Atari got CSYNC wrong on more than the Falcon ...

[leonk]

They also got it wrong on the Atari Mega STE.

A Mega STE came in today with no composite video / csync out the multi-DIN port. I traced the problem to the 470uF capacitor (sounds familiar Falcon fixers?) I've attached the section from the schematics.

C511 is a 470uF 16V capacitor. Note the +ve end is towards the 75 ohm resistor. That is wrong. Here's why. We know that the 75ohm resistor is there because by convention this sets up the 75ohm termination. The 470uF cap is there to prevent signal bounce. Using 470uF is a bit of an overkill (Most Sony AV ICs like the CXA1145 suggest using 220uF) But in all datasheets, the +ve leg is always towards the source. Here Atari has the +ve leg towards the output!

When I replaced the cap, in the correct orientation, my SCART RGBs cable started working and the PVM reported that CSYNC was back. :)

Screenshot 2024-05-17 at 8.43.43 PM.png

[Steve]

Nice find. This is also an issue for STe boards too. So that's three different computers they stuffed this up on lol.

[sporniket]

Absolutely, that's why I mentionned this fact on my conversion of the STE's schematic for C415, mentionning both how it is on the ST, and the topics of this forum :

Capture d’écran du 2024-05-18 14-33-28.png

[Hellraiseruk]

Funnily enough I have an STE which works fine over RF, but no signal over RGB SCART on my LCD - using a CRT I get a picture but it's rolling all over the place, which sounds just like a lack of sync signal... I have dug out a 470uf cap, I'll snip out the old one and blob this on the correct way round, see if it makes a difference.

Cheers