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.

[Kepler]

I have 4.96v on the 5v rail and 11.86v on the 12v rail.

[robcfg]

That should be good enough.

Anything weird in the -12v rail?

[Kepler]

Only -12.05v.

[robcfg]

I asked on the Dragon 32/64 Owners/Users group on Facebook, and Chris Poacher replied the following:

I had a bad 6821P keyboard controller on mine that gave a similar error. They are super cheap to buy.

Pity these don't come socketed...

[Kepler]

Mine are socketed now
I did it a few days ago after speaking to Chris. I swapped them round and it made no difference...so unless both are bad, I don't think that is the problem.

[dublevay]

Can I ask what the serial number on the bottom of this D64 is please? And were there any other boards inside it?

[Kepler]

Sure. It is 005837.

When I received it, this board was inside, it was not connected:

IMG_20260312_092857.jpg (5.97 MiB) Viewed 2068 times

IMG_20260312_092922.jpg (6.1 MiB) Viewed 2068 times

There were RAM ICs soldered on each motherboard RAM IC and there were some pins soldered on to various points of the RAM and logic ICs. Presumably wires from the board connected to the pins. I removed them all as I had no way of determining which wire connected to which pin.

[dublevay]

I thought so. You have Bob Hall's other MMU computer. I have the two Dragon 32 ones from the same seller. Your board may have tiny mods on it, so you'll need to really scrutinise it. Mike is our current expert on this. Basically, these computers were modded to have 256K RAM and be able to run OS9 level 2.