Testing transistor on power supply

[Trueburton]

Re: Testing transistor on power supply

I only get OL with every combination i place the pins in diode mode

[budm]

Re: Testing transistor on power supply

16N55G is N-CH Power MOSFET which usually failed as short circuit between Source and Drain pin.

https://www.badcaps.net/forum/printt...1&pp=20&page=2

[Trueburton]

Re: Testing transistor on power supply

I only get OL on all connnections

[budm]

Re: Testing transistor on power supply

So you have two of those MOSFETs, right? Both of them show the same reading?
You should see the Body Diode reading (about 0.6~0.7V) between The Source and the Drain pin in diode mode with Black probe on the Drain and Red probe on the Source pin

[Alastair E]

Re: Testing transistor on power supply

Bung that mosfet back in, nowt wrong with it, They go Short practically never open--And then sort out the mistake you made with the SMD resistor/capacitor on the MC board.

Missing parts sure to make it malfunction!

[dick_barton]

Re: Testing transistor on power supply

I only get OL with every combination i place the pins in diode mode

That's not what you said in post #1

#1
I suggest taking Alastair's advice.

[Trueburton]

Re: Testing transistor on power supply

Sounds good, I'll put them back in. Back to post #16 about my missing parts. Any one know what that is or how I can find out what it is. I get a resistance reading of 1K Ohm. Thanks

[dick_barton]

Re: Testing transistor on power supply

There appears to be two small boards soldered upright on the PSU. Are they identical and if so you can check on the second unit. If they are not, sorry cannot help.

[Trueburton]

Re: Testing transistor on power supply

They are not identical but very close. They are so small there isn't any markings on them or if there is you can't see with the naked eye.

[Trueburton]

Re: Testing transistor on power supply

Update:
I put everything back together and the TV works! however, At first the screen flickered a few times but eventually went away. After 10 mins of TV being on, all fans running, I blow F301 on the power supply. Why? Is it the MOSFETs? What else could have been damaged when this fuse blew? Please help, Thanks!

[vinceroger69]

Re: Testing transistor on power supply

did you fit the correct value resistor that was missing to the mc201/301 module are you sure your soldering is good on the ics and also where the module mount to the main power pcb? if you removed any mosfets etc too test did you replace the heat sink compound?

[Trueburton]

Re: Testing transistor on power supply

I replaced the missing resistor with a 1K ohm as that is what the same resistor on the other board tested at. Im not 100% sure that it was correct though. Also had a very hard time soldering that small of resistor in place. Any tricks? I checked all my solder connections of the MC boards to PCB. They dont look great but non are bridged and conntinuity checks out on each pin to the trace its soldered to. The mofsets i reinstalled assuming they were good and did place back with the heat sink. Thanks for the quick reply.

[jetadm123]

Re: Testing transistor on power supply

F301 is a 5A 250v fuse. When it blows, it's usually because one or both of the 16N55G mosfets have shorted out. I suggest you go back and test them for shorts. No need to remove them for testing for the time being.

[Trueburton]

Re: Testing transistor on power supply

I probed the 3 mosfets on the top side of the board. All three give different readings.
Testing these in circuit with board out of TV. Using voltmeter to check continuity from center pin the pin on each side of center.
One is closed between all three pins.
One is closed between two and open on the other.
One is open between all pins.

[budm]

Re: Testing transistor on power supply

Take them out and retest them off the board. Look up the spec sheet so you can tell what pin is short to which pin, those MOSFETs have SOURCE, GATE, and DRAIN pins.
You should also use meter in Ohms mode and report the resistance readings, make sure to indicate if the reading is in Ohm, K Ohm, M Ohm.

[Trueburton]

Re: Testing transistor on power supply

Could not find correct data sheet. However, here are my readings. I will call the left pin 1, middle 2, right pin 3.
Q301- 1-2=121 ohms
1-3= 121 ohms
2-3= 0.8 ohms
Q302- 1-2= 3.3 ohms
1-3= 3.3 ohms
2-3 = 0.8 ohms
Q351 1-2= OL
1-3= OL
2-3= 2.7 M Ohms

[budm]

Re: Testing transistor on power supply

Q301, 302 are shorted.

[Trueburton]

Re: Testing transistor on power supply

That's what I figured. Thanks for confirming. What causes them to short? The Mc301?

[jetadm123]

Re: Testing transistor on power supply

Was mc301 the one you changed the resistor on? If yes, that could be the source of your problem. Someone else had mentioned that mc201 and mc301 are different, so your measurement of the resistor to replace the missing resistor (post #32) might have been wrong.

[Trueburton]

Re: Testing transistor on power supply

Yes, mc301 is the one I changed resistor on. But I cannot find any information or datasheet for this board! A aah! Thanks for the help so far. I'm not giving up yet!