Thank you for your time cjs and Pentarou, it is greatly appreciated.
Ah, so some of those voltages are ac, that explains why I didn't get a proper reading. Unfortunately I have a cheap multimeter which has only 600 and 200 ac voltage settings and displays only 0.00 when I try to measure these.
Using command "color 0, 0, 0" I set the screen to black, but get no reading propably due to my meter.
About the picture, you started reading the male connector pins upside down I think. I was hesitant to post the picture for this reason as it looks really ugly and is difficult to take a picture with all connections showing. I have however checked them multiple times and everyting is as I stated previously. I am using a scart extension from the female end of my adapter to OSSC, I have tried 3 separate known working ones, but it has made no difference.
Don't own a PVM.
I did try to connect JP21 Green pin to my monitor composite in (actually a retrotink 2x classic), and it is actually trying to react. Screen goes black for a few seconds, but then it returns to its color bar test screen. Tried multiple times, no difference.
I am considering buying an oscilloscope, they seem easy enough (https://www.youtube.com/watch?v=t3SSlUDtqTA&t=402s),
but don't know if I care enough to do it. I've pretty much lost hope of getting RGB out working, it's just that I'd like to figure out where the problem is, since obviously something is wrong.
Unfortunately I have a cheap multimeter which has only 600 and 200 ac voltage settings and displays only 0.00 when I try to measure these.
Well, don't get too sad over it. Even some expensive multimeters don't measure this kind of stuff well, or at all. It's not that they're bad, it's just that it ain't the kind of tool you'd use for a job like this.
I did try to connect JP21 Green pin to my monitor composite in (actually a retrotink 2x classic), and it is actually trying to react. Screen goes black for a few seconds, but then it returns to its color bar test screen. Tried multiple times, no difference.
Well, you'd learn more from a proper CRT, but it's not worth buying one just for this. But I'd say it's likely you don't have sync on green.
I am considering buying an oscilloscope, they seem easy enough (https://www.youtube.com/watch?v=t3SSlUDtqTA&t=402s), but don't know if I care enough to do it.
From a quick glance, that looks like a great video, and I really would encourage you to get one of those cheap 'scopes and further debug your problem if learning more about how video works interests you. And of course you'll be able to use it to debug things further back inside the machine if the problem is with the machine itself.
Sometimes this kind of thing is as simple as a broken leg on a capacitor on a motherboard. My TRS-80 Model I had broken video for a long time until I finally sat down with a 'scope, traced things back, and found that a tiny blob of solder in the right place fixed everything.
[Measure it on AC, but if you don't get a reading, don't be surprised.
If you test it in DC you'll get ~0.6V.
About the picture, you started reading the male connector pins upside down I think.
Yes, sorry... I used the socket numbering, which is mirrored, for plug and socket.
.
Anyway, before you give up, you can do a quick test by connecting a single wire from the composite output (Yellow RCA) to the OSSC SCART pin 20, (leave the R,G,B & GND wires in place) and then see if the OSSC is able to output a signal.
[Measure [the CVBS/sync signal] on AC, but if you don't get a reading, don't be surprised.
If you test it in DC you'll get ~0.6V.
I'm curious; why is that?
[Measure it on AC, but if you don't get a reading, don't be surprised.
If you test it in DC you'll get ~0.6V.
Anyway, before you give up, you can do a quick test by connecting a single wire from the composite output (Yellow RCA) to the OSSC SCART pin 20, (leave the R,G,B & GND wires in place) and then see if the OSSC is able to output a signal.
Measured DC voltage between scart end pin 20 and scart end plug shield and I did indeed get around 0.6v that steadily started dropping from there.
I also did the composite to scart pin 20 test. Did it by holding my composite wire to pin 20, no reaction from OSSC.
Repeated the same test after removing pin 10 -> 20 connection, no reaction from OSSC.
It's strange that the voltage is steadily dropping, it should only fluctuate a little. Maybe the GND connection is flaky?
But anyway, assuming your composite output is working I don't know what else to try.
Sorry. 
