Weird characters on dragon 32 startup screen

[jmellidg]

Hi folks,

I'm repairing an old dragon 32, and on startup it seems to do it well but with strange characters in the welcome message. I'm afraid that something can be wrong with memory, what do you think?? Do you know what chip can be responsible of??

Thank you!!!

 

[Eudimorphodon]

It certainly looks suspiciously like bit 0 is stuck low from the video system's point of view... but I don't think it's RAM, because if that bit were stuck low for the CPU I'm pretty sure the system would not have been able to boot. If you enter BASIC commands do they function properly, the text output is just broken? (What you're seeing there is every "odd" character in the ASCII chart is being displayed as the "even" character below it.)

Looking at a schematic of the Dragon I would guess... IC13, the 74LS273 latch on the data pins in front of IC12, the 6847? But I've never seen a Dragon in the flesh before, so take that with a huge grain of salt.

 

[jmellidg]

Hi, thank you!! You're right as far as I can see, the problem are:

(Correct character - display character)
) - (, 1 - 0, 9 - 8, A - @, C - B, E - D, G - F, I - H,
K - J, M - L, O - N, S - R, Y - X

BAD
), 1, 9, A, C, E, G, I, K, M, O, S, Y

GOOD
(, 0, 2, 6, 8, B, D, L, N, P, R, T,

 

[jmellidg]

I'm realised that the video is monochrome, but I think there are different issues, what do you think?? I've taken a video of the startup process, so with your help I'll try to fine tuning the problem before desoldering chips

Thank you for your help

 

[jmellidg]

Hey, I've changed the 74LS273 as Eudimorphodon said and now it works fine!! No colour at all but I'm waiting to change the LM1889 and wait for the result

Thank yor very much Eudimorphodon for your help!!!!

 

[jmellidg]

The dragon

 

[Eudimorphodon]

Hey, I've changed the 74LS273 as Eudimorphodon said and now it works fine!! No colour at all but I'm waiting to change the LM1889 and wait for the result

Thank yor very much Eudimorphodon for your help!!!!

Woot! I guess maybe I've managed to pick up a few things about reading schematics over the last few years. Cool that it fixed it.

Just for sanity's sake, have you tried the machine on a different TV? I gather that the Dragon has kind of weird video output (they basically hacked an NTSC-only video chip to generate a kind of "fake PAL"), I wonder if the LCD you're using isn't quite able to grok it.

 

[jmellidg]

Ehhh nope, I'll try if course, and if you get two in a row I'll pay you a couple of beers, or whatever!!

 

[Eudimorphodon]

That's a thing that sucks about trying to use some of these 80's machines on modern TVs/composite adapters. A lot of them had very not-according-to-Hoyle modulation that analog TVs usually digested without complaint but modern scalers have no idea how to play nice with.