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 808 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 792 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 604 times

and pin 6:

Pin 6

IMG_20260309_172101.jpg (6.94 MiB) Viewed 604 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.

[robcfg]

I may have got that wrong as the pins are not labeled in the schematics.

That said, if pin 5 of the 74LS02 and pin 25 of 783 have continuity, then pin 6 of the 74LS02 should have continuity to pin 32 of the 6809.

I'm going to check that on my D64 too.

[robcfg]

Ok, so on my machine, things are connected like this:

SAM pin 35 -> 74LS02 pin 2
SAM pin 35 -> 74LS02 pin 3
Depending on the position of the LK1 jumper, either:
74LS02 pin 1 -> All 4164 Ram chips pin 9 (A7)
or
74LS02 pins 2 and 3 -> All 4164 Ram chips pin 9 (A7)

SAM pin 25 -> 74LS02 pin 5
6809 pin 32 -> 74LS02 pin 6
74LS02 pin 4 -> 74LS138 (IC40) pin 5

6809 pin 34 -> 74LS02 pin 11
SAM pin 25 -> 74LS02 pin 12
74LS02 pin 13 -> 74LS138 (IC40) pin 4

All these combinations should have continuity.

How did you test the ram? And, do you have any replacements you can test?

[Kepler]

Thanks Rob.
All of those connections are fine.
I have tested the RAM ICs again using my RAM tester and they are fine too. I could test each one in my C64 to be absolutely certain.
However, what concerns me is:
1. Why there is no output on pin 4 of the 74LS02.
2. Even my diagnostic cartridge doesn't work which presumably should work even with bad RAM?