Emerson LC391EM3 - Constant High Pitch Sound

[ECU_Techie]

Hi all!

So I have an Emerson 39" tv with a model number of: LC391EM3. The problem is the screen will not come on and once plugged in, there is a constant high pitched whine coming from the power board. The area it is coming from is circled in the attached photo.

Once you unplug the flex going to the main board from the power board the whining stops. I am leaning towards a bad PSU board that is not putting enough power out for the main board, but then again I thought I would ask for some input on this as well.

- Jeff

[budm]

Re: Emerson LC391EM3 - Constant High Pitch Sound

You have shorted circuit that drawing too much current from the power supply.
You need to check ALL the diodes in the power supply, check the resistance between GND and the Voltage output pins of the power supply to see if any one of them shows very low resistance. Do not try to power up the unit otherwise you may cause more damages.
http://www.shopjimmy.com/magnavox-a2...pply-unit.htm#
Magnavox / Emerson A21T0MPW-001 Power Supply for 39MF412B/F7 / LC391EM3
Part Number: A21T0MPW-001
Board Number(s): BA21T0F0102 1, A21T0MPW, A21T1MPW

[vinceroger69]

Re: Emerson LC391EM3 - Constant High Pitch Sound

These are the parts included in the repair kit for this fault so maybe check all these parts too
Part Number: A21N0MPW-001 Repair Kit
Kit Includes
Description Quantity Board Location
Fuse 1 F601
Resistor 1 R611
Transistors 3 Q501, 601, 602
Diodes 8 D601, 602, 603, 604, 607, 608, 609, 653
Capacitor 1 C605

[tbsmith]

Re: Emerson LC391EM3 - Constant High Pitch Sound

I have the same TV with the same whine when the main board is plugged in to the power supply. Did you ever find your problem? I see people on ebay that sell repair kits, one specifically for the whine and won't turn on. It includes what looks like 3 surface mount caps and is cheap enough to try, but I'm not even sure it is the main board or the power supply.

[tbsmith]

Re: Emerson LC391EM3 - Constant High Pitch Sound

I should mention that I pulled and checked all the parts that vinceroger69 mentioned and they all test fine.

[neilc6]

Re: Emerson LC391EM3 - Constant High Pitch Sound

Trying to find my old thread. I recall I found a service manual and there was a shorted chip cap on one of the regulator rails on the main board. You should be able to measure a short on the supply voltage input of the main board to ground.

Add - found it. Philips and Emerson are the same. https://www.badcaps.net/forum/showthread.php?t=76666

[tbsmith]

Re: Emerson LC391EM3 - Constant High Pitch Sound

Thanks neilc6. Unfortunately, my ps and main boards are different from those in that thread, so troubleshooting is tough, especially without a schematic. I did check resistance from the various power supplies to ground with the main board connected and disconnected. I did see low resistance (24 ohms) at the P-ON-12V when the main board was connected. Disconnected it was 200 ohms or higher, changing as the capacitor charged.

[RJARRRPCGP]

Re: Emerson LC391EM3 - Constant High Pitch Sound

That Emerson TV, looks like shit! Sure not like the Emerson plasma TV that we have here!

Also, you're describing a squealing sound, like my PowMax LPxxxx series 300W PSU, (back when Leadman didn't look as bad, as that was in the early-2000s) when I was hoping that it would be better than the first Deer PSU that I had! But I think it wasn't really!
(except no dark brown spots all over the place, unlike the first Deer PSU that I had)

[ivtec]

Re: Emerson LC391EM3 - Constant High Pitch Sound

Also most likely be a bad inductor or transformer.

[tbsmith]

Re: Emerson LC391EM3 - Constant High Pitch Sound

I fixed it! I found the service manual/schematic in another post for a different model, but same boards. Without that, it would have ended up in the trash. The problem was on the main board, specifically C3103 (a tiny surface mount capacitor near IC3008) was shorted. I found it by checking the resistances of the various power supplies (or rather their loads) and the P-ON-12V was 10 ohms. Pulled 2 other caps before finding the bad one.

I'm reposting the service manual in case anyone else is looking for it.

A question for the experts: I don't have a replacement capacitor and it powers up fine without it. The question is, is it necessary for long term reliability? It is 4.7uF and is in parallel to a 47uF electrolytic on the Vin line supplying a voltage regulator. There is another 4.7uF cap in parallel, but is 2-3" away.

Thanks!
T

[neilc6]

Re: Emerson LC391EM3 - Constant High Pitch Sound

The caps are for filtering noise on the supply voltage rails for each regulator and are placed next to each IC. Will work without it but if you want to replace it, Digikey ships USPS to US with no min or Ebay may have sellers that ship small volumes.

[alfatv]

Re: Emerson LC391EM3 - Constant High Pitch Sound

A question for the experts: I don't have a replacement capacitor and it powers up fine without it. The question is, is it necessary for long term reliability?

T

This cap is at input of buck regulator.
Without it:
- regulator operation may be unstable
- electrolytic cap next to it will be stressed
- unwanted electromagnetic (radio) interference is created

If possible, use larger size (physically bigger, not capacitance) capacitor with voltage rating of 25V or more.
There are some not populated caps in parallel, you can install those too, more is better.

[tbsmith]

Re: Emerson LC391EM3 - Constant High Pitch Sound

Thanks for the info and explanations! I found a 6.7uF SMD cap that is a little bigger physically, but was able to solder it in. All back together and ready for duty. Hopefully the difference in capacitance isn't going to be a problem...if so, should I just remove it and go without?

Ya'll are great!