Kogan TV PSU PWM controller not turning on?

[rocket]

Have a failed single board TV (a Kogan 'smart tv') and am having difficulty why the PSU refuses to turn on after a replacement of AFAIK the damaged components including mosfet and its associated power resistor, pwm controller, main fuse, one of the optocouplers and a couple of small diodes and zeners.

(The failure of the PSU was my fault - accidentally shorted the board). The schematic of the PSU is mostly the same as this one I found:

http://www.gzweix.com/article/Upload...0161772032.jpg

I noticed that the PWM controller (6755) Vdd pin 7 would fluctuate between around 8 - 13v (measured only with a dmm), indicating that the 6755 is probably attempting to restart but without success. I have no idea why the voltage won't go to 15-16v in order to turn on the controller and start the PSU. It's possible that the subcircuit which includes the V3 transistor isn't working to provide Vdd once the 6755 turns on but to be honest I don't quite understand how this is supposed to work (it may well be that there is a failure of V3 which presumably should turn on when operating). I've also changed NW7.

Any help in trouble shooting this would be much welcomed! I'm at a loss of what to do next.

[budm]

Re: Kogan TV PSU PWM controller not turning on?

Did you check the capacitance and the ESR of that startup cap connected to the VCC pin6 (through linear regulator)? Pin 8 Vin is just for line voltage detection if line voltage is too low.
Shorted rectifier or load on the secondary side can also cause it to be in the stratup-shutdown mode also.
The IC could have been damaged also when the power MOSFET shorted out.
V3 transistor is setup as a simple linear voltage regulator.
Do you see voltage fluctuating on start up cap C9, and Voltage on pin 6?
Vdd must be 16V typ (15~17 is the range) for the circuit to start up.

[rocket]

Re: Kogan TV PSU PWM controller not turning on?

Wow - thanks for the quick reply and tips, I will do that when I get a chance later today.

A couple of quick q's:

I'll check the voltage on C9, but on pin 6 of the 6755 it fluctuates between 8v and about 13V. The 6755 has already been replaced and the above pin 6 measurement is with new ic in place.

I'll also remove C9 later and test the esr. Is this the startup cap you were referring to or is it C4/C11?

[budm]

Re: Kogan TV PSU PWM controller not turning on?

There should be voltage at C9 if the IC did turn on ad send the Gate drive signal to drive the MOSTFET to drive the transformer to produce the Voltage on the AUX winding that produce that produces the Voltage for C9 through the rectifier diode connected to the AUX winding.
Did you verify that the resistors/diode network for the Gate drive is OK, also the resistor on the Source pin of the MOSFET is OK?

[rocket]

Re: Kogan TV PSU PWM controller not turning on?

I've finally had some time to take more measurements and checks:

On C9 I get a very slowly increasing voltage (starts at about 0.5v and then increasing slowly as it took 5 minutes to ramp up to about 0.7v at which point I stopped checking). C9 measures ok to value but I realised I don't have the equipment to test its ESR. There is also a smd cap in parallel (value unknown and not shown in the schematic) which I assume is to lower the overal ESR of the filter.

The resistors/diode on the gate of the mosfet all tested ok (diode removed and checked).

The power resistor between the mosfet source and ground was already replaced as the failing mosfet had killed it.

Also the voltage at pin 6 of the pwm controller fluctuates from 9 - 14v (not 8-13v as I reported earlier).

[kaboom]

Re: Kogan TV PSU PWM controller not turning on?

Replace the three electrolytic caps after the 12 and 24v rectifiers.

Margins are 13 and 27v, respectively (less junction drops). It won't take much ripple or overvoltage to trigger the overvoltage lockout.