Emerson LF320EM4F

[JamisonM]

I have an Emerson LF320EM4F that seems totally dead. I made some measurements using the the metal tab on the corner of the board as ground. The 21v, 3.3v and 13v lines are all dead.

J145(Reset) Is 2.6v and J171(Protect3) is 0.073v. On CN1001, I get 2.2v. On the Primary side, I get 173.8v at the main filter cap. I've looked the board over for any physical damage and there are three places I've identified. On the secondary, D660 seems to have had it's face blown off.

On the primary side, Q602 also has a chunk of itself missing. Right next to it, C609 has darkened areas to each side of it. I didn't even notice this until I looked at the images I took. I worry that there may be a short in the transformer since this one seems to be an uncommon part, but my understanding is this is rare. After that, I wonder if the transformer isn't being fed properly considering the damage to Q602. I'm not sure either way.

According to a service manual for a very similar TV, the rectifying diodes on the primary side, Q601, Q602, and T601 could be at fault. I don't think the diaodes are the problem considering the measurment on the main filter cap. This makes me think the problem is with the transformer or the damaged Q6o2.

[culcuhain]

Re: Emerson LF320EM4F

Looks like D660 has the chunk missing. 5th picture. Bottom right. Arrow pointing to it.

[CapLeaker]

Re: Emerson LF320EM4F

My guess is, that there are more components bad yet. Also check the resistors in the primary. Search for "Emerson" here in badcaps. I think there will be some diodes and zener diodes bad in the primary and some surface mount diode on the secondary, plus some resistors on the primary too. Also check the gate drive circuit on the mosfets in the primary.

here is an LC320EM2, just to give you an idea... :
https://www.badcaps.net/forum/showth...hlight=emerson

[JamisonM]

Re: Emerson LF320EM4F

Sorry for the late reply. I went through the whole primary side and tested all the diodes and resistors I could. I set my meter to auto and tested all the resistors. For testing all the diodes, I set my meter to diode mode and tested the forward and reverse bias.

Resistors
R611 is 0.6. Supposed to be 0.36.
R601 is 1.052M. Supposed to be 1.2M.
R614, R603 & R604 all read low. They should be 470K, but measure ~310K.
R622 is supposed to be 0, but reads 0.4.
R623 is 0.4 and supposed to be 10K.
R608 is 1.479k and supposed to be 1.5K.
R612 measured 1.152K and should be 5.6K
R609 measured 98.3. Should be 100.
R621 is supposed to be 560, measured 542.
R606 and R607 is ~325. It's supposed to be 330.
R613 measures 177.2. Supposed to be 180.
Diodes
D608 is marked open in the datasheet. 0.001v both ways.
D607 is marked open as well. 0.004v both ways.
D603 forward is 0.53v. Reverse is 1.992v.
D602 forward is 0.533v. Reverse is 1.987v.
D604 forward is 0.524v. Reverse is 1.977v.
D601 forward is 0.51v. Reverse is 1.984v.
D610 forward is 0.605v. Reverse is 0v.
D621 forward is 0.601v. Reverse is 0v.
D609A forward is .0.219v. Reverse is 0.915v.

For all the other diodes, I'm not sure how to test them. They're all zener diodes. This includes D605, D607A, D623, and D608. I'm also not sure how to test the two transistors(Q602 & Q621) or the mosfet(Q601) either. I'm fairly sure that one capacitor is fried(C609).

[CapLeaker]

Re: Emerson LF320EM4F

With a DMM you can only test the zener diodes for short or open. You can't properly test the diode for the "knee" like that. However. you don't really need to either, as they usually fall short, or if they are totally blown away, open as well.
Bad resistor are going to 99% up in value of resistance or open circuit.

As for the mosfets, test them in diode mode (some check it with resistance, I guess what ever suits you). They fail usually short. Maybe youtube for how to check mosfets. Also when a mosfet is shorted to the gate, check the gate drive circuit. Other than that, you might risk a new mosfet.

Next: Only because you identified a potentially bad part in circuit, doesn't mean it is automatically bad, as it depends on the circuit the part is in. So in this case, you have to isolate the part, by either unsoldering it partially (lift a leg of the diode) or the whole thing.

[Andrew F. Ali]

Re: Emerson LF320EM4F

From my experience with these Emerson boards. the least headache is to purchase and replace the defective power board. Replacing defective components just end up being costly and disgusting...That is from my neck of the woods.

[culcuhain]

Re: Emerson LF320EM4F

Are you taking measurements with the components still on the board or are you removing them to test?

[ivtec]

Re: Emerson LF320EM4F

Sorry for the late reply. I went through the whole primary side and tested all the diodes and resistors I could. I set my meter to auto and tested all the resistors. For testing all the diodes, I set my meter to diode mode and tested the forward and reverse bias.

Resistors
R611 is 0.6. Supposed to be 0.36.
R601 is 1.052M. Supposed to be 1.2M.
R614, R603 & R604 all read low. They should be 470K, but measure ~310K.
R622 is supposed to be 0, but reads 0.4.
R623 is 0.4 and supposed to be 10K.
R608 is 1.479k and supposed to be 1.5K.
R612 measured 1.152K and should be 5.6K
R609 measured 98.3. Should be 100.
R621 is supposed to be 560, measured 542.
R606 and R607 is ~325. It's supposed to be 330.
R613 measures 177.2. Supposed to be 180.
Diodes
D608 is marked open in the datasheet. 0.001v both ways.
D607 is marked open as well. 0.004v both ways.
D603 forward is 0.53v. Reverse is 1.992v.
D602 forward is 0.533v. Reverse is 1.987v.
D604 forward is 0.524v. Reverse is 1.977v.
D601 forward is 0.51v. Reverse is 1.984v.
D610 forward is 0.605v. Reverse is 0v.
D621 forward is 0.601v. Reverse is 0v.
D609A forward is .0.219v. Reverse is 0.915v.

For all the other diodes, I'm not sure how to test them. They're all zener diodes. This includes D605, D607A, D623, and D608. I'm also not sure how to test the two transistors(Q602 & Q621) or the mosfet(Q601) either. I'm fairly sure that one capacitor is fried(C609).

if know how to troubleshoot electronics then use this service manual and measure circuit.

https://cdn.badcaps-static.com/pdfs/...31d80cb26b.pdf

[CapLeaker]

Re: Emerson LF320EM4F

I had more time to look at this what you measured more closely.

"D608 is marked open in the datasheet. 0.001v both ways."

That's because of R611. Look at the circuit. This is what I was talking about being careful. So you have to lift a leg of R611 and D608 ( again, look at the datasheet, there is a D608A and a D608 (36V zener)).

Right now you have no idea, if you measure an open resistor and a shorted zener diode, or a good resistor and a shorted diode, etc.