Emerson LC320EM2 No power after Kit Replacement

[David53]

After replacing most common shorted components from a shorted (Q600) mosfet and blown fuse I still have no startup. I measure changing voltages at the drain. I also measures 2.7 volts at the gate. Anyone else have any luck repairing these Emerson sets after the repair kit has been installed. D640 the smd has been replaced along with R608, D610, F601, D607, D604, IC601, Q601 and Q600. I have attached the closest schematic I have for the tvs. Bad thing is I have 2 of these sets in the shop needing repaired for the same problem. I have fixed several of the them in the past with the kit I found on ebay, but not this time. Any ideas are most appreciated. I have also read through the other threads on this model tv. Thanks.

David

[paulstef]

Re: Emerson LC320EM2 No power after Kit Replacement

What do you mean by changing voltage? At what frequency? Can you post oscillographs?

[David53]

Re: Emerson LC320EM2 No power after Kit Replacement

Measuring from hot chassis ground the voltage starts at 0 volts and moves rapidly up to over 20 to 30 volts and cycles over and over. No power light and does not start. We are at full line voltage. You can see the voltages moving from lower to higher and cycling over and over. Thanks for any help. This one has me stumped.

David

[paulstef]

Re: Emerson LC320EM2 No power after Kit Replacement

Look at the attached diagram. I presume you don't have an isolated differential probe to measure on both primary and secondary, right?

My suggestion: Measure drain of Q600 to hot ground. Then measure pin 15 of T600 to cold ground. Try to relate these two oscillographs and post them here. This will get us started.

Since your main MOSFET is kind of switching it seems it charges the gate through the 3 startup resistors of 220k but its not enough to create its own power supply then and start oscillation.

You may have too much load on one of the secondaries (or short) or some problem in the feedback loop.

Measure also the voltages indicated in the diagram I attached. We need to relate them to the main switching element Q600.

[David53]

Re: Emerson LC320EM2 No power after Kit Replacement

I don't have any oscilloscope images to post, but I have the following voltages
IC601 pin1= 6.94 volts
IC601 pin2= 5.72 volts
IC601 pin3= .08 volts
Ic601 pin 4= .11 volts

Q600 Drain charges to 14.6 and cycles
Q600 Gate is 2.7 volts
Q600 Source 0 volts

D645 Cathode in Secondary supply 8.26 volts

Q601 Emitter= 0 volts
Q601 Base =.08 volts
Q601 Collector = 2.7 volts

[paulstef]

Re: Emerson LC320EM2 No power after Kit Replacement

It' going to be hard without scope.

What is the voltage at the cathode of D646 (to cold ground)? Does it vary?

[David53]

Re: Emerson LC320EM2 No power after Kit Replacement

D646 Cathode to Cold ground 9.5 volts

[paulstef]

Re: Emerson LC320EM2 No power after Kit Replacement

Does it vary or is it stable??

[David53]

Re: Emerson LC320EM2 No power after Kit Replacement

Very Stable holding at exactly 9.4 volts at present

[paulstef]

Re: Emerson LC320EM2 No power after Kit Replacement

TBH I don't know how to do that without a scope.

1. Measure anode of D642 and cathodes of D643 and D660 to cold ground.

2. You say on cathode of D609 to hot ground you have almost no voltage?

[David53]

Re: Emerson LC320EM2 No power after Kit Replacement

D642 Anode 3.6 volts
D643 Cathode 2.9 volts
D660 Cathode 13.9 volts

Cathode of D609 to hot ground is .3 volts

I have a scope, but it has been rarely used in over 20 years and not sure if I could even get it operational enough to work.

[David53]

Re: Emerson LC320EM2 No power after Kit Replacement

Oops I made a mistake D642 Anode is -3.6 volts

[paulstef]

Re: Emerson LC320EM2 No power after Kit Replacement

Q600 gate is stable at 2.7V? Then why does the drain vary from 0 to 15V?

From what I see if the output voltage on the secondary is higher than what it should be (around 24V) the optocoupler IC601 will turn on. Since your voltage is much lower the opto output should be open. If it's open then C609 would be charged through D609 everytime Q600 turn on.

It looks like it's something on the primary side.

[David53]

Re: Emerson LC320EM2 No power after Kit Replacement

Gate of Q600 is very stable at exactly 2.75 volts. Any idea what would keep Q600 coming up to the 165 volts? I have tried to measure every component in the primary...but not sure what I should be looking for now? I too do not believe that the secondary is the problem. I have checked all the diodes in the secondary. As usual with these set D640 was initially shorted causing multiple components to be open or shorted. If I can find the problem with these 2 sets I know a lot of others having the same problems. Thanks for staying with me and trying to help..

David

[paulstef]

Re: Emerson LC320EM2 No power after Kit Replacement

D640 was initially shorted? That's very bad for everything that comes after that. What's the voltage at the cathode of D640 now?

[paulstef]

Re: Emerson LC320EM2 No power after Kit Replacement

I think there is no way around. You need the scope. If there is voltage on the secondary Q600 must be switching. So its gate cannot be stable. If this is measured with a DC multimeter you may just be reading the average of the pulses at the gate.

[David53]

Re: Emerson LC320EM2 No power after Kit Replacement

D640 Cathode is 6.9 volts and steady. I connected the old scope to the drain of Q600 and Hot chassis ground. The best I can tell it looks like a sawtooth waveform at possibly 450 mv. Here I will show my ignorance of using a scope. On the 50mv scale it is 9 full divisions in amplitude. The top is slightly rounded, then gives the appearance of an upside down sawblade and goes straight up to the slightly round top of the uppermost amplitude. Hope you haven't fell out laughing at my ignorance. All those years ago now I wished I had learned how to use a scope.

[paulstef]

Re: Emerson LC320EM2 No power after Kit Replacement

First of all, you do remember that if this is not an isolated scope then probe ground is connected to AC ground and from there could be connected to AC neutral?

When you are sure your scope cannot damage anything then we can continue taking measurements.

Post the scope pics here. For example the drain of Q600.

[budm]

Re: Emerson LC320EM2 No power after Kit Replacement

If the scope is NOT floated (no safety ground on the scope, dangerous), when he puts the scope probe on the primary circuit ground (not the safety ground), he will see nice spark and may blow up the primary rectifier diode. He should put the DUT on the isolation transformer that has isolated Hot and Neutral but still have safety ground.
What is the ground ref. are you using?

[David53]

Re: Emerson LC320EM2 No power after Kit Replacement

Working on getting pictures...I put the scope probe on the Drain of Q600 and the ground lead on hot chassis ground. Also checked the gated No loud pops so far. Of course both of you have scared about a years life out of me. :-)