PC Engine - Graphics Error after console warms up?

[MasterOfBiscuits]

Hi all - have an isssue with a console I have been restoring that has me stumped. It's a PC Engine GT that has has a full recap and seems to work fine, however after around 5 minutes of loading a game the graphics start to error. The sprites will move out of place and sometimes disappear from the screen completely. The backgrounds tend to remain unaffected, it seems to be sprites that only have the problem, and only after a few minutes.

I have some spare RAM chips here so I tried replacing the VRAM but there was no change. I'm thinking to try replacing the main RAM as well so have some chips on order, but does anyone have any idea what could cause this issue? There doesn't seem to be any obvious damage on the mainboard.

[Syntax]

Try replacing the Hu chip that outputs RGB and also try running the system without the copper shield in place underneath and see if it has any effect on the glitching or time it takes to happen. Ive had systems do this with failing Hus and just the capacitive charge from your body to the Hu can effect it.
The Hu's are pretty sensitive to heat also so preheat and use the lowest temp you can on the hot air to get it off if you can.

[MasterOfBiscuits]

I have a spare white PC Engine here so I decided to swap the HUC6260 over from that - no change.

So I swapped over the HUC6270 as well - still no change.

I'm really puzzled. The only thing left I can think of is the main RAM, so will try swapping that over.

[kamiboy]

This sort of behavior sounds like a cold solder joint problem. Prolly on a trace between the graphics processing side and the vram.

You could prolly find the general location by letting the unit warm up until it glitches then spray parts of the mobo with cold spray to see where produces the greatest effects.

[MasterOfBiscuits]

This sort of behavior sounds like a cold solder joint problem. Prolly on a trace between the graphics processing side and the vram.

You could prolly find the general location by letting the unit warm up until it glitches then spray parts of the mobo with cold spray to see where produces the greatest effects.

Thanks for the suggestion, I've checked for issues and been over a lot of the joints but haven't found any problems.

I'm now starting to think this may be a grounding issue, with some interference getting into the data curcuits. Sometimes the game will fail to start, however if I put my multimeter probe on a ground point it will start. The screen also seems to show interference that gets worse as the console stays on before it basically crashes.

Any opinions on this and possible solutions?

[kamiboy]

I suppose it could be an issue with a faulty or noisy power adapter. Have you tried swapping that?