lisa 2 startup problems

I recently acquired a Lisa 2 (Macintosh XL) with the 2/5 motherboard, old IO board with the coprocessor socket and battery pack and 800k Floppy ROM, CPU board with ROM's dated 1985 and 2 x 512k RAM cards. The problem is, it won't start. The motherboard was badly corroded but I cleaned the corrosion with vinegar and alcohol and it looks decent now, I could not find any broken circuits.

What I already tested:
- Power supply is OK (+5V standby voltage present, replaced all
electrolytic capacitors)
- on the COP421 chip I can measure the power button signal when I press the
power button but the state of pin 24 remains the same
- i can "force" start the power supply by applying 5V on the ON pin of the
COP421 (pin 24).
- the power supply starts with no problems, correct voltages present on the
I/O board

• I can only get a grey picture with some horizontal lines but no sound and no video
• unfortunately I have no other boards to switch out for testing.
• does anybody have a clue where to start digging for reparing this lisa?

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