Magnavox LCD 47mf437/37 turning on after fix, but high pitched whine

[withalligators]

Hey guys, I tried posting this in an existing thread but got no replies, so I'll try a new one.

Found a Magnavox LCD 47mf437/37, LED lit up when plugged in but wouldn't turn on. When power button was pressed, the LED would change color, still wouldn't turn on, but would chirp.

Has power supply 715T2454-2 S/N 07386. It looks a bit different from the other ones I've seen pictured on various threads. For starters, the 3 tall caps at C952, C951, C953 were all PF nichicon caps, and the one at C955 was a VR series Nichicon. The soldering job looked sloppy so not sure if it was a repair job or a revision of sorts.

I replaced the two zener diodes at ZD904 and ZD902 because they were burnt. I also replaced all the other caps (besides the 4 Nichicon and big 450V Capxon one) on the board with the same rating Panasonic or Nichicon ones.

The TV now works, but there is a very high pitched whine when it is plugged in, and slowly dissipates after it is unplugged.

I've just taken the heat shield off the main board, and found three swollen caps at C113, C131, and C830. I am obviously going to add these caps to the list of things to replace. Should I check other caps on the main?

I'd like to sell this TV, but don't want to sell something that might crap out. What else can I replace to make this thing bulletproof?

I've seen some people replace:
diodes D925, D912, D907, D926, D928
Mosfets Q901, Q902, Q917

Those Mosfets are kind of expensive, so before I replace, how do I test? I have a DMM and an ESR meter on its way to me (finally).

Cheers,
Alex

[withalligators]

Re: Magnavox LCD 47mf437/37 turning on after fix, but high pitched whine

Ok, well, I pulled every cap from the main board and ended up replacing the three aforementioned bad ones, and 2 others that tested ok but were so-so ESR wise. Still has a high pitch whine when you plug it in that gets so high it becomes inaudible. Then After you unplug you can hear it come back down. Not comfortable selling it.

Going to test the mosfets, but not sure how. Pulled Q901 and put it on a smart tester, which identified the gate, source, and drain properly. Also said C=12.9nF and Vt=4.2V. Does that mean it's good? Here is the datasheet.

Cheers,
Alex

[withalligators]

Re: Magnavox LCD 47mf437/37 turning on after fix, but high pitched whine

So, for giggles, I pulled the 2200uf 25V VR series nichicon cap from the board and it's reading 1854uf, Vloss of 1.5%, and an ESR of 0 ohms. That uF reading seems low. Could that be causing a whine?

[Caleb]

Re: Magnavox LCD 47mf437/37 turning on after fix, but high pitched whine

do any of the inductors get hot? Often whine under load is caused by inductors. It could be that a cap is not doing a proper filtering job and causing the inductor to work harder. you may be able to isolate to a single spot using a toilet paper roll to you ear to narrow down the source. If its not overloaded you can quiet an inductor with a little clear nail varnish or super glue. Since you also had bad diodes make sure there are no other leaky diodes in the circuit

[withalligators]

Re: Magnavox LCD 47mf437/37 turning on after fix, but high pitched whine

OK, here's a bit more detail: When I plug it in, it builds up this very high pitch squeal. When I turn it on, it makes some sounds, and then goes quiet (actually, I think it moves to a higher pitch squeal, but it's so faint, I can't be sure I'm not imagining it). If I turn it off, the squeal slowly returns. I can't tell for sure, but I think the sound is emanating from underneath the large transformer. Nothing seems to get terribly hot, but I'm not sure what I can and cannot touch.

[philliesfan30m]

Re: Magnavox LCD 47mf437/37 turning on after fix, but high pitched whine

i had 1 i thought i had fixed but had the same problem. for me it was d912 that was causing the whine after replacing it went away. that doesn't mean the same diode is the problem but my guess is that it is one of the diodes you didn't replace. i would desolder the ones you didn't replace and check them.

[withalligators]

Re: Magnavox LCD 47mf437/37 turning on after fix, but high pitched whine

Coolio. I'll look into it for sure tomorrow. Thanks!

[withalligators]

Re: Magnavox LCD 47mf437/37 turning on after fix, but high pitched whine

some of the diode's seemed a little leaky, but I went ahead and replaced them all and the other Zener diode. I also did the large 2200uf cap. No more squeal, or at least, it's above my perceptive range if there is one.

This set has the worst image retention I've ever seen. Shutting it off seems to help more than running any fullscreen flashing colors or anything, does that indicate there's a problem with one of the boards, or is this TV just kind of crappy?

Cheers,
Alex

[Caleb]

Re: Magnavox LCD 47mf437/37 turning on after fix, but high pitched whine

Not the best quality set, but I worked on a 37" in this series and didnt notice any retention issues, Im guessing the tcon may not be resetting the pixels fully?

[withalligators]

Re: Magnavox LCD 47mf437/37 turning on after fix, but high pitched whine

That sounds right, but rather than deal with it, I'm selling it as is with a warning to the buyer.