Emerson TV Buzzing-Won't turn on LF391EM4F

[Shane711]

I have this Emerson LF391EM4F that buzzes. High pitched squealing. It is coming from the transformer on the power supply but the problem is the main board. Switching out the main board fixes the TV.

I also put glue on the transformer to prevent if from vibrating. The square choke on the top is loose. Even when the TV is fixed it still hums slightly. Adding the glue quiets the noise but not 100%.

I actually have a few Emerson TVs doing the same thing.

Checking voltages on the main board and there is no voltages going through the board to the voltage regulators.

I was wondering if anyone had experience with this problem and knew what part on the main board is bad? It seems to me something is turning on and off at a high frequency.

Maybe I could test all the chips for shorts but I think it still good to post everything on this web site so that someone else can benefit from it all.

[budm]

Re: Emerson TV Buzzing-Won't turn on LF391EM4F

You are on the right track on looking for shorted circuit or something that has very low resistance and loading down the power supply. I would check the AUDIO amp chips, and also take the resistance reading on the power supply lines filter cap to see any one of them shows low resistance.

[Shane711]

Re: Emerson TV Buzzing-Won't turn on LF391EM4F

Well I changed just about ever cap. The originals were slightly low in capacitance and the ESR on just about everyone was around 2 ohms. Not sure if that is bad but I was replacing them with ones around .50 ohms ESR.

The audio chip seems suspicious. I am not sure if I was testing it correctly.
It is TPA3110D2. I have included the datasheet for it.

Pin 8 seems to be the ground. and I am getting a short from pin 8 to pins 18, 19, 23, 24. Seems like a lot of shorts to be a good chip. What do you think?

[budm]

Re: Emerson TV Buzzing-Won't turn on LF391EM4F

I am talking about the resistance testing between the two legs of the cap in the circuit to see if the loads which are connected in parallel with the filter caps have low resistance (load), not the ESR of the cap.
From the spec sheet page 20:
Check between PVCCL pin 27, 28 and GND (pin 29).
Check between PVCCR pin 15, 16 and GND (pin 29).
Check between Pins 25, 23, 20, 18 and GND (pin 29).
Pin 8 AGND (Analog GND) probably tied to pin 29 PowerPAD, pin 24, 19 PGND.

"Pin 8 seems to be the ground. and I am getting a short from pin 8 to pins 18, 19, 23, 24. Seems like a lot of shorts to be a good chip. What do you think?" It does look like you do have shorted circuit on output pin 18, 23.

[Shane711]

Re: Emerson TV Buzzing-Won't turn on LF391EM4F

Are you referring to the filter cap on the power supply? I am not sure where the one on the main board is but I did find a short. I also tested the good board and it is not shorted so I have found a problem now all I have to do is find out what is causing it.

The short is at pins 3 and 4 of IC3605 and also at cap 3632. I have removed several parts and the short is still present. I have marked the spots with red in the picture below.

I tested the audio chip like you said and I found no shorts when I did that.

I wonder if it is the main chip MSD8200DS-S1. I found these for $4.00 each and if that is the problem I could replace it. I might have to remove it from the board to test it. And if I remove it and the short is gone then that should tell me it is bad.

[CapLeaker]

Re: Emerson TV Buzzing-Won't turn on LF391EM4F

I cant read what's on IC3605 in order to pull up a spec sheet.

[Shane711]

Re: Emerson TV Buzzing-Won't turn on LF391EM4F

IC3605 is RT8293AL. I already removed this IC from the board and the short is still present. IC3605 does not have any shorts. The short seems to be scattered across the board. I can't pin point its source.

[CapLeaker]

Re: Emerson TV Buzzing-Won't turn on LF391EM4F

The RT8293AL is a step down converter. So the output rail of that IC is shorted to GND.
2 ways of finding the short:
#1: a very sensitive Ohm meter or a Leak Seeker etc.
#2: Put a external PSU on that rail, dump some alcohol on and around suspected components and push an amp or 2 through it. Something must get warm, the alcohol will disappear.
#3: Track the output trace, take something off the trace that is in series, like an inductor, bad cap etc. and follow the short, as you know something shorts the output to GND.

I can't trace it on the pic. Maybe you can.

[CapLeaker]

Re: Emerson TV Buzzing-Won't turn on LF391EM4F

Maybe there is a diode on the back of the board? Check some other IC's that would use that voltage rail.

[Shane711]

Re: Emerson TV Buzzing-Won't turn on LF391EM4F

I checked all the diodes and the tiny transistors too. I have a variable power supply so I will put power to the rail at 1 amp and try to heat something up. You said put the power to pin 3 and 4 of IC3605, yes?

