Dragon 64 repair

[Kepler]

Right, I couldn't resist desoldering IC20 & I 25 and checking them...they proved to be fine.

I have also tried a cartridge and the display does change but not to anything legible or coherent.

Here is a picture of underside. I can't see any dry joints or anything else suspicious but perhaps someone else can.

IMG_20260307_102832.jpg (7.42 MiB) Viewed 519 times

[Alastair]

Hello,
is that a damaged track within the red circle? Also I'm not sure of the horizontal line crossing the track within the yellow circle.

D64_Solder_Side.jpg (856.44 KiB) Viewed 503 times

[Kepler]

Hi Alistair,
Unfortunately, neither of those are breaks in the tracks...

[Kepler]

Update:
I have socketed and swapped the two 6821Ps and that has made no difference.
I also bought a Diagnostic cartridge that just produces gobbledygook.
I my scoping, I have noticed something that seems wrong. IC36 has no output on pin 4 but does have inputs on pin 5:

Pin 5

IMG_20260309_172053.jpg (6.98 MiB) Viewed 315 times

and pin 6:

Pin 6

IMG_20260309_172101.jpg (6.94 MiB) Viewed 315 times

I have tried a different 74LS02 with the same result.

[robcfg]

It seems that pin 4 should output the NOR operation between the R/W signal (pin 32 of the 6809) and the SAM S2 signal (pin 25 of the 74LS783.

Could you try checking if there's good continuity between pin 5 of the 74LS02 and pin 32 of the 6809, and from pin 6 of the 74LS02 and pin 25 of the 74LS783?

[Kepler]

There is no continuity between pin 5 of the 74LS02 and pin 32 of the 6809. Are you sure that is correct?

There is continuity between pin 5 of 74LS02 and pin 25 of 783. But there is none between pin 6 of 74LS02 and pin 25 of 783.