This is an Assy no. 250469 Rev. 3 short board. The machine would start up with a completely black screen.
I checked that the machine had power all the way through the power switch. Everything seemed fine. I then removed the SID as the machine would run fine without it (but of course without any sound). Still no change. I then swapped the VIC-II and the 8701 at U20. Still no change.
I then carefully unsoldered the MOS 8500 MPU at U6 and mounted a socket.
I then tested the chip in a working machine and good things happened! The MPU chip was broken after all! As I did not have a spare MOS 8500 at hand I temporarily swapped it with a working MOS 6510 MPU. I do not know if leaving in the older generation 6510 chip is something I would do permanetly, but for faultfinding it should be fine.
And the sweet blue screen was back!
A new MOS 8500 MPU chip was ordered from Ebay and the machine had been resurrected.
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