Still poor picture quality on 520 STM despite ST VIDEO DAC buffer board

[burn]

Hello everyone,

I've been reading here for years, but this is my first post.

I have a 520 STM equipped with various upgrades. However, the image quality in ST Low and ST Mid is very poor. So I gave the 520 STM the DAC VIDEO buffer board.
Unfortunately, this has not improved the picture quality. Even with an unmodified 520 ST+ and 1040 STE, the picture quality is significantly better.

Here are a few photos of the 520STM.

You can see a lot of vertical stripes here.

Output of 520 STM with VIDEO DAC buffer board

P1080432.JPG (256.79 KiB) Viewed 7767 times

For comparison: The 520 ST+ is equipped with a C025915-20 GLUE, which causes the desktop to slide to the right edge. But better picture than on the 520 STM with VIDEO DAC buffer board

Output of unmodified 520 ST+

P1080431.JPG (162.1 KiB) Viewed 7767 times

installed VIDEO DAC Buffer Board

P1080422.JPG (420.67 KiB) Viewed 7767 times

without VIDEO DAC Buffer Board

P1080429.JPG (413.12 KiB) Viewed 7767 times

Capacitors near the monitor port of the 520 STM

P1080434.JPG (224.88 KiB) Viewed 7767 times

The complete board of the 520 STM

P1080437.JPG (436.28 KiB) Viewed 7767 times

Does anyone have any tips on what I could do to get the picture quality I would expect from the VIDEO DAC buffer board?

Regards

[PhilC]

@burn when I fitted mine I joined the RGB wires right next to the monitor socket.

Also it may be that you need to adjust the horizontal width on your monitor, this can often take out the banding to some extent.

[burn]

I have now used the RGB pads near the monitor port. Without success. The stripes (which flicker a little, by the way) are still there.

It is not due to the monitor settings. I have already tried everything possible with the settings.

P1080440.JPG (417.22 KiB) Viewed 7552 times

[coonsgm]

It looks like you're working with an LCD. In my case, my 520STM had jailbars on the SC1224 CRT monitor. The video DAC buffer board fixed that up fantastic.

When connecting to 15khz capable LCD screens (without going through an OSSC), I've not had any of my ST's that haven't had some amount of jailbars show up. Maybe my expectations are too low, but I was very happy to have a clean signal to the CRT now.

[exxos]

Interference could be coming from additional sources anywhere. It is possible because the RF modulator chip and box is still there or maybe additional circuitry in the RF can which isn't present on later machines could be coupling noise to the video circuit.

[burn]

As I was planning to remove the RF part later anyway to make room for extensions, I have now brought this forward. But it takes a while, the components don't like to be desoldered.

[burn]

Unfortunately, removing the RF part did not bring any improvement.

removed RF-Part

P1080447.JPG (423.6 KiB) Viewed 1963 times

I then thought that the interference could be somewhere in the area of the monitor port. To rule out the monitor connection, I got a DB15 breakout board and connected it directly to the ST VIDEO DAC buffer board. I got V-Sync and H-Sync from 2 pins of the RF modulator and lo and behold, the picture is now crystal clear.

Next, I connected the breakout board directly to the ST's monitor port. Again, the picture is crystal clear!

So the problem seems to be caused by the exxos ST2VGA adapter.

DB15 Breakout

P1080445.JPG (338.22 KiB) Viewed 1963 times

[Steve]

That ST 2 VGA adapter worked perfectly on the STfm though right? This rings a bell for me, I remember when I got my small STm machine, the Exxos adapter didn't work at all, I ended up getting a different branded adapter. I think it's something to do with the voltages on an STm video output being different to an STfm... I can't remember the details.

[exxos]

I'd assume the problem is with the 12v which powers the adapter. There's something like 10k in series with the 12v on the monitor port. Changing it to something around 1k - 4.7k may solve the problem. This assumes 12v is stable to begin with.

[burn]

I measure about 16k from pin 8 of the monitor port to ground.
From pin 8 it first goes to R128 with 1.2k. I also measure this at this resistor and in the 12V line from the monitor port to the power supply port.
If I follow the 12V line further, I end up next at C102 and C103, which both go to ground. When I measure there again, about 15k is displayed.

520ST_12V.jpg (466.27 KiB) Viewed 1914 times

I have also measured my 520ST+, there it is only 1.1k from pin 8 of the monitor port to ground. The ST2VGA adapter works well there.

Replacing R128 with a lower value certainly makes no sense. I won't get below 15k either.

There seems to be something wrong with C102 and/or C103. What can I usefully replace them with?
Or is there a better solution?