Emerson LC320EM2 No power after Kit Replacement

[David53]

Re: Emerson LC320EM2 No power after Kit Replacement

Do you think R608 might be the culprit??

[paulstef]

Re: Emerson LC320EM2 No power after Kit Replacement

R608 shows 0.18R in the schematic. If it originally was 0.18 it should be replaced with the same value.

If D640 was shorted then there was negative voltage on C637 and D645 was conducting forward for some time. There is a whole lot of other components after that. Did you check D645?

[paulstef]

Re: Emerson LC320EM2 No power after Kit Replacement

Do you think R608 might be the culprit??

If it's replaced with a higher value than it should be it seems that when Q600 turns on the voltage drop on R608 would turn off the gate earlier than it should via D607 and Q601. It seems! :-)

[David53]

Re: Emerson LC320EM2 No power after Kit Replacement

D645 Checks good...also all other diodes on the secondary have also been checked. They are all on the back side of the board.

[tom66]

Re: Emerson LC320EM2 No power after Kit Replacement

This is a self-oscillating PSU (ringing-choke converter)

* The initial startup pulse comes from the 220K Rs R603, R604, R600.
* Once the MOSFET begins conducting (Vgs>2V) current flows through T600 pins 3-5 building a field up in the transformer.
* The sudden increase in primary current causes a reflected current across pins 7~8 of T600 which drives the gate fully into saturation (about 20V) limited by zener D604.
* Once the primary current hits a certain limit (defined by approx 0.7V/R608), the transistor Q601 will start discharging the MOSFET gate.
* As the MOSFET gate discharges below the threshold the primary current begins to drop and this causes a large negative pulse across pins 7~8 of T600 which rapidly discharges the FET gate, improving efficiency.

It's quite a clever circuit, but it has no intelligence. It'll do it's best to drive a dead short, like a shorted output diode, quickly resulting in the destruction of the MOSFET due to overcurrent or overstress.

[paulstef]

Re: Emerson LC320EM2 No power after Kit Replacement

This is a self-oscillating PSU (ringing-choke converter)

* The initial startup pulse comes from the 220K Rs R603, R604, R600.
* Once the MOSFET begins conducting (Vgs>2V) current flows through T600 pins 3-5 building a field up in the transformer.
* The sudden increase in primary current causes a reflected current across pins 7~8 of T600 which drives the gate fully into saturation (about 20V) limited by zener D604.
* Once the primary current hits a certain limit (defined by approx 0.7V/R608), the transistor Q601 will start discharging the MOSFET gate.
* As the MOSFET gate discharges below the threshold the primary current begins to drop and this causes a large negative pulse across pins 7~8 of T600 which rapidly discharges the FET gate, improving efficiency.

It's quite a clever circuit, but it has no intelligence. It'll do it's best to drive a dead short, like a shorted output diode, quickly resulting in the destruction of the MOSFET due to overcurrent or overstress.

Thanks for that explanation.

[David53]

Re: Emerson LC320EM2 No power after Kit Replacement

Thanks for explaining how it works. I still haven't gotten it to work. What are your thoughts on removing secondary diodes 1 at a time to see if mosfet in primary runs at 165 volts? Just a thought.

[R_J]

Re: Emerson LC320EM2 No power after Kit Replacement

Do you have 165v across C602,3,4 Main filter? If you do than the voltage is droping accross the primary, Is the primary of the transformer open?
If you don't have 165V on the main filter I wold check for an open diode in the bridge circuit or open R602/611.
If the fet is drawing so much current as to drop the 165V B+ down then the fuse would be blowing.
I would open D640 and see if the primary comes up. You could disconnect the secondary diodes but I would leave the diodes D644, D646, D637 connected as they supply voltage for the feed back circuit (optoisolator)

[David53]

Re: Emerson LC320EM2 No power after Kit Replacement

Back to trying to repair this set. Below is the voltages I get on the Secondary of T600.
Off Transformer pin 14 D640 Cathode 7.3v
Off Transformer pin 10 D649 Anode -20.2v
Off Transformer pin 15 D646 Cathode 9.8v
Off Transformer pin 15 D634 Cathode 10v
Off Transformer pin 13 D642 Anode -3.4v
Off Transformer pin 10 D643 Cathode 3.7v
Off Transformer pin 9 D660 Cathode 14.1v

IC601 Pin 1 6.9v
Ic601 Pin 2 5.7v

Thanks for any help..

