Atari Mega ST Video Output problem

[wietze]

Hi there guys,

Ive had a non-working Mega ST for a while and decided to give it a hand. Ive replaced both the 14 pin video-out connector and socketed (and replaced) the shifter; and now I have video output again!

However, I come into the following problem. The first few seconds that the machine is on, I get a nice and crisp white screen; but after like 5-10 seconds, I start to get interference on the screen (grey/black lines) and after 20 or so seconds the complete screen is black. The machine keeps running (I tested this by running a demo, that just kept giving sound output).

When I power down the machine, and let it sit for a while, I get a white screen again (for a few seconds at least). And if I do not let it sit, but just power cycle; the black screen or the interference screen just remains. So it seems to recover to its initial point after a while.

Any of you guys encountered this before, and got some tips/tricks for me to diagnose whats up? It'd be nice if I can save this machine.

Kind regards,
Wietze

[rubber_jonnie]

Perhaps a recap of the videou out circuit?

[exxos]

Have you recapped the PSU ?

[wietze]

Perhaps a recap of the videou out circuit?

Ive checked the shifter schematics, am I right to assume in the schematics R stand for resistor and C stand for capacitor? As such, it seems the video circuit has 3 capacitors, C64, C65 and C70 that I could possibly replace.

[wietze]

Have you recapped the PSU ?

I have not recapped the PSU, but I kinda assumed the PSU wasnt the problem, because I have tried several (confirmed working) PSU's, and the machine has this problem with all of them.

But to logically rule it out; rather than assuming, I suppose I can turn on the machine with 1 psu, then when the screen is black, replace with other psu and try again, and if the screen is white again straight off the bat; it can be related to the PSU.

[rubber_jonnie]

Perhaps a recap of the videou out circuit?

Ive checked the shifter schematics, am I right to assume in the schematics R stand for resistor and C stand for capacitor? As such, it seems the video circuit has 3 capacitors, C64, C65 and C70 that I could possibly replace.

Given the age of the machine, and the cheap capacitors used, then it is worth doing, just ensure you use capacitors (Yes they will be marked C on the schematics) of the same voltage rating or higher and buy decent capacitors with a low ESR such as Panasonic. Use the same value capacitor too.

PSU, yes you can try different ones and see if it makes a difference, but I'd always recommend a recap because once again, given the age and source of the originals, even if they look fine they may have dried up internally. I usually use a kit from exxos because it also includes other components you might not consider like an upgraded rectifier.

It sounds like the machine runs ok as you get sound, what display are you connecting it to?

[wietze]

It sounds like the machine runs ok as you get sound, what display are you connecting it to?

I will have a look at the capacitors when Im near the machine, and see what I can replace.

I use the SC1224 as a monitor.

[stephen_usher]

Actually, given the symptoms and it getting worse as the machine warms up and then is OK after cooling down again it would suggest to me a dry joint somewhere.

[wietze]

Hej guys,

Ive replaced the Electrolyte Capacitors around the circuit and reflowed the shifter pins. Ive also checked continuity between GLU/MMU/SHIFTER for the DE, and they were fine.

I had the idea of connecting the board to ST HIGH, and this also gave me screen for a period of time, before becoming distored and dying out. In contrary to what I posted before, when I turn on the machine with display now; I do no longer get screen; its just black.

Ive checked the shifter chip itself in another machine, and it was confirmed working there.

When I boot the machine with an executable demo in the auto folder; the demo executes; I hear music and all that. Which leads me to believe the machine is up and running. When I connect the machine to my OSSC, the OSSC displays that there is a 50hz vertical and 15.6kz horizontal signal, so it seems all is fine there too.

This leads me to believe that its more related to the `output' side of the shifter to the 14 pin din plug. What I cannot explain, is that how could the RGB output die out and no longer give screen; whilst the mono display would give me some output later that evening; before dieing out too. I am too much of an electronic novice to deduce that into an indication to where to look in the output side of things on the circuitry.

What I intend to do next:
- measure all resistors and continuity for the output part of the schematic, on the broken display machine, and do a base measurement on a known working other mega st; perhaps this gives me some leads.

I will attach pictures and movies in a seperate post; for those interested, as a refernce. Any input and tips are welcome!

Kind regards,
Wietze

[rubber_jonnie]

If you haven't already, download the schematics and trace from the Shifter to the output socket, see what components are along the way and check each item. I'd continuity check all the PCB tracks as well. Also worth a look at the soldering under magnification, because as @stephen_usher suggested, you may have a bad solder joint. It could also be something that is providing the Shifter with a signal that's dying.