Magnavox 42" 42mf521d/37 CHIRPING

[cola2681]

I am attempting to repair a 42mf521d/37 lcd magnavox tv for a friend and also increase my electronics skill level. The tv will not power up and makes a "chirping" coming from the psu. I removed the psu and found 4 bad capacitors.
3 X 1500uF 35 volt
1 X 2200uF 25 volt
replaced all 4 and no luck. I did some more researching and the most likely cause i saw for the chirping was a bad horizontal output transistor or ic
Any help would be very appreciated in this repair or a safe place to or
a replacement board. Board # 715T2243-1
I do have the psu removed and on my work table and still chirps when hooked up alone

[tom66]

Re: Magnavox 42" 42mf521d/37 CHIRPING

No such thing as a horizontal output transistor in an LCD or plasma TV (that's CRT and CRT-RPTV.)

A common issue with this set is a shorted diode on the primary which does often cause a chirping sound.

How about some pictures?

[cola2681]

Re: Magnavox 42" 42mf521d/37 CHIRPING

Here is a picture of the power supply. this is my first switch mode power supply
repair so i will probably be a little slow but i am doing as much research as i can on this board.

[tom66]

Re: Magnavox 42" 42mf521d/37 CHIRPING

Check these diodes on diode test mode. Report readings for each both ways. Board must be disconnected.

[budm]

Re: Magnavox 42" 42mf521d/37 CHIRPING

Yes those darn snubber diodes we seen so many of them that failed in this power supply.
By the way, what is the chassis number of this TV?

[cola2681]

Re: Magnavox 42" 42mf521d/37 CHIRPING

Ok here are the readings with the board disconnected

far left diode = .018 , .019

top diode = .066 , .062

middle bottom diode = .031 , .031

far right diode = .029 , .029

What exactly do you mean chassis number and where is it located?

[tom66]

Re: Magnavox 42" 42mf521d/37 CHIRPING

All appear shorted.
Your chassis number is indicated on the model sticker (where you read 42MF521D/37), e.g. Q422.3E.

[cola2681]

Re: Magnavox 42" 42mf521d/37 CHIRPING

Chassis = L06 E

[tom66]

Re: Magnavox 42" 42mf521d/37 CHIRPING

I believe this to be your chassis (can't find any reference to L06E as a chassis)
http://www.tgohome.com/ServiceManual...&Search=Submit

Can we have picture of the model sticker?

[Techstar]

Re: Magnavox 42" 42mf521d/37 CHIRPING

They appear shorted but i don't think they are , they are in circuit readings.

[tom66]

Re: Magnavox 42" 42mf521d/37 CHIRPING

At least three out of four of them should not appear shorted in circuit test according to the schematic.

[cola2681]

Re: Magnavox 42" 42mf521d/37 CHIRPING

What do you mean "they are in circut reading" and what values normally
should i be looking for when testing these diodes.

[martyparish]

Re: Magnavox 42" 42mf521d/37 CHIRPING

He means when testing the diodes without removing them from the circuit board. Sometimes the layout of the parts can give false readings while "in circuit".

Usually, if you suspect a bad diode, then you would remove it and retest it standalone.

[budm]

Re: Magnavox 42" 42mf521d/37 CHIRPING

Best and most accurate way to test the components is to remove them for the board and test them.

[Techstar]

Re: Magnavox 42" 42mf521d/37 CHIRPING

Yes diodes must be pulled on 1 leg to get an accurate reading testing in circuit is not always reliable . I have measured diodes in circuit that measured virtually dead short , remove 1 leg from the circuit and it measures just fine.
With practice though techs working on the same boards day in day out
just know what is normal and what is not without pulling..

[cola2681]

Re: Magnavox 42" 42mf521d/37 CHIRPING

Ok so i removed the diodes from within circut and tested them

diode 6054 = .008 , .008

diode 6055 = .007 , .007

diode 6056 = .460 , no reading

diode 6062 = .474 , no reading

so it appears diode 6054 and 6055 are bad with a part number of
LT6246 P6KE120A
both diodes are the same

[tom66]

Re: Magnavox 42" 42mf521d/37 CHIRPING

M'kay, get some of these (advise more than two as they're easy to misplace):
http://www.digikey.com/product-detai...ACT-ND/2770358
+$2.75 or so shipping.

[cola2681]

Re: Magnavox 42" 42mf521d/37 CHIRPING

Ok thank you for digikey info. just ordered....now the waiting game.

[Hotrodder899]

Re: Magnavox 42" 42mf521d/37 CHIRPING

I have the exact same tv with the same exact problem. Should I start my own thread?

[tom66]

Re: Magnavox 42" 42mf521d/37 CHIRPING

It's best, just in case your problem has a slightly different cause and so we can avoid getting mixed up.