Help repairing Dragon 64

[llopis]

I tried piggybacking after you mentioned that, but I didn't have high hopes since the Dout on all the pins showed activity. In any case, it made no difference.

So today I finished socketing the remaining 4 RAM ICs, and to nobody's surprise, I'm still getting the green screen with black border

Any suggestions on what to try next? It seems to me that it has to be memory related. Could that be caused by the SAM chip? Is it a likely failure point? If so, any suggestions where to buy a new one?

I'm open to ideas. Thanks.

[sixxie]

Have you visited IC25? 74LS244. It buffers RAM reads (Dout) to the CPU - testing by modifying XRoar suggests that if you could write but not read, you'd get a cleared screen with occasional glitches.

Edit: to follow up on that, read failure in any single bit causes a blank screen followed by:

bit 7 - no glitch (steady green)
bit 6 - no glitch
bit 5 - glitch (occasional flashes of data)
bit 4 - no glitch
bit 3 - visible corruption (strings of obvious bad data)
bit 2 - visible corruption
bit 1 - visible corruption
bit 0 - no glitch

Further edit: that's if the result is pulled low. Here's the equivalents if the bit is pulled high:

bit 7 - visible corruption
bit 6 - no glitch
bit 5 - no glitch
bit 4 - visible corruption
bit 3 - no glitch
bit 2 - visible corruption
bit 1 - visible corruption
bit 0 - visible corruption

[llopis]

Have you visited IC25? 74LS244. It buffers RAM reads (Dout) to the CPU - testing by modifying XRoar suggests that if you could write but not read, you'd get a cleared screen with occasional glitches.

That's very interesting! Especially those tables.

The fact that when I pulled some chips I got some specific characters on screen makes me think the 74LS244 must be working (it just interpreted a disconnected line as high or low). But maybe not. It's easy enough to test (I have a spare one laying around since they're used in the Amstrad CPC as well), but I'll wait until I'm more convinced before socket yet another chip

Is it reasonable to discard the SAM as a possible suspect? After all, we are reading some values and displaying them on screen correctly (thinking of those unconnected memory lines).

I'm also wondering if maybe one of the two ROMs failed. The one that works is the one with the characters, and the code to initialize the CRTC. Maybe the one that failed contains the BASIC code? (I don't even know how they're split, I'm just speculating based on the results of what I'm seeing).

Thanks for all the help so far. I've been learning a lot about Dragon architecture so far. Now if I can only get it to work in the end...

[robcfg]

The Dragon has no character rom.

The character set is defined in the VDG chip, the MC6847. Although you can attach a custom character rom, like on the Dragon 200-E.

If I remember right, one rom is the basic for 32k mode and the second one, the basic for 64k mode.

That said, boot your machine and try typing EXEC and press return to see if something changes.

[sixxie]

The fact that when I pulled some chips I got some specific characters on screen makes me think the 74LS244 must be working (it just interpreted a disconnected line as high or low). But maybe not. It's easy enough to test (I have a spare one laying around since they're used in the Amstrad CPC as well), but I'll wait until I'm more convinced before socket yet another chip

Data to the VDG goes a different way than to the CPU, so it's still possible!

..ciaran

[llopis]

That said, boot your machine and try typing EXEC and press return to see if something changes.

I didn't notice anything happening when I typed that.

What's the jumper labeled LK1 near the memory on the PCB? Just curious.

[sixxie]

What's the jumper labeled LK1 near the memory on the PCB? Just curious.

Never tried it, but it sort of looks like it turns your 64K machine into a 32K machines, using the other half of RAM as the base 32K - presumably for testing purposes...

[llopis]

Today I got the new SAM chip. I put it in and... nothing. Still the same.

I decided to try the 74LS244 since a failure there could cause these symptoms and... nothing

Is there really no ROM for full computer diagnostics? Or at least a quick and dirty RAM-testing program that can be executed from the ROM? Or someone with good knowledge of 6809 assembly and the Dragon system to write one (there's always hope ).

[sixxie]

Ah I'd been wondering if you'd made any progress with this. Shame...

I guess with pretty much every major bit replaced already, you're now down
to checking the traces? To and from the 74LS244 might be a good start.

..ciaran

[llopis]

I realized there *is* a diagnostics ROM!
http://archive.worldofdragon.org/archiv ... ms/Dragon/

I have no idea what it does as there's no documentation, but I figured I would try it. I assume it goes in IC34 (left ROM, the one labeled 18). I popped it in, turn it on, the screen got garbled for a few seconds and then settled on this pattern:

norom.jpg (68.73 KiB) Viewed 8362 times

Here I was thinking it was one of those tests that reports problems in a graphical way to get around video issues, but then I removed the ROM completely, turned the computer back on, and got pretty much the same thing! So apparently the diagnostic ROM isn't working at all? (I saved it on a 27C128, which is what the original ROMs come in).

Also, does that pattern for the computer without ROM give us any hints as to what might be wrong? It's curious that the screen is totally out of sync for a couple of seconds before settling in and more or less syncing.