David

[Caleb]

Re: Emerson LC320EM2 No power after Kit Replacement

I have one of these in as well, mine seems to have the same basic faults. I don't have any compatible zener diodes so I will have to wait for them to get here to test further. But if you need some voltages to cross reference I would be happy to oblige.

[David53]

Re: Emerson LC320EM2 No power after Kit Replacement

I always find D640 on the bottom of the circuit board shorted. Then Q600 and Q601 next are an automatic replacement if D640 has shorted. Also several zeners need to be replaced. Most of the power supplies have been repairable, but I still have 2 that are Dogs!

I still have not found the problem with the 2 power supplies that are trying to start with the previous voltages listed. Thanks for any help. Maybe someone will find the cure.

David

[Caleb]

Re: Emerson LC320EM2 No power after Kit Replacement

I got my first order of zener diodes in last night and swapped out what I thought was the last failed component, using the light bulb in place of the fuse it glows brightly even though resistance checks of the primary show it to be ok. Using my ohm meter on amp to measure current draw (9amps) lead to a squealing power supply and two more zeners in the secondary letting out their magic smoke.
So far I replaced
q600 -shorted
r608 -open
d640 -shorted
d604 -shorted
and now

d645 is shorted (1ZB22 22v 1w)
d648 is shorted (1ZB36 36v 1w)

interestingly enough both tested ok before I powered it up, Im not really sure why they failed as the voltage on the rails they are on was 16v and 25v so neither should have been conducting. My only thought is there is some major ripple riding those rails. I will have to put it on the scope when my other order of zeners gets here. circled known bad components with red, blue around the usual suspects that test ok on mine

[David53]

Re: Emerson LC320EM2 No power after Kit Replacement

Be sure to check D610 next to the Mosfet out of circuit. I think it shows shorted in circuit, but I also find it bad many times when tested out of circuit. I like to replace Q601 also as I have seen it cause a lot of problems.

[David53]

Re: Emerson LC320EM2 No power after Kit Replacement

I hope someone finds a cure for the problem of not powering on after a kit replacement. I still have 2 of these sets unrepaired. Thanks.

[Jonesy123]

Re: Emerson LC320EM2 No power after Kit Replacement

I have one of these in also, replaced all component that everyone speaks of. I found a tiny Zener diode connected to the gate circuit for the driver transistor that was shorted 4.3v waiting to place a parts order to see if that finally makes er tick.

[johnboy1313]

Re: Emerson LC320EM2 No power after Kit Replacement

What diode number was it?

[Jonesy123]

Re: Emerson LC320EM2 No power after Kit Replacement

D605 if I'm not mistaking.

[David53]

Re: Emerson LC320EM2 No power after Kit Replacement

Let us know how this works out for you....Thanks

[Caleb]

Re: Emerson LC320EM2 No power after Kit Replacement

Well after several hours of frustration and about 12$ worth of parts and my set is now working. I ended up burning out those large cement resistors as well as one new mosfet and zener d604. Ended up replacing the opto and the driving transistor as well. I dropped the value on d604 to 24v as the only pin compatible transistor I had on hand was rated for 30v instead of 35v. Attaching scope shots of drain and gate of q600 (50v per division)

[johnboy1313]

Re: Emerson LC320EM2 No power after Kit Replacement

Nice. At this point, I've changed Q600, Q601, D604, D607, D610, D640, R602, R605, R606, R608, and R611 with no positive results. The lamp burns as bright as ever.