STM display Sync issues...

[Crounder]

Hi guys,

First time posting here. I'm wondering whether anyone can help me.

I have an Atari STM (the short one) expanded to 4MB RAM running TOS 1.04. I'm having some sync issues when connecting to a number of displays, adapters, etc, which I'm not seeing when connecting my 1040STE using the same displays, adapters, etc.

Setup
To eliminate as many variables as possible, I've taken the board out of the case, laid it on an anti-static surface, and plugged in power, a mouse, a 13PIN > VGA adapter then into a computer display. I also have an ACSI2STM hard drive attached.

Symptoms
When I boot the ST, providing I don't touch the board, the display comes up fine. I can run games, productivity software, etc and it'll run just fine. However, the minute I bump the board, even just a little, I lose sync. If I keep moving the motherboard, the sync will come back. If sync is gone for long enough, when the display finally comes back I'll have bombs and will need to restart.

Troubleshooting
To investigate further, I've done a number of things:
1) Swapped the 13PIN > VGA cable with a 13PIN > Colour/Mono adapter (the one with a 3.5mm jack), in turn plugged into an OSSC, before plugging into the computer display. Repeating the above actions that trigger the problem, I can clearly see on the OSSC LCD the signal and refresh rate jumping all over the place from the steady 15.76Khz/35.76Khz signal to lower numbers. If I move the board about, it eventually locks., but it's very intermittent.
2) Measured the voltage at board level to see whether the PSU has any problems. The 5V and 12V rails are rock solid. As I nudge the board to trigger the issue, even though the sync is fluctuating, the voltage remains rock solid. This tells me that the PSU is not the problem.
3) I've looked at all the caps on the board and none seem to be bulging or are loose in any way.
4) I repeated all setups (output adapter cables, displays) using my 1040STE and I have zero issues, however much I push it around.

Does anyone know what the problem could be, or which area I should be looking at?

Thanks in advance!

[rubber_jonnie]

Have you looked at the video output socket to see if there are any bad solder joints? How about checking to see if the Shifter is in a socket, and if so, removing it, spraying a little Deoxit or Electrolube on the socket and making sure the IC is properly seated?

Given your suggestion that touching/moving the PCB causes things to go bad, my main thought it is some sort of mechanical issue and I'd check the board thoroughly for bad/cold solder joints.

[stephen_usher]

Indeed, the first thing to do is reseat the Shifter.