Here are some meansurements on the ic uc3842.
after the bridge rectifier vss is 320V. Mos-Fet no short.

Service manual
https://docs.dev-docs.org/htm/search.php?find=_F

short video

https://www.youtube.com/shorts/dkYyM0O3wrI

tt0ny wrote on 2024-05-18, 14:40:

The pwm controller is a uc3842an ic. At the output pin 6 of the ic this oscillation occurs every 1 sec. The signal go to the mosfet. Does it look like a shutdown by a protection circuit?

IMO it looks more like the PWM IC is not getting power and just runs the startup cap dry, then the cycle repeats. It could be due to a problem on secondary side but it's not the only explanation.

First I would check or even replace the electrolytic caps that are in the power supply of the 3842. The schematic photo is not that great, I can't read many of the parts numbers but I think it's C131. Sits on the right of the IC, next to a diode, is connected to pin 7. Which according to annotation is 10-15V, that's too low. There should be 16V and preferably a bit more to make sure the IC turns on and stays on. When the voltage here drops below about 10V the IC will shut down due to undervoltage.

The way this works is initially there is some power supplied to the PWM circuit via that RT191(?) resistor which charges the C131 cap, once it has enough energy stored and the voltage rises above 16V the IC will turn on and should be able to power itself via that extra winding from the main transfomer. If the cap is now degraded it might not have enough capacity to power the chip before it can self-sustain itself. That being said there is a protection circuit, all that stuff to the left of the IC, but the schematic is not readable enough for me to tell how it works. I assume it's opto-coupled to the secondary, and Q130(?) can trigger Q131/Q132 (probably a latch) to pull the pin 7 down to GND and thus stop the PSU. Usually though this type of circuit will not self-reset unless power-cycled. So if the PWM chip seems turning on/off I would consider it a supply problem, not protection kicking in.

Thanks for the info.
according to the service documentation, i connected the 15v external voltage to pin 7. The ref voltage on pin 8 should then be 5V. It did not do that. The 3842 is defective!
replaced the IC and connected the 15V to pin 7. Now the voltage is
at pin 8 is 5V. Nevertheless, nothing moves on the monitor. I will disconnect the power supplies on the secondary side individually in the next few days and see if the PWM works again.

tt0ny wrote on 2024-05-19, 21:02:

according to the service documentation, i connected the 15v external voltage to pin 7.

For 3842 it must be over 16V. Up to 30 because that's the absolute max rating and besides I bet the cap is only 25V. So try 20V.

Resolved!
You are right the 3842 is ok it works with > 16V.
The defect is the horizontal Power Transistor BUW13A. Replaced and the Monitor live 😀
Thx