Element Elchw321 will not turn on

[CapLeaker]

Re: Element Elchw321 will not turn on

look for bad solders. try forcing the psu, or make sure BLon is there.

[zape]

Re: Element Elchw321 will not turn on

SW ON signal is persent, and I see no bad joints.
Circled is the FET I replaced.
Do you think it must match the one below?
Which characteristics are important to match?

[zape]

Re: Element Elchw321 will not turn on

FAN7313 17 pin = 3.9v which means BCT/FT Fault-Voltage

Any ideas how to troubleshoot this?
I ordered original FETs already

[R_J]

Re: Element Elchw321 will not turn on

If the fets don't operate, I would think the ic detects a fault, likely SCP. It thinks there is a short circuit killing the voltage, so it shuts down.

[CapLeaker]

Re: Element Elchw321 will not turn on

If the original FET was shorted to the gate, look at the gate drive circuit.

[zape]

Re: Element Elchw321 will not turn on

Gate drive circuit looks OK
And it turns on sometimes, the problem is intermittent.

[zape]

Re: Element Elchw321 will not turn on

I would think the ic detects a fault, likely SCP.

It definitely detects a fault, which is indicated by high voltage on pin 17 (3.9v) When the backlight works it reads 1.2v
How to narrow it down to the root cause?

[CapLeaker]

Re: Element Elchw321 will not turn on

If I understand this correctly, most of the time the backlight on the TV does not turn on at all, but sometimes you have backlight. Do they stay on as long as you want or do they turn off by themselves after a while?

[zape]

Re: Element Elchw321 will not turn on

Yes, sometimes the backlight work just fine.
I didn't wait long, but it stayed for about two minutes. I also power cycled the TV several times in a row and it would also turn on every time.
But sometimes it would not turn on at all. I was not able to find any correlation yet and yes I did check visually for cracked solder joints and was not able to locate any.
I also verified that there are no significantly dark spots on the screen - the backlight seem to work OK when it turns on

[Unspun01]

Re: Element Elchw321 will not turn on

V803 and V804 appear to be working together as two halves of a resonant converter. If one failed (V803), you should replace with the same matching transistor (FDPF7N50U) , or replace BOTH V803 & V804 together with a PAIR of matching specs transistors.

There is a possibility that you are triggering protection because the gate driving circuitry is driving the two different transistors identically, but the transistors are performing differently and protection doesn't like it.

If you already replaced V803 with a 2SK3569, then I suggest you replace V804 with a 2SK3569 also (and keep the other FDPF7N50U for spare). These two transistors V803 and V804 should function as a pair, and should be matching specs.

also, double-check the diodes in your gate driving circuits. The fact that you accidentally triggered something by simply touching your probe leads means you may have a damaged component, or bad solder joint.

[zape]

Re: Element Elchw321 will not turn on

Yeah... I guess I'll wait for original FDPF7N50U to come and see if it fixes the problem.

[zape]

Re: Element Elchw321 will not turn on

FDPF7N50U fixed it
Thanks you all for your input

[R_J]

Re: Element Elchw321 will not turn on

That's good to here, so it was just a mismatch of the two fets that caused the problem. Likely the different on resistance

[Unspun01]

Re: Element Elchw321 will not turn on

Glad to hear you fixed it.