TV REPAIR
They are 500V 5A 1.4ohm, with a body diode inside. What I don't understand, had never seen before, it that there is more that just the body diode. Look at the datasheet attached below... the symbol they give shows what looks like a DIAC between the Gate and the Source pins ?!
They don't talk about it in the text, strange... If I don't know what that do and how important it is, I might cause damage to the board if I replace that MOSFET with another one that does not have this DIAC in it...
Not just the font; the casing plastic appears to be a slightly different colour. That and the observation about the heatsink compound infers there may be a thermal issue. TO-220P have a pretty horrible thermal resistance compared to regular TO-220.
Oh yes good point ! Sorry I failed to get that in your prior message, told you just woke up, need an hour to sharpen up !
OK, so did a quick test before I go working on my plinths. Powered up the TV and measured voltages at the test points conveniently provided on the board.
One reads 190V no ripple, good DC.
Other one reads only 133V, no ripple either, good DC.
So the MOSFET and the power stuff is good, it can generate HV... just not enough of it...
From working on the other TV last time, I know this is normal behaviour for a working board but failed LED strips...
The board at startup puts out 200V+ to get the LED's going, then lowers the voltage to a bit lower than 200V, as it regulates the current and lowers the voltage as necessary.
The driver chip monitors the current and if it senses a short or open-circuit condition, it lowers the voltage by a few tens of volts like it does here, as a precaution, to be below the voltage drop of the diodes.
Did another test : measured Vgs on both MOSFETs, thinking the AVG DMM value would reflect the duty cycle being applied to the gate, hence tell me what the driver chip is actually asking. That is, do I get a low voltage because the chip asks for it, hence it is sensing a fault, or does the chip want to give me proper voltage but it fails because of external reasons.
The one that puts out 190V, the working string, I get 11V AVG at the gate. The other one that puts out only 133V, I get only 8.5V, which seems consistent...
So looks like the chip is working fine and just doing what's supposed to be doing when a string fails.
So that means my luck is out, didn't last long did it...
So yes, bad luck, we do have indeed two strings failed at the same time... so either the owner lied and it happened one string then later the other one (owners always lie to me...), or, it's a cascaded failure. One string failed and caused the other to follow suit.
I don't know. For example say the original fault was an LED going open circuit in one string. Instantly that doubles the current in the remaining string, because they are wired in //, and that blows the LEDs in that string.... unless the driver chip can react fast enough to lower the voltage in time, but can it.... LEDs blow instantly, they don't wait for Mister driver chip to react...
OK so it's like the other TV then, need to take the LCD apart again to get to the LED strips inside, oh dear.......
Before I do that I will ask permission from the owner... if I damage the LCD during disassembly/re-assembly, I don't want him to be mad at me. So either he is OK taking the risk, or I give him back and he can take it to a professional shop that will charge him 250 for the job... assuming he can even find such a shop, locally at that, as he is of course not going to package and ship such a monster TV, too much money and trouble.
So hopefully he will see it as a "nothing to lose, go for it, whatever happens happens ! " case... if not well I will get to reclaim the bench space sooner that I expected.... and I wouldn't be complaining... this thing just is way too big to be practical to repair. Way.. too... big. Makes my Tek boat anchors look like toys.