Pro-2021 LCD trouble

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AC9BX

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Jun 11, 2011
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Lockport, IL
Yes, I have the ol' 2021. This is regarding the display, not the backlight.
The row of bank indicators 1-9, the numbers and the block below number 1, are very transparent. The bottom row of segments of the frequency digital are a little weak but not worrisome. The "bank" and "monitor" words are weak. "Scan", "program", "manual", the remaining digit segments, bank 10, and 2-10 blocks are all good.
I'm wondering if this is a failure of the LCD module or the drive to it.
I noticed the driver chip has 3 bias lines. This might explain why certain parts work and others do not. I assume this display is AC driven (as it probably should be) because I measure 2.4 volts on every pin of the display when trying to measure DC (I was just poking at it). So there's a pulse width or phase problem?
I'm looking for insight into what may be going on and how the display is supposed to function.
I haven't yet put the scope on it to see what's going on.
I can't imagine either of the parts are still available. Any ideas?
 

AC9BX

Member
Joined
Jun 11, 2011
Messages
326
Location
Lockport, IL
I have the service manual. It doesn't offer any such troubleshooting.
Yes, there's a GRE CPU featuring outputs directly to each LCD segment plus 4 commons along with connections to the various controls and keyboard matrix. If only there was documentation on this chip available. It appears to have some fashion of voltage divider supplying the LCD bias inputs.
 
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