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 918 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 902 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 714 times

and pin 6:

Pin 6

IMG_20260309_172101.jpg (6.94 MiB) Viewed 714 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?

[robcfg]

This is a really weird fault...

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

Let's do a quick summary.

- All chips appear to be behaving correctly as well as the connections between them. (I'd try the connection between pin 32 of the 6809 and pin 15 of the 783, just in case).
- 74LS02 has no output on pin 4. Does it mean it's always low? It should change as the R/W signal changes.

The pin 4 issue could mean that the 74LS02, the 6809 or the 783 have faults, or the connections between them don't work, as pin 4 should output the R/W signal NOR'ed with the 783 S2 signal, and I'd expect at least the R/W signal to change frequently.

Maybe it's time to check the clock pins and to check the voltages at the 9-pin connector between the boards.

See the Power Regulator Board to Main Board Ribbon section here: http://dragon32.info/info/psu.html

WARNING! Don't swap the power boards as you could destroy probably your 32 by injecting -12v instead of -5v!

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