PC Engine GT - Contrast Issue

[chaoticjelly]

Hi Folks,

I'm trying to troubleshoot a contrast issue on a PC Engine GT.

The issue is that the contrast/brightness will not go up to its maximum, it seems to get to a certain point and then turning the variable resistor doesn't increase the contrast any further so the screen is slightly too dark - a PC Engine GT without this issue will go as far up as pure white screen.

The PCE GT has all of the SMT capacitors replaced, this hasn't solved the contrast issue.

I've traced the tracks that supply the contrast variable resistor and I can't seem to find anything interesting.. I measured the voltages compared to a PCE GT without this issue and the PCE GT without the issue seems to have slightly higher voltages supplying the contrast variable resistor.

I've tried the screen from another PCE GT and the issue still remains, so it is definitely a fault on the PCE GT mainboard.

I have a few pictures, the contrast variable resistor is the part marked C701, on the right hand side of most of the pictures.

I was just wondering if anyone else had the same issue, and how they managed to fix it, I did do a search of the forum but couldn't find anything specifically relating to this contrast issue. Or if anyone has some schematics, or can explain where and how the contrast signal is generated on the mainboard, that would really help!