Suggestion for repairing a Commodore Plus4

I'm trying to fix a plus4 and looking for some suggestions

Plus 4 would turn on, display picture but after 10-15 minutes of operation:
- lose video signal for a second (composite)
- reset itself randomly

I have replaced the capacitors and reseat CPU and TED, with no success in fixing the problem. After a while operating in these conditions, the sreen suddenly went garbage, and then black. Since then it's displaying only black screen

My guess is shorted TED or CPU. I have noticed that CPU and TED replacements are quite expensive, so I don't wanna go random. Any sugegstion to better identify the fault?

 

Standard troubleshooting - go with the service manual flowchart first.

Obviously check +5v and +9v lines (and that chips are getting correct voltage). Garbled graphics check the BASIC ROM. You could do the video check but that usually assumes something on screen.

Black screen was common and was the TED (or sometimes CPU, yes). RIP Plus/4 in that case.