exxos's Atari, Amiga & retro forum

Hi,

I have a stock Atari 1040STFM that has a slight video quality problem (noticeable during games, not desktop use) that I wanted to correct overall video quality.

I've installed the video DAC board and am getting interference and bad image. I've removed the 3 resistors and transistors.

Any hints on how to debug the issue?

Thanks,
Erez

Might be the monitor itself or video cable or interference on RGB lines on the ST itself.

You can see interference lines on the desktop.. But you say the problem isn't on desktop, in which case, why post the no problematic image?

I think I wasn't clear in my original note. Prior to installing the DAC board, I had good desktop image and medium gaming image quality.
This may mean monitor, RGB cable and monitor->RGB connector are working fine. The RGB cable and monitor also serve my Falcon and everything is great.

Then I installed the DAC video board and have the attached image quality and interference.

You would have to wire your video direct to the DAC board to rule out interference from RGB traces. Beyond that, I only know of cable and monitor issues. Or maybe the IMP shifter is causing problems.

EDIT:
Possible your missing the 47pf? caps next to the RGB socket?

Are you referring to those caps? If so, I have them installed. What would be the best way to connect the RGB signals directly to the monitor? I guess I need other signals from the main connector.

Also, if the new RGB signals are the same as before just cleaner and previously the image quality was good, why would there suddenly be interferences? Might there be something that influences the DAC vid board that I should remove? (I've installed another 100uF cap as the previous one what too tall, is that redundant?)

whomper wrote: 01 Jun 2025 15:34 Are you referring to those caps? If so, I have them installed.

Yes.

What would be the best way to connect the RGB signals directly to the monitor? I guess I need other signals from the main connector.

You would have to why a new RGB connector direct to the DAC board somehow. This bypasses all the noisy signals between the DAC and motherboard RGB connector. Generally having the caps is enough to solve that problem anyway.

Also, if the new RGB signals are the same as before just cleaner and previously the image quality was good, why would there suddenly be interferences?

Good question, I don't know. Its first time I have heard of such a problem.

Might there be something that influences the DAC vid board that I should remove? (I've installed another 100uF cap as the previous one what too tall, is that redundant?)

It is better to have the cap, generally when I removed mine, it did not make any difference, but there is bulk capacitance on the DAC board anyway.

The IMP chips are known to be "noisy". Maybe the extra SMT caps is causing more spikes on the power rail now the DAC board is applied. But its only a guess. Atari added a diode like a 4148 in series with the IMP chips power rails for such issues. But don't recall it ever being done on a shifter. You could try adding more bulk capacitance, like 1,000uF or more on the shifter power rail. Or try a non-IMP shifter to see if that solves the problem.

Are you using an old PSU or have you changed capacitor on it?

exxos wrote: 01 Jun 2025 19:25 The IMP chips are known to be "noisy". Maybe the extra SMT caps is causing more spikes on the power rail now the DAC board is applied. But its only a guess. Atari added a diode like a 4148 in series with the IMP chips power rails for such issues. But don't recall it ever being done on a shifter. You could try adding more bulk capacitance, like 1,000uF or more on the shifter power rail. Or try a non-IMP shifter to see if that solves the problem.

I may want to try to add such a cap or diode, which pins should I connect then in the what direction?

DoG wrote: 01 Jun 2025 21:18 Are you using an old PSU or have you changed capacitor on it?

I bought it used half a year ago and it looks like it was recapped as there are blacks dots marked on a few of the caps. I also tested the PSU and it looks like it delivers a stable signal.

Black dots on the capacitors normally indicate the original and should be replaced.

5V goes in on the left and the right side, goes to pin 40 of the shifter, you will have to gently bend it outwards from the socket to solder.