Samsung pn64f5300af dead

[freakaftr8]

So I'm repairing this TV for somebody that came in dead main fuse blown on power supply found two voltage regulators MOSFETs shorted on hot side location qp802c and qp804c. They are K18A60V. Replaced with subs. Worked for a day and then heard something pop. Surprisingly they are still good in the fuse is still OK. However what I am presented with now is a red standby light in the TV will not turn on I'm getting 5v standby from the power supply to the mainboard. 175 volts DC at the main filter caps does not change. I have ruled out mainboard. I jumped PS on to 5 volts and then ground and also tried vs on with the same results just a red standby light and it will not come out of standby regardless if i try it with the remote or the button on the TV

Power supply model BN44-00618A. This TV and its power supply or both seemingly rare

[ReeceyBurger123]

Re: Samsung pn64f5300af dead

Any thing else test short also did you replace the gate drive circuit for the fets when you replaced them ?

[freakaftr8]

Re: Samsung pn64f5300af dead

Looks as if I made an error. The MOSFETs are replaced where 600volt 13 amp. The originals for 600 volt 18 amp. Looks like they may have been starving for amperage and the heatsink got too darn hot and pop the MOSFETs again I'm only getting one leg from gate reading direct short.

Since you didn't blow the main fuse I'm assuming the stand by transformer still operate sense its on a seperate circuit

[freakaftr8]

Re: Samsung pn64f5300af dead

So i replaced the MOSFETs and I got the same problem. I do have standby 5v the only other pin that has any voltage on it is PS_ON at 4.3 volts. Now with this power supply in order to get it to power up do you ground PS_ON to ground?

[freakaftr8]

Re: Samsung pn64f5300af dead

Any thing else test short also did you replace the gate drive circuit for the fets when you replaced them ?

No I didn't. What should I be looking at?

[freakaftr8]

Re: Samsung pn64f5300af dead

Okay I'm at a total loss obviously the PFC circuit is working because I'm getting stand by voltage. I'm getting 5 I'll stand by and 4 volts plus on the power supply on pin.
So I'm getting those too and I tried to jump ring power supply on the ground and 5 volts with no avail. So what's next? The circuit that turns on the switch mode?

[freakaftr8]

Re: Samsung pn64f5300af dead

I don't mean to sound like a noob but is there anything I'm overlooking on trying to force the power supply on?

[jetadm123]

Re: Samsung pn64f5300af dead

1) Don't know if this power supply requires ground or +5v to the PS_ON pin to turn the supply on. Did you measure any voltage (should be around 4V) on the PS_ON pin? If yes, then you use a 220 ohm resistor from that pin to ground to turn on the supply.

2) You should measure the gate drive components of the mosfets that shorted for proper values. These are the smd resistors/diode soldered around the gate lead of the mosfets.

3) Also, check for bad solder joints on the transformers.

[awyatt2]

Re: Samsung pn64f5300af dead

the standby usually comes from a separate section of the powersupply, meaning the pfc is separate from the 5v section. now go back to post 2 have you checked the gate drive components?

[Hamie]

Re: Samsung pn64f5300af dead

Usually Samsung plasmas are ps-on to ground.

[freakaftr8]

Re: Samsung pn64f5300af dead

Okay yes I'm measuring 4.3 volt at the ps_on pin. However no configuration that I do by jumping turns on the power supply. So this obviously rules out mainboard. I did find a couple small SMD components MOSFETs off the gate drive circuit of both of those transistors one had a hole in it in the other one was shorting 100 ohms to center pin. However I found replacements and it's still doing the same thing.

[jetadm123]

Re: Samsung pn64f5300af dead

The pfc controller chip could be damaged. What's the voltage on it's vcc pin?

[freakaftr8]

Re: Samsung pn64f5300af dead

Good call. I'll check it out.