PC Engine GT Dim screen

[phipscube]

Hi all, I'm hoping somebody can help me.

I have a PC Engine GT that i'm trying to fix (for free) for my friend. It works fine apart from the screen is very dim. I tried to recap the unit and that helped a little in that once it's been on for a few minutes the screen gradually gets to the correct brightness. If the unit is turned off then on when it's warm the screen is still bright. Leaving it off for 10 minutes then on again. It's back to dark and needing to warm up again.

I have verified the screen is ok with my own GT unit so it's something on the board.

Any ideas?

[Keith Courage]

 check the underside of where the two 4.7 uf capacitors are.  sometimes the old capacitors can leak so badly that they go through to the other side of the board. This old goop can short out some of the parts. this can be fixed by wiping the old goop off and then resoldering the connections on the parts that it was covering.

[phipscube]

Thanks for the suggestion Keith. When I looked again more closely around that area I could see some potential gunk on the legs of D500 and also D502. I took the two 4.7uF caps off again and I could smell fish as I could when I first removed the old caps, so I think that's a sign I didn't clean up so well first time. I also removed D500 and 502, cleaned all the board with alcohol and a tooth brush. Put all the components back and assembled. Still the same fault :/ But I have now noticed the brightness doesn't raise and lower smoothly via the brightness pot, it jitters a bit and is momentarily a little bright then suddenly dark. So wonder if the pot is bad? I don't have any contact cleaner but think maybe I should try getting some and cleaning it. Not sure if that would cause the brightness to gradually increase though.

I also noticed the coil that is covered in white sealant has part of the top broken off. The history of this unit is that the friend who has it now bought it of another guy we know "Possibly needing a recap". But when I opened it, half of the caps had already been replaced and the coil was how you see it.... but apart from that small top piece it seems intact. I bought the cap kit from Console5 and they are usually ok. I replaced all the caps again including the ones changed.

Some photos of the area I cleaned up

You can see the chipped top of the coil here









First turn on... screen pot at full brightness



After 1 min



After 2 mins.. seems normal (screen almost totally white on full)

[Keith Courage]

I don't have any express systems open right now. so I am unable to take a picture of the area that I am thinking could be the issue. I will be working on one in the next day or two. Will post pictures then.

[Keith Courage]

every time I've come across a turbo Express where the brightness cannot be changed and the screen looks dim it turns out to be old cap corrosion in this area (circled in red). if there is indeed old capacitor leakage here, clean off the leakage with a toothbrush and then resolder the contacts. the majority of the time just using flux alone to reflow the solder over the contacts is good enough.


also, just in case, using batteries that are almost dead will cause similar symptoms. Especially not being able to change the brightness.

[phipscube]

Thanks Keith, that's great! I will try it and see if it helps

[phipscube]

I have now cleaned up and resoldered the components you circled....... and the brightness comes straight away now! Really awesome, i'm so pleased Thanks for the help Keith!

There is one small niggle still left with this machine and I want to try and fix it too. When moving the brightness pot its not increasing and decreasing consistently, it kinda still does what I described above with it jittering (suddenly bright, then darker then brighter even when turning slowly up). I'm thinking the pot might need contact cleaner (I don't have any)? Or it could be worn out? Maybe another component? I tried to re-solder the connections on the pot but that hasn't helped. If its left alone the screen stays fine once you get it right so I could just say that's good enough, but i'm a bit of a perfectionist.

The great thing now is it has instant brightness though!

[phipscube]

OK I actually managed to clean the pot with some alcohol and now all works fine. My friend will be really happy. Thanks again for the help Keith Courage