520 ST repair after overvoltage

[burn]

Hello everyone,

due to a really stupid mistake, my 520 ST (short version) unfortunately got 12V on the 5V rail. This destroyed some components, which I am now trying to identify and replace.
With the help of another 520 ST I found out that the CLUE is defective. I have now replaced it.
After switching on the ST, the serial console of the diagnostic module immediately gave a lot of RAM errors and one of the RAM chips became hot. The STM 4MB RAM UPGRADE from exxos is installed in this ST.

I was able to find 2 new RAM chips of the same type on the internet and replaced both chips. The ST now boots into the menu of the diagnostic module and the RAM test runs without errors. However, only on the serial console, because the screen output does not work.

The shifter seems to be OK, also the MMU and DMA. These work in the other ST.

Using a ST2VGA adapter I get no picture at all, neither RGB nor monochrome.
But on a CRT monitor I get this output:

Video Output.gif

Without the diagnostic module the ST starts up to the desktop. The mouse pointer can be moved. So the ST seems to be running in principle.

I replaced the 74HC74 near the shifter on suspicion, but this did not bring any change.

Unfortunately, I am now at a bit of a loss and hope that you have a few more tips on how I should proceed with the troubleshooting.
Thanks.

[stephen_usher]

It looks to me as if the sync pulses aren't being generated (GLUE), or the monitors are trying to use the composite sync and Q10, the transistor which combines them, or the diodes used to feed the signal (CR9 and CR10) have died.

(This assumes that it is the modulator free version and not the STM.)

[burn]

I removed the modulator and all associated components at the beginning of the year. After that there were no problems with the image output until the overvoltage accident.

To rule it out, I fitted Q10, CR9, CR10 as well as R129, R130 and R131 as suggested. Unfortunately without improvement.

I have not yet been able to measure whether the sync signals are generated by the GLUE as I did not have an oscilloscope. But now I have bought one.

Apparently no usable signal is generated at H-Sync. V-Sync seems to be ok.

H-Sync broken.png

V-Sync broken machine.png

The GLUE chip seems to be OK. Because when I install it in another machine, a correct H-Sync is generated by this GLUE.

H-Sync ok.png

Does the GLUE need signals from other components to generate the H-Sync? What can I check next?

[exxos]

HSYNC sometimes has other logic involved...

Capture.PNG