Hi!
An indication of a high voltage but after a few minutes, 4
minutes i think, it suddenly shutdown.
Do you have a picture during this time?
I suspect you don't. Are the CRT filaments lit?
I measured a voltage but when the monitor
dies up, the voltage also disappears.
1. NEVER Measure the HOT collector voltage with the deflection
working! You may, if you own a good old VOM with a 1500V (or greater)
scale, and ONLY IN THIS SCALE!
2. If the voltage disappears it is either a power supply problem or
the controller is shutting down sensing some other fault. Can you turn
the set back on with the power switch? do you need to press once or
twice?
I think, there is some other problem causing the micro to blank the
video and to shutdown. Maybe vertical deflection: Turn op the SCREEN
Control on the flyback (with the set on and the deflection running)
and see if you get a raster or a horizontal line.
The H-yoke is very unlikely at fault, the V-yoke is possible (in
theory) but I've never heard of a V-yoke failure. It would suspect the
Vertical output stages then (or a service switch)