092673Problem with 520STfm monitor output

[myriadcs]

Hi Everyone

 

I have recently found 520STfm on local auction. After connecting to TV (via SCART Cable) it did not display anything but grey screen. I have opened it and found quite a lot of rust on disk drive and a bit on motherboard. After cleaning everything and reseating chips I was able to see green TOS screen Little success However, I noticed strange screen shake every 5-10 seconds. Here is recording how it looks

I guess something is wrong with motherboard (C103253 Rev.1) but not sure where to start/what to replace. Any suggestions? Edited January 29, 2019 by myriadcs

[ParanoidLittleMan]

I had some blankings similar to what can see on your video when made ST - SCART cable after schematic in Atari ST ProfiBuch - and similar ones can see online. On pin 16 of SCART (RGB blanking) goes signal Vsync from ST (pin 12 of video conn,). But that's not good for all TVs, obviously. Instead Vsync connect there pin 8 of ST video conn. This solved problem in my case. Newer LCD/LED TV.

I remember that for some old TVs, in 80-es, 90-es current from that pin 8 (what is 12V via resistor) was not enough, and needed to use +5V or +12V via smaller resistor. Some TVs may need to connect switching voltage to pin 8 of SCART, to switch in RGB mode

[myriadcs]

Thanks for suggestions.I actually bought cable so had to open it now to verify. Looks like producer used this diagram

https://www.flashback-computing.com/screenshots/Atari%20-%20scart.jpg

However don't see any resistors

[ParanoidLittleMan]

Resistors are not relevant if you don't use some power line. What is on pin 8 of Atari video conn. goes already via resistor.

Try with connecting that line - currently connected to pin 8 of SCART to pin 16 of SCART, and disconnect what comes from pin 12 of Atari there. That worked for me too.

[myriadcs]

I have modified cable and tried connecting Atari pin 8 to Scart pin 16 (and disconnecting Atari pin 12 to scart pin 16). It's not "shaky" but image is kind of blurred.

 

Then I tried Atari pin 8 to Scart pin 8 (as it was originally) and Atari pin 12 disconnected.

Better than previous but still not good. Similar to Atari Pin 8 and 12 not connected at all.

 

What else should I try?

[ParanoidLittleMan]

It is blurred because works in composite mode. Try with connecting Atari 8 to both SCART 8 and 16. If that's not good will need some +5V line via resistor about 1K to pin 16 .

My monitor switches in RGB mode without adding anything to pin 8, but that's probably not typical.