Black screen (but sync?) on Mega STe

[Pacman]

For some reason my Mega STe screen is black when I turn on its power.
It's attached to a hires VGA monitor and the screen does show "640x400/71.5 Hz" (info text from the monitor itself) so apparently something is working. I also tried pressing some keys and I get to hear the usual keyclick sound.
So suspecting that the home-made VGA cable has contact issues (I checked the connections, and after some resoldering it's OK again -one of the R, G or B lines had a bad connection to the "mono" pin) I pulled out my trusty Atari SM-144 and experienced the same thing (totally black screen).

Apart from the 13-pin DIN connector on the MSTE motherboard (cold solder joints, broken traces due to to repeated unplugging throughout the years), is there anything else worth checking?

Prior to the screen going all black the next day I turned it on I noticed distorted text (i.e. slightly "ugly/chunky" looking text and graphics), and at times the screen looked like it extended beyond the right hand side of the desktop. At the time I thought the VGA cable was to blame, but maybe not. I had the blitter turned off in the TOS 2.06 menu when trying all this out.

[frank.lukas]

There are 33 Ohm resistors on the Vsync and Hsync lines check the schematics. The resistors can be faulty ...

[Pacman]

Found them!
R531 and R533 both measure around 33K and go through inductors L502 and L503. I checked these since too I've had bad experiences (and frustrated troubleshooting in the past) when one of these had shorted to GND. But these appear OK.

Screen Shot 2021-04-27 at 11.51.12.png

[derkom]

R531 and R533 both measure around 33K and go through inductors L502 and L503.

Presumably you mean 33 Ω here?

[Pacman]

Oops!
Yes, absolutely. I rechecked and they're indeed both 33 Ohms.

Here's what comes up on my VGA LCD screen (hires mode) when I power up the MSTE. Other than that the rest of the screen is entirely black. Same thing with the SM-144 (obviously not with the info overlay as shown here though):

20210427-114130_P4270051.jpg

[stephen_usher]

That's high resolution. Have you got a mono monitor you can check things with?

Alternatively check the line for monitor switching.

[Pacman]

You mean colour monitor?
No, just two mono-capable monitors -an Atari SM-144 and an NEC 1990SXi.

Good idea to check the mono detect line. It should go to GND with a mono monitor, right?

[derkom]

Good idea to check the mono detect line. It should go to GND with a mono monitor, right?

Correct, and it's pulled high otherwise. But since you are getting mono sync, it certainly follows that you should find the mono detect line is working properly.

[Pacman]

Are there any signals/voltages/waveforms etc. I can check for in order to track down what's missing in the video signal and where the problem might originate from?

I never got the serial connection to work with the diagnostic cartridge or I'd connect it to my STe and confirm that the rest of the computer works fine, but from what I can tell it appears to be OK (the usual power up procedure, keyclick etc.)-

[exxos]

It sounds like you have a RAM fault going by your first post..