This machine is an Assy No. 250425 long board.
The startup screen was filled with scrambled characters.
This particular fault has previously been related to a faulty CHARACTER ROM at U5 (MOS 901225-01 chip) on another long board (link).
Changing the Character ROM did no cure the fault. I therefore tested the machine with Jupiter Lander on cartridge as this game will run even with a broken Character ROM.
However, this caused a greyed out screen much like the one found in another machine (link).
On that particular repair job, the problem was related to the PLA. I therefore changed the PLA (MOS 906114-01 chip) with a more modern version – the PLAnkton as described here in further details (link).
…and back came the sweet blue screen!
The scrambled startup screen needed these two replacement ‘chips’ before it was back to normal operation! I guess the PLA can cause issues that seems like they come from other chips…
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