts? What could have gone wrong? I was very careful when I removed/replaced the lcd and made sure all the solder joints were continuous with a multimeter. Is it possible there is still water causing shorts?

Thanks,

Andy


#6

Hi,

it seems the I/O assembly of your first kbd/dsp combination is faulty,

and/or the zebra stripe between the CPU board and the main pcb is bad.

Or maybe one or more contacts got too hot while resoldering the display...

The I/O assembly is the block where the battery and port connctors are located on.

Raymond

#7

Five to ten minutes is all you need in an ultrasonic cleaner.

ANY moisture inside the keyboard will cause problems. Four days should have been enough time though if you shook it out well afterwards and pressed all the keys to expell the water inside. If in doubt, the 41 service manual (on CD5 or the DVD) details the keyboard connections, I would ohm out the matrix to see if there is leakage anywhere.

Residual flux on the display connections will cause trouble as well, they need to be spotless. Clean with isopropyl alcohol or the proper flux remover for the type of solder used. Do not allow the flux remover to get into the keys or on the display window or you'll have intermittant keys and white haze on the lcd.

If your units logic board to keyboard interconnects are the early pink elastomeric strips, try exchanging them with the later gold metal type. They tend to be very problematic.

#8

I printed the keyboard pinout from the service manual. After testing many of the keys I decided they were probably all okay.

Then I removed the display again. Next I removed the two c-shaped clips which hold the lcd-sandwich together. I squeezed the clips and put them back on in hopes they would provide a little more clamping force. Then I re-soldered the lcd back in place, put it all back together and it works. I'm not sure what was wrong before ?.

Now I have a very very clean 41cv with a "2" key that works much better!

Thanks for the help,
Andy


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