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Recapping a quite leaky LC475 - Video is red


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I received an LC 475 to recap. I’ve recapped like 100 boards but I haven’t encountered this issue nor do I have any sort of scope to diagnose it. 
 

The white signal is being output as red. Right from switch on. The on-board RAM also isn’t being recognized. 
 

It came to me as needing recap because it had no video output. It was missing the PRAM battery (which means it often won’t work anyways), and the factory caps were leaking quite bad. After I removed the caps I noticed significant leakage at the pads but it cleaned up quite well. 
 

A few of the ICs were corroded with green. I thoroughly cleaned with IPA99 and a toothbrush. 
 

I have swapped out VRAM. I have reset PRAM. I have changed monitor resolutions and settings to gray and black and white. No change. It appears to function in every other way. 
 

I think the video issue is also related to onboard RAM not working. 
 

Thoughts?

75A66D7E-6FA3-4014-9D6F-78AF85319BBD.jpeg

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I'm not sure how color values are encoded, but my guess would be that something is wrong with the signal lines for the bits that must be set to get out of the red values.

 

This could be a problem with data lines in the VRAM, but it could also be signals feeding the DAC.   There should be a video DAC on the logic board, and perhaps some of the data lines to it aren't working, so that pixels all have values of "red" when they arrive in digital form.

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  • 2 weeks later...

You got three lines going out to the display port for red, green and blue signals. An always-red picture would suggest that only the red signal is making it to the monitor. Now why that would happen, might be some corroded traces on the board, might just be a dirty display port...

 

Judging from your picture, you have different tones of red on it, so I would say the problem lies within the final video signal, somewhere between the DAC and the display port.

Edited by Arisotura
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