Another nother Duo white screen. Please?

[Ergot_Cholera]

As the title with no response. I read another recent issue like this (Desh?) and THESteve suggests that the C6270 is bad. I replaced it and got audio and glitched graphics on some Hucards, I thought this might be a RAM issue so I replaced the 2 SRM20256 and the SRM2264 but there is no change.
I thought it might be a problem with the C6260 so I replaced this and thought I may as well do the C6280 as well, but there is still no change.
The CD bios seems to start as the disc activity light comes on and the glitched screen flashes when I press the run button. I cannot check discs though as there isn't currently a laser unit installed.
I have done all the usual like cleaning the board, replacing the caps, checking the voltage regulators, checking traces and vias.
I removed the card slot and checked the traces and vias and continuity appeared fine.
Kato and Ken has perfect audio but in other games the audio is faster than normal and others just give a hum sound and blank screen.

I forgot to mention that I had removed IC520. I reinstalled it and plugged in the laser unit black plug and the motor turns the gear that drives the laser along the sled so most functions seem to be working apart from the visual output.
I even tried a Supergrafx game in it which just gave a solid green screen.
I'm just hoping the oracle might chime in now as he seems to have the knowledge for every eventuality.

[Ergot_Cholera]

Here are photos of the two screens I get when powering on the Duo to what should be the BiOS screen.

[thesteve]

ok so the 6270 was dead (replacement revived system)
odds are you have an open address between the 6270 and its ram (touching the ram will cause the display to change)