DIM LCD HP-11

[aj04062]

I posted this question a while ago and am reposting to see if there are any new ideas.

I picked up a Brazilian HP-11C that has a very dim LCD. One forum member suggested the Capacitor. Can someone confirm this is the cause?

If it is, can someone identify it for me?

SN is 2825B29156

[Randy]

There are two caps, one across the battery and one in the rc clock oscillator. The battery bypass would have no effect but if the clock circuit is not the correct speed, it could affect the contrast.

The simplest means to test is to run the "X" key self-test. Typical time is about 25 seconds to complete.

[aj04062]

using a stopwatch it took about 13.5 seconds for the display to read all 8's.

Does this mean the clock is too fast. Does too fast result in a dim lcd or the other way around?

BTW, I ran the same test on my 12C and it finished about the same time. It's display is fine. (SN MY81707265)

I have a donor 12C from singapore (SN 3519S05250) which is built with the same 1 pc board. I want to determine if I can fix this with caps or if I should tear into it to replace the LCD. I prefer to do this as a last resort but will do so if it is the cause.

Thanks,
Adam

Edited: 3 July 2010, 10:46 a.m.

[Randy]

How many chips in the 15C? Two or three?

[aj04062]

on the donor 12C or the 11C (the unit I want to fix is a 11C)? Both have 1 main chip. If you mean the caps, I'll have to pull them apart again to confirm, but I think they both have 2 caps.

[Randy]

The time you saw is correct for the one chip variant. Most likely a bad LCD but there is no way for sure to know other than to swap it out.

[aj04062]

Randy,

The donor 12C does not work so I do no know if the LCD is good. Do you know of a way to energize the display to test?

Adam

[Randy]

The only way to test is on a working calculator.

[aj04062]

Randy,

Can you confirm for me that the LCDs are interchangeable? Donor 12C on top, 11C to fix on bottom. I'd like to be sure before I cut apart the 11C

[Randy]

Yes, they should have the same pinout.

[aj04062]

Eureka!
WAS:

NOW:

Bad LCD.

Edited: 10 July 2010, 9:30 p.m.

[Bill Zimmerly]

Cool ... there's nothing like that feeling of accomplishment after a successful repair project. Time for a good beer!