Philips 42PFL3704D/F7 Dead

[kevinm34232]

Philips 42PFL3704D/F7 Serial: YA2A Chassis FL9.2

TV is dead, no standby. Checked main fuse good, and then noticed no AC voltage at bridge rectifier. Checked resistor R1602 left of main fuse and resistance measured 8.37 with meter set to 20M ohms.

Could it just be the resistor??

[tom66]

Re: Philips 42PFL3704D/F7 Dead

That is an EMI safety resistor (MAINS to EARTH) and should essentially appear open under test, it is not at fault.

There is a relay on the board, it needs to click in before you see power at the main bridge. Above it there are four diodes -- the standby bridge rectifier implemented using individual diodes. Post a picture of the bottom of this area.

I assume you have checked the fuse.

[kevinm34232]

Re: Philips 42PFL3704D/F7 Dead

Thanks Tom. Fuse was good. Will pull the board up when I get back home. Also those 4 larger diodes test good right above the EMI..

[budm]

Re: Philips 42PFL3704D/F7 Dead

Service manual:

[tom66]

Re: Philips 42PFL3704D/F7 Dead

Ugggggggg it uses a self-oscillating converter. Like Sylvania sets. Fun!!

[kevinm34232]

Re: Philips 42PFL3704D/F7 Dead

Ok so Im using the flow sheets in the SM, the diodes 1205-1208 test good. Check Q1203 and Q1204. Q1204 was showing short between all pins in circuit, I removed Q1203 and Q1204, both have about 13ohms between pins 1 and 3 (C-E and G-S). High resistance between other pins.
Not sure how to tell if these are good with meter..





Also by removing Q1203 and Q1204 now diodes 1209 1210 and 1211 test good in circuit which didn't before.
D1212 still does not I need to lift a leg on that one.
(Lifted leg D1212 tests good)

[tom66]

Re: Philips 42PFL3704D/F7 Dead

Sounds bad to me. They should be checked out of circuit.

[kevinm34232]

Re: Philips 42PFL3704D/F7 Dead

Yea it looks like Q1204 is definetely bad and Im suspecting the larger Q1203 is also bad..

[tom66]

Re: Philips 42PFL3704D/F7 Dead

What do the tests show?

[kevinm34232]

Re: Philips 42PFL3704D/F7 Dead

I don't have analog multimeter to test the N-Channel MOSFET, if you know a way to test it with the DMM let me know.

The Bi-Polar transistor has low resistance between Collector & Emitter so according to my info that is shorted.

Having trouble finding the Bi-Polar transistor Q1204 (TRANSISTOR KTC3199-GR-AT/P Part #NQS4KTC3199P)

Sears parts direct has it but shipping is ridiculous, Mouser has the other one..

[tom66]

Re: Philips 42PFL3704D/F7 Dead

Test it on ohms. Anything you can do on an analogue meter, you can do on a digital one. You need to test gate-source. It should appear open circuit.

[kevinm34232]

Re: Philips 42PFL3704D/F7 Dead

Gate to Source shows shorted, about 17ohms. This is removed from board.

[tom66]

Re: Philips 42PFL3704D/F7 Dead

Replace the MOSFET then. You can use almost any part as it is in the standby circuit, pretty much non critical.

[kevinm34232]

Re: Philips 42PFL3704D/F7 Dead

I can find the MOSFET on Mouser for $0.76, its the other one that's shorted that I cannot. Is there a subsitute I can use for this NPN transistor:

http://pdf1.alldatasheet.com/datashe...TC3199-GR.html

[tom66]

Re: Philips 42PFL3704D/F7 Dead

Any NPN with >30Vceo will do... i.e. 2N2222. Which you can buy in bulk for about $0.02.

[kevinm34232]

Re: Philips 42PFL3704D/F7 Dead

Thanks Tom, just placed order.

BTW is this a common failure that you know of? Would the shorting MOSFET take out the other small transistor? Everything else around them seems to test good..

[tom66]

Re: Philips 42PFL3704D/F7 Dead

I've never had to repair a self-oscillating converter, which could be considered a blessing. Gate-source shorts on MOSFETs are very rare, but usually due to poor design of the surrounding circuit exceeding the absolute maximum of 25V or 30V on the gate relative to source.

[kevinm34232]

Re: Philips 42PFL3704D/F7 Dead

Replaced the 2 transistors, still dead. Still no AC at bridge rectifier. This problem should be localized to the standby circuit correct?

[budm]

Re: Philips 42PFL3704D/F7 Dead

"Still no AC at bridge rectifier" The bridge is fed through the fuse F1601.

[kevinm34232]

Re: Philips 42PFL3704D/F7 Dead

Fuse is good, all diodes in area are good.. Im gonna check all the resistor values later. The 2 transistors were bad maybe something else close by shorted out too..