Problems with ST VIDEO DAC BUFFER PCB 0253 and ATARI 520ST

[sportingt]

HI,
I assembled the PCB and put it into my 520st.
Now I have no color resolution. Monochrome works fine, so I created no big damage on the system. On my tv I see a very lightly color picture. Fragment an some contours.
Do you have an idea what could be wrong?
Or how I can troubleshoot this?
I would like to avoid a deconstruction.

ST VIDEO DAC 01_small.jpg

ST VIDEO DAC 02_small.jpg

Thanks Thorsten

[exxos]

Did you followed the instructions https://exxosforum.co.uk/atari/last/VIDEODAC/ as you appear to still have the original RGB drivers installed. I don't remember after all this time if that would cause problems or not.

dac.jpg

Your wiring looks different, but I don't know if that's correct as I don't know if the circuit is difference between STs ( I would assume not but I haven't tested other revisions) .

[PhilC]

@sportingt you have to disconnect the existing DA circuit from the monitor port as well as solder in the RGB output, so if you haven’t done that it may be what’s causing the issue.

[sportingt]

Hello,

I checked the difference between 1040ST and 520ST of schematic.
1040St has only 3 27Ohm resistors in the RGB output circuit before the connector.

The 520ST has 3 additional capacitors (C117,C118,C119) to ground.
(see pictures below)

May be this the reason, so that I had to remove the 3 capacitors also?

Video out 520ST foto.jpg

Video out 520ST.jpg

[exxos]

The 520ST has 3 additional capacitors (C117,C118,C119) to ground.
(see pictures below)

May be this the reason, so that I had to remove the 3 capacitors also?

A good find! 470pF seems way to high to me. 47pF is more sane. I think I used something like 33pF on the DAC.

I just looked on the STF board and there's no caps at all. So I suggest removing those 3 caps and see what happens then.

EDIT:
Seems Atari couldn't make their minds up with those caps. There there then not there , then different values :roll:

[Steve]

@sportingt out of interest, what are all those cables soldered to your shifter? Some custom memory expansion?

[sportingt]

Yes, it is a custom 72-pin SIMM 4MB memory expansion.

[sportingt]

My additional idea is to use the DAC PCB as an outbreak board for the shifter in later projects. It is ideal to get the needed signals from shifter without soldering direct on the shifter chip legs.

[nokturnal]

That's weird issue. I didn't have to remove any capacitors in 520ST+ on video connector, but I didn't have modulator, so maybe this board has different components than mine?

[exxos]

That's weird issue. I didn't have to remove any capacitors in 520ST+ on video connector, but I didn't have modulator, so maybe this board has different components than mine?

It seems to be a mixed bag on if they are fitted and what value. 470pf if that's the value seems to high to me.