Mosfets on monitor inverter board getting too hot

[Ahura Mazda]

Hello everybody, I came across one monitor which I was told makes buzzing sounds (for which I assume are inverter transformers), and sometimes wouldn't turn on. I have to say, with monitor working, I couldn't replicate the buzzing sound after replacing the bad caps, practically all of them, but I am worried about dark spots on the board indicating really high mosfet temperatures. After an hour, the enclosed box with the board is extremely hot to the touch (the reason for bad caps, surely). The board dates back to 2006.

I was thinking whether mosfets Q5 and Q6 have degraded over time and their internal resistance has increased? Does replacing the mosfets with even lower Rds(on) could solve the issue or could there be another reason for such high temperatures? Could some asymmetry or something else I overlooked?

Thank you, any suggestion would be highly appreciated.

[madires]

MOSFETs can become hot by driving them too slowly, i.e. when they stay longer in the resistive part of their operation curve. You could check the quick-discharge based on Q3 and Q4, and also R11 which limits the drive current.

[TheMG]

I'm guessing that monitor has very high hours of use on it?

A lot of older monitors do run quite hot and that's just the way they were designed, to keep the costs down as well as the physical size, so not much in terms of heat dissipation, and that type of PCB material does discolor like that over time with elevated temperatures.

Also I believe I read somewhere that as the CCFL lamps age their operating voltage increases slightly, and they run hotter.

[Ahura Mazda]

I'm guessing that monitor has very high hours of use on it?

A lot of older monitors do run quite hot and that's just the way they were designed, to keep the costs down as well as the physical size, so not much in terms of heat dissipation, and that type of PCB material does discolor like that over time with elevated temperatures.

Also I believe I read somewhere that as the CCFL lamps age their operating voltage increases slightly, and they run hotter.

Well, that is interesting, I couldn't find the info regarding the operating votage increase, that is interesting and would explain this if true. And yeah, I'm not sure for hours used, but it is safe to assume so.

MOSFETs can become hot by driving them too slowly, i.e. when they stay longer in the resistive part of their operation curve. You could check the quick-discharge based on Q3 and Q4, and also R11 which limits the drive current.

Mmm, that is true. I don't know how to check for quick discharge, but I did remove the top metal cover which was enclosing the board (literally making it like an oven) and I did use it all day today to see whether buzzing would return. And, the temps now seem quite reasonable. Still fairly hot, yes, but much warmer than before.
The main source of temps as I've observed today is the switching transistor on live side. Since the monitor won't be on much daily, I really don't want to bother with making suitable heatsink for it.


Thank you for your help. 

[JayArr]

I would replace all four lamps and don't forget the high voltage caps, C13 and C14.