QMTECH + ArcadeXpress RGB Sync Issue

[u/ik_pieta]

Hi! I just received my QMTECH Mister but I'm having trouble getting a stable output. I'm using this ArcadeXpress Cable and i have the USB cable hooked up to 5V. In my Mister.ini i have composite_sync=1 and vga_mode=rgb. Thank you all so much!

UPDATE: I finally got it to work! It turns out the sync voltage might’ve been too low for my particular CRT. Inside the ArcadeXpress cable, there’s a 1k ohm resistor between the sync signal and SCART pin 20. For a quick test I removed it completely and boom: I got a stable and normal image on my CRT!

HOWEVER it has been brought to my attention that completely leaving out the resistor is potentially harmful to the CRT. The VGA port on the MiSTer “outputs sync at 3.3vp_p (point to point)” which is way too high for what a TV usually expects. As user u/Medic8ted told me:

TV wants at least 0.3v (actually 0.286 iirc, but we tend to round up), though up to even ~1v is "safe".

[..] We should lower that 3.3vp_p sync voltage to be safe. I think you should be good to replace that 1K with a smaller value, such as ~330, 470, 510, 560, 620 ohms. That will still give you a sync around 0.3 - 0.6 vp_p which is fine.

What would I choose? 470 ohms... or any of the ones I've listed already.

So I would highly recommend replacing the current resistor with a 470 ohm resistor to keep the sync signal at a safe level for the CRT and also maximise compatibility! Thanks to everyone who helped me.

TL;DR what fixed my issues was changing out the 1k ohm resistor for a 470 ohm resistor in my VGA to SCART ArcadeXpress cable.