Emerson LC320EM2 No power after Kit Replacement

[paulstef]

Re: Emerson LC320EM2 No power after Kit Replacement

Here is how think it works:

1. Q600, Q601 are off
2. at power-up Q600s gate is charged via R603/4/0
3. above gate threshold of Q600 it turns on powering up coil 3/5
4. the same voltage appears on 8/7
5. the secondaries don't have (usable) voltage yet since they are all inverted (flyback)
6. once D605 conducts Q601 conducts also and discharges Q601s gate
7. coil 3/5 is de-energized
8. the transformer developes output voltages rectified by the various diodes

9. D605 cannot conduct if D609 conducts, i.e. if IC601 is on, i.e. if the voltage on the secondary for that branch is higher than 24V (I'm missing something here, not an obvious circuit, sorry)

You don't have voltage on the cathode of D609, why not?

[paulstef]

Re: Emerson LC320EM2 No power after Kit Replacement

Well if the TV is isolated from grid through the transformer, all is good.

Let's start from the beginning. I would check the following with a scope.

1. voltage on the main caps C603 etc
2. voltage on drain or pin 5 of the transformer (you did this already)
3. voltage on pin 8, you also did this, but while the waveform can be similar to drain of Q600 its voltage should be much lower.

4. the waveform you posted on pin 15 I think it was looks weird. All transformer waveforms directly measured on the coils should look similar. So 15 to 11 should look similar to 8 to 7 for example.

[budm]

Re: Emerson LC320EM2 No power after Kit Replacement

No, not to worry, safety is my first concern. Since you have isolation transformer and the TV is plugged into it, and the Neutral of the Isolation is not tied to safety Ground, then you are fine to put the scope ground lead to the primary side ground ref point, may be that is you are doing but I did not or missed understand your set up. Sorry about that.

[David53]

Re: Emerson LC320EM2 No power after Kit Replacement

I don't totally consider myself a beginner...Sorry I seem so dumb. I troubleshoot mostly with a meter. Been doing this for some 40 odd years. Yes I do have an isolation transformer. I do not crisscross grounds when troubleshooting. I was always under the impression that Hot chassis ground meant that that section of the power supply was live ac and that cold chassis ground was after it had gone throught the transformer and was using the rest of the chassis for ground. Two totally seperate supplies in one sense. I guess I am a beginner because I do not grasp how the mosfet ramps up to start the set. I do understand also if you do not want to help, I am sorry if I caused you any inconvienence. Paulstef the scope images look the same on the drain and the pin 8 of the transformer. budm, I do enjoy your posts because you are very knowledgeable. I have always had a little trouble using a scope. At 60 years old its not as easy as it used to be. Maybe someone can figure out what is wrong with these Emerson, Funai, Philips sets. Again I am very sorry.

[paulstef]

Re: Emerson LC320EM2 No power after Kit Replacement

David , a little illustration:

probe ground is connected to earth through the ground pin of the scope. Earth is connected to neutral somewhere in your house. So if you put the ground of your probe to primary side ground you will short a diode of the bridge rectifier. So please don't do it. Or use a isolation transformer for the TV.

You can work on the secondary side.

Maybe we concentrate on the diode 640 that was short? What parts of the circuit that comes after that did you swap?

[budm]

Re: Emerson LC320EM2 No power after Kit Replacement

This is getting dangerous for the beginner here. I have no more suggestion due to safety concern.

[paulstef]

Re: Emerson LC320EM2 No power after Kit Replacement

That's why you need an isolation transformer for the TV or isolated probes or a handheld scope.

Do you have an isolation transformer? If you don't then there is only one way which works but is not recommended (dangerous) to measure correctly on the primary side.

[budm]

Re: Emerson LC320EM2 No power after Kit Replacement

Grounds.
If you measure the AC voltage with the meter between the chassis and the Primary side ground ref, you will see 60VAC or more.

[budm]

Re: Emerson LC320EM2 No power after Kit Replacement

Yes, there are R/C/D snubber, the spike still can be high enough to damage input circuit of the scope, especially if the circuit is not functioning like it should and using the wrong ground. He keep using the word HOT Chassis Ground.

[paulstef]

Re: Emerson LC320EM2 No power after Kit Replacement

The amplitude and the big faint spikes reading do not look right, not good idea to look at the D signal, the spike can be close to 800V or more. That waveform you are seeing is what you get when not using the correct ref ground point for Primary.
Safety Ground is not circuit ground for Primary circuits.

There is a voltage clamp from 3 to 5 of the transformer. But I'm worried about the ground too.

[paulstef]

Re: Emerson LC320EM2 No power after Kit Replacement

The gate signal looks very different from the first photograph you took. Can you zoom in on the peak of the drain, just leaving 1 or two periods on the screen?

Also measure drain of Q600 and pin 8 of the transformer at the same time.

[David53]

Re: Emerson LC320EM2 No power after Kit Replacement

.2 volts 1st on left and 1 on the pic on right.

Picture on the right is coming out of the T600 Transformer at pin 15.

Using Hot Chassis Ground on picture on left....For Drain and Gate of Q600 and using Cold chassis ground for T600 picture. What am I doing wrong???

[budm]

Re: Emerson LC320EM2 No power after Kit Replacement

The amplitude and the big faint spikes reading do not look right, not good idea to look at the D signal, the spike can be close to 800V or more. That waveform you are seeing is what you get when not using the correct ref ground point for Primary.
Safety Ground is not circuit ground for Primary circuits.

[tom66]

Re: Emerson LC320EM2 No power after Kit Replacement

What is the volt scale on pic 1 and 2? For pic 1, .2V/DIV does not seem right. Also, you used an isolating transformer for this?

[David53]

Re: Emerson LC320EM2 No power after Kit Replacement

Probes are 10 to 1 attenuation...Attached are Scope photos of Top Drain and Bottom Gate of Q600. Thanks
Also included Pin 15 of T600 Cold Ground. Diamond pattern with only 1 probe.

[paulstef]

Re: Emerson LC320EM2 No power after Kit Replacement

I'll come later. In the meantime you could also measure gate and drain at the same time. The probes are not 100:1 probes by any chance?

Their attenuation should be printed on them.

[David53]

Re: Emerson LC320EM2 No power after Kit Replacement

Here are oscilloscope pictures from the Drain and Gate of Q600

[paulstef]

Re: Emerson LC320EM2 No power after Kit Replacement

What is the oscilloscope model?

[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. :-)

[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?