Dragon 64 repair

[Kepler]

Hi all,

I recently acquired a Dragon 64 - after a mere 40 years of waiting - which unfortunately does not work.
I have therefore been attempting to repair it. It produces a green screen on startup with a few characters.

So far I have checked the following:

1. The 6809 CPU (using my Dragon 32)
2. 74LS783N (using my Dragon 32)
3. All the RAM (RAM tester)
4. IC35 & IC36 (TL866)
6. 6847P - I have checked the DA01-08 lines with my scope and receive a nice square wave of different frequencies.

I am not sure what to look at next. Any suggestions?

Thanks.

[robcfg]

Hi, and welcome to the Dragon Archive!

Could you post a picture of what appears on the screen. Just to check because certain error patters are quite recognizable.

You've already done a good job checking components, but knowing you have a TL866, I'd check the rom chips IC17 and IC18, maybe dump them (as 27128) and post them here (as a zip file) so we can verify the contents.

If they turn out to be Ok, then it would be time to start looking for corroded socket pins or cold/broken solder joints.

Cheers!

[Kepler]

Hi Rob,

Thank you for the response. It is great to be hear. The Dragon is a wonderful machine and one that I would really like to explore, once I get this one working!
Here is a screenshot of the display on startup:

IMG_20260306_092930.jpg (6.76 MiB) Viewed 76 times

I did check the ROMs previously but have nevertheless uploaded a zip of them.

[sixxie]

ROMs check out.

But I don't recognise that failure...

Couple of random thoughts to check: make sure there's a jumper across one half of LK1 (bottom-right), maybe try moving it to the "other" side. I think if it were not present at all you'd see something different though. Check E/Q from SAM are getting to the CPU ok. You know the crystal's ok as the SAM generates timing for the VDG from it.

Then as Rob says, start looking for any dodgy-looking.

And if all that fails, how far are you from Cambridge at the end of the month?

[Kepler]

Moving LK1 from South to North does affect the display. According to the technical manual, that is the 32/64k switch.
By SAM, do you mean the 783?
I'm afraid I don't live anywhere near Cambridge but I am happy to post this if my attempts prove fruitless...

[sixxie]

Moving LK1 from South to North does affect the display. According to the technical manual, that is the 32/64k switch.

Yeah it swaps the top/bottom 32K so in theory may have demonstrated different fails (though you'd tested RAM).

By SAM, do you mean the 783?

Yep, so pin 14 of the SAM to pin 34 of the CPU, and pin 13 of the SAM to pin 35 of the CPU. The clock signals driving the CPU.

I'm afraid I don't live anywhere near Cambridge but I am happy to post this if my attempts prove fruitless...

Ah no, I more mean that there's a Dragon meet-up there on the 28th/29th of this month, and someone present may be able to help.

[Kepler]

OK.
This is the 32k screen:

IMG_20260306_112232.jpg (1.41 MiB) Viewed 49 times

...and after 30 seconds I hear a click:

IMG_20260306_112358.jpg (7.48 MiB) Viewed 49 times

Pin 35 of the CPU:

IMG_20260306_112125.jpg (7.08 MiB) Viewed 49 times

Pin 34 of the CPU:

IMG_20260306_112125.jpg (7.08 MiB) Viewed 49 times

[robcfg]

The screen with all Q's looks like it's been jumping around memory and hit the CLOAD/M functions, which would clear the whole screen, make the relay click and put the cursor on the top left corner.

Looking at the schematics (http://dragondata.co.uk/tech/circuit-diag/index.html), and assuming the CPU (6809), The SAM (Synchronous Address Multiplexer, 74LS783N), the RAM chips, the ROM chips and the video chip (6847P) are OK, there's only IC25 (74LS244) and IC20 (74LS273) that have anything to do with RAM access.

I think that IC25 may be the most suspicious of these two, but both can be actually failing...

Maybe Prime or Dublevay could shed a bit of light on the subject.

While we are at it, can you check the markings on your PCB and take a look here to see if it's on our list of scanned boards? https://archive.worldofdragon.org/index ... d_Pictures

[Kepler]

OK. I will socket IC20 and 25 tomorrow. In the meantime, is there a better way (more logical way) of determining if those ICs are working correctly using a scope? The approach of desoldering and using a logic tester seems slightly extreme.
This is my board:

IMG_20260305_145142-edit-20260305145155.jpg (6.78 MiB) Viewed 22 times

By the way, I have also been told that a faulty 6821P (keyboard controller) can sometimes also cause such errors...

[robcfg]

Looking for a way to check them before having to desolder them is actually a good point.

Maybe it would be best if you could wait a bit till we get hand of our hardware guys or we come with a way to do it.

Do you have any game cartridge by any chance? It may be worth if you have one to see what happens if you insert it and then turn the machine on.

Oh, and would you be so kind to take a nice picture like the one you posted but of the solder side of your motherboard?

Turns out, we have a revision 2A board but not yours