Dragon 64 repair

[Kepler]

This is a really weird fault

I agree!

If you put the 6890 and the 74LS783 from your Dragon 32 in your Dragon 64, does it show the same problem?

Yes.

I'd try the connection between pin 32 of the 6809 and pin 15 of the 783, just in case

The connection is fine.

Does it mean it's always low?

Yes, always low with no fluctuation.

Maybe it's time to check the clock pins

From the 3 clocks, the only one that doesn't register is the 14.218MHz to pin 5 of the 783. But then it doesn't register on my Dragon 32 either. When I touch the pin with the probe, it results in a garbled screen.

Also if you have a couple of power bricks, try powering your 64 with both of them and see if there's any difference.

No difference, I'm afraid.

[robcfg]

Have you tried measuring the voltages at the 9 pin connector between the boards?

If the 5v it's marginal or low, it may create such havoc.