wow thanks for so much detail.
I'm sure something is wrong, cause now when I re-test, voltages seem way off?!
On the probes there is a probe attenuation slider, with 1x and 10x positions. Slide it back to 10x (or maybe it is the other way around! Has been a while). There is a probe attenuation setting in the software as well. RetroBob covers this in his video (you linked it above).
While you are at it, take a little time to calibrate the square-wave of the scope. Bob also shows you how to do this in his video.
In the pic you posted we can see the H sync pulses clearly, but you need to also see the V sync. It will be easier if you "zoom out" by using a larger time scale (like 500us or 1ms), pause and centre on the V sync, then "zoom in" (to a shorter time scale like 50 or 20 or 10us). Drawback is detail is not as good this way. You can also use the trigger function on the scope to try an get it to lock onto the Vsync period, though this can be difficult as the scope can get confused between H & V sync.
Alternatively you can leave the scope at 100us and just randomly hit pause until you get lucky and it stops with the V sync period visible.
This is from R200 series card with VGA -> RGB/S cable, and sync plugged directly into oscilloscope
I dont really have a way of testing the sync while plugged in, i assume id need a T connector or something?
Yeah, it can be a bit tricky to get the probes securely in the right spot for the output to TV. Best to do your measurements "under load", while connected. In the industry they get around this by using BNC T connectors. You can then use an RCA to BNC adapters to connect your signal to the scope and then carry on to the TV. I know the Svideo doesn't use RCA connectors, but composite video does and you could still run the Y signal through to the scope and then to composite and still get a B&W picture.
Here is another video with RetroBob and Ste from HD Retrovision demonstrating how to use a Rigol scope. You'll see they use the BNC T-connectors. They also use some kind of custom SCART breakout board to get the signal out of SCART
You'd still have to hack a connection somehow.
When I took my measurements before, I was using my component-modded China TV. For that TV I had added some ceramic capacitors (104 I think) in series on the input, and 75ohm terminator resistors (to ground). Anyway, I left the termination resistor legs exposed and raised *slightly* at the signal end, leaving a small arch, which made it easy to hook the probe leads on. Obviously the back of the TV was off so I could get the probe leads in there.
I wish I could take some photos of how I attached the probes to the TV, but I've since dismantled the whole thing! I actually took that screenshot I posted earlier of the sync months ago. I'm currently re-organising my work areas to get back more space.
You'd have to take the back off and have a good look if you want to do similar to what I did. Try to find a spot (like a component leg) on the signal path before the next component after the termination resistor (probably a capacitor in series). If you can't find a good spot the probe leads to, you could try lightly soldering a resistor leg near to where your Y signal terminator is, so you have something to hook your probe to.
The good news is that you don't need to have the TV turned on to do this. In fact, you can leave it off while using the scope, it won't make a lot of difference, the signal termination is still there.
A less invasive way to do this, without opening the TV, would be to hack a svideo cable. Cut it in half and use a small piece of breadboard to reconnect the signal paths (Y, C, GND and shield ground) between the halves. Put in some loops, using old cutoff component legs, to make it easy to hook your probes onto.
You could also take the covers off your transcoder and see if you can find a good spot to access the Y signal from there. It would have to be either at the signal outputs or just before.