MAGNAVOX no picture powers LED 47MF437B/37

[tom66]

Re: MAGNAVOX no picture powers LED 47MF437B/37

In circuit measurements are never guaranteed to be accurate -- if they test good out of circuit they're probably good.

[c.teaman]

Re: MAGNAVOX no picture powers LED 47MF437B/37

Thanks for the reply. Where should I look next to repair my tv? After replacing all of the caps, it still wont power up and I still get the chirping. The ZD diodes referred to earlier in this thread all test fine...

[tom66]

Re: MAGNAVOX no picture powers LED 47MF437B/37

When you say they test fine, what do you read?

[c.teaman]

Re: MAGNAVOX no picture powers LED 47MF437B/37

between 450-550 on each, infite (1) backwards...

[c.teaman]

Re: MAGNAVOX no picture powers LED 47MF437B/37

I have spares for some of them, would it do any good to just throw them in?

[tom66]

Re: MAGNAVOX no picture powers LED 47MF437B/37

Those readings look good. The other possibility is a bad startup cap. This is a small capacitor on the primary side of the supply, normally near the controller IC.

[c.teaman]

Re: MAGNAVOX no picture powers LED 47MF437B/37

Thank you very much, I'll replace that and see what happens. Would anybody be able to pinpoint it's location for me? I would upload a picture of my own, but my camera isn't nice enough. Would it be possible to show where it is on one the pictures already on this thread?

[tom66]

Re: MAGNAVOX no picture powers LED 47MF437B/37

Assuming it is this board.
https://www.badcaps.net/forum/attach...2&d=1321230837

Your startup caps are indicated below, replace all of them.

[c.teaman]

Re: MAGNAVOX no picture powers LED 47MF437B/37

Thanks so much, I really appreciate it

[budm]

Re: MAGNAVOX no picture powers LED 47MF437B/37

Diagram

[c.teaman]

Re: MAGNAVOX no picture powers LED 47MF437B/37

I'm sorry to keep asking questions... is it acceptable to replace a 6.8 uF/50V cap with a 6.8uF/35V? I'm removing the capXon and replacing it with a rubycon

[tom66]

Re: MAGNAVOX no picture powers LED 47MF437B/37

Depends where it is. If it is a startup cap probably OK but that's not a general rule.

[c.teaman]

Re: MAGNAVOX no picture powers LED 47MF437B/37

So, I have now replaced every cap on the board... it still chirps and won't power on. The only thing that seems to indicate a problem is the three diodes that show bad numbers in the circuit. All three test fine out of the board. Any ideas? And am I damaging the caps I have just put in when I test it?

[budm]

Re: MAGNAVOX no picture powers LED 47MF437B/37

What are the designators of the the diode so you can look at the diagram to see how they are being used in the circuits?

[c.teaman]

Re: MAGNAVOX no picture powers LED 47MF437B/37

D912 0.001, 0.001

D926 0.365, 1.755 and climbing
D928 0.355, 1.755 and climbing

the last two are located right by the three 1500 uF caps

[tom66]

Re: MAGNAVOX no picture powers LED 47MF437B/37

Can you highlight D912 for us -- looks bad...

[c.teaman]

Re: MAGNAVOX no picture powers LED 47MF437B/37

D912 tested fine when it was out of the board... .560, 1.0

I also noticed that Scatman was getting the same exact readings on D912 and it ended being resolved for him when he replaced zd902 & zd904. They were both completely destroyed... mine are functional.

[tom66]

Re: MAGNAVOX no picture powers LED 47MF437B/37

Perhaps it is best to replace them, as it has almost always proved to be the issue with these TVs.

[c.teaman]

Re: MAGNAVOX no picture powers LED 47MF437B/37

Ok, I'm stuck... I'm just going to run through real quick what I've done... hopefully someone knows something I'm not finding.

All electrolytic caps replaced. All of them.
Fuse continuity test. Good to go
Replaced ZD902 and ZD904.
D912 0.001, 0.001 in circuit; but test normal outside circuit.
D926 0.365, 1.755 and climbing in circuit, normal outside circuit.
D928 0.355, 1.755 and climbing in circuit, normal outside circuit.

No progress. When plugged in, the red light comes on. Upon pressing the on button, it turns green and chirps. After about 5 seconds, the green light goes out, turns back to red, and the chirping continues.

Now what? Do I need to check out Q901? I'm not ever sure I know how to test it... I think its a MOSFET. I only have my DMM.

Anything input would be greatly appreciated. Thanks for all your help thus far, tom66

[budm]

Re: MAGNAVOX no picture powers LED 47MF437B/37

OK, as I had suggested to look at the SCH I provided, you will see that D912 is connected in parallel with SOURCE resistor R949 0.1 Ohm, so it will show like short on the board.
D926, 928 are connected in parallel with Source/Drain of MOSFET Q917, so if the pads where the D926/928 are connected to still show short, then Q917 is shorted.