My Thomson TV won't turn on.

Thank you, it works. I was desperate.

Hello, it's an internal power supply problem
return to service (unless your knowledge of electronics allows you to possibly find the fault)

Hello, you need to check the power supply and especially the high voltage (danger, beware!!!!!!). If your knowledge is basic, do not venture into it; as the name suggests, it's high voltage. Even with the TV turned off and unplugged, you risk a serious electric shock: capacitors can remain charged!!!! (the voltage at the level of the tube vacuum can reach 25,000 V!!!). It can be replaced, but they are becoming harder to find. If that is the cause, it needs to be changed.
Also check the solder joints on the chassis at the high voltage level, to see if an arc appears at its level or if it is cracked...
Another possibility: faulty tube holder: that won't cost much.
For your information: https://forums.futura-sciences.com/depannage/109259-television-ecran-noir-ok-sifflement-tht-ok-led-rouge-allume.html
High voltage: https://www.indipc.fr/informatique,tht,connectique,page,boutique,mod,boutique,gps,1332.fr.html
TV failure file: https://www.lelectronique.com/depannage/technique/cas-de-sifflement.html

Thank you for your quick response. Which part of the feeding system needs to be monitored? And regarding the high voltage, what voltage are we dealing with?

See the links I indicated, everything is explained, there are even photos
Voltage at the suction cup level: 25000 v
Do a Google search: 'tht tv failure', you'll find what you need
The pen trick to detect the fault is basic but it works
tht: here at the bottom left (black block with thick red and black wires):https://imageshack.com/

Ok great! Thank you for your answers and the time spent, wishing you a good evening...

A little extra: photo of the THT and its suction cup: http://www.sobelec-tn.com/img/p/3401-3313-large.jpg
Parts: also here: http://www.sobelec-tn.com/product.php?id_product=3401
If you have any other questions or need further details, don't hesitate to ask
Have a good evening