[CapLeaker]

Re: Emerson TV Buzzing-Won't turn on LF391EM4F

Yes, pin 3 and 4. That cap, the inductor and that IC make a small PSU L_C circuit. If you look at the datasheet of the IC, there is an example circuit given. With that I am judging that something on the output rail of that IC is shorting to GND. Too bad there is no schematic available... Take that cap off, solder some wires to the pins and see what happens once you push some current through it. If it is a small little non heat sinked device it wont need much to warm up. However if it is some thing with a large GND tab, it may need more current and more time to warm up and reveal itself. Before I would do this, I would try to isolate a few IC's on that power rail if that is possible. That all depends on how far you are willing to go.

I actually did find a service manual! Look at page 52! That will show you how things work in there.... and it traces to IC3001 (just what I thought it would). Check D3606 if it is shorted, trace VDDC+1.26V.

[Shane711]

Re: Emerson TV Buzzing-Won't turn on LF391EM4F

I found the short, looks like it might have been my fault. It was a solder bridge.

The board still makes the PS squeal.
I am thinking that IC3605 or IC3607 (RT8293AL) could be bad even though there is no short. But also I suspect the audio chip because on the board I am repairing the heat sink for the audio chip was missing, that could be an indication it is bad.

I think I will remove one IC at a time and see if anything changes and swap them out. I had a PS board one time with no power. It turned out to be a bad transistor that switchs the power on. The transistor was not shorted but it just would not turn on. Replaced it with a new one and all was good.

[CapLeaker]

Re: Emerson TV Buzzing-Won't turn on LF391EM4F

why do you suspect all that already. Fire the TV up and test everything. If you have audio, the audio IC can't be bad etc.
As for the squeal... What squeals?

[Shane711]

Re: Emerson TV Buzzing-Won't turn on LF391EM4F

I did test the board and it is not fixed yet. I know because I have a board that works but this second board does not. Maybe you can figure out what is causing it. Otherwise I am going to have to keep switching stuff around until it does something different.

So with the bad board in the TV the big yellow transformer on the Power Supply Board vibrates loud enough that it makes a squealing noise, you know that noise transformers make when the coils are vibrating. With the good board it is perfectly quiet and the TV works perfect. With the bad board I get nothing, not even the power light on the front of the TV.

Something on the main board is causing the power supply to vibrate. I have to test the voltages again now that the short is gone.

I got the TV fixed but I don't want to throw this main board in the trash, I want to find out why because I have other Emerson's with the same problem and I know I will be getting more. Plus I did not see anything on Badcaps referring to this problem so I figured it would be a good topic to discus. I got another TV that has stumped me for about 2 years, I will be getting back to that one soon. I finally know how to post a new Thread, YippIe!

I also like to say because of BadCaps (budm, CapLeaker, tom66) my experience and knowledge has increase by 10 fold. I do not know how many TVs I have fixed but I do know I started somewhere about Nov 2014. Of course I have been working on electronics since I was 3 because my Dad was into it. And that was many moons ago.

[CapLeaker]

Re: Emerson TV Buzzing-Won't turn on LF391EM4F

Ok... well... it is fair to say that the PSU is fine. Since you actually have a working main board and the schematics to go with it you should be set to go. However, here is another idea you could do. Take a piece of tape and block off the parts on the ribbon cable for the 13V amp supply. Turn the TV on and see what happens.

[Shane711]

Re: Emerson TV Buzzing-Won't turn on LF391EM4F

I taped up the four contacts on the ribbon cable for the 13V Amp and it didn't make any difference, I guess this means it is not the audio IC. I think I may have it figured out soon but I got other projects that are higher priority.

I will remove IC3605 and then IC3607 to see if it makes a difference, if it does I will start swapping parts around. I could just order new parts but by the time they get delivered I would have lost interest.

[CapLeaker]

Re: Emerson TV Buzzing-Won't turn on LF391EM4F

Cover up the 3.3v rail and recheck. I think it is pin 8.

[Shane711]

Re: Emerson TV Buzzing-Won't turn on LF391EM4F

According to the diagram it is pin 16. I covered it and there was no change. Makes me think pin 16 is not the 3.3v

I am not at my shop so I will have to bring it to my shop tomorrow and check the voltages with my meter.

[Shane711]

Re: Emerson TV Buzzing-Won't turn on LF391EM4F

According to the service manual the main IC on the board (IC3001) controls the TV turning on and off. The 3.3V goes directly to IC3001. It would seem to me that is the bad chip. If I am reading the service manual correctly.

Although there could be another component causing the TV to cycle on/off at a high rate and that could be the cause of the vibrating transformer. Maybe one of those tiny transistors or a cap near the IC3001.

I will make sure I recheck all the components around the IC3001 again and then I will change the remaining caps. Remove IC3605 and test it . And then IC3607 and test it. If there is any changes I will remove one from another board and replace it. If all this does not work then I will order a NEW IC3001 and see what happens. If this doesn't fix it I will be so upset.

[CapLeaker]

Re: Emerson TV Buzzing-Won't turn on LF391EM4F

Do the same thing to all the voltage rails one by one, so we can try to isolate the problem. You also can try to identify and compare what resistance you get between a voltage rail and GND and compare that to the good main board. BTW: you were right with pin 16.