Emerson LC320EM2 No power after Kit Replacement

[Andrew F. Ali]

Re: Emerson LC320EM2 No power after Kit Replacement

I have been following this post and I am really glad someone got this power board fixed. I have changed ALL of the parts suggested and still cannot get one to work. I have Four TVsets-1 Emerson and 3 Sylvania.....Have failed to fix these boards.

[Caleb]

Re: Emerson LC320EM2 No power after Kit Replacement

Mine was doing the same thing johnboy, even though my multimeter showed no shorts. I ended up damaging some of the components I had already replaced trying to get the culprit to heat up. I found another transistor off an old projection power board (think it was an c2336) that had same pinout, even though my cricket said the other transistor was fine. d604 had failed leaky after initial replacement and was keeping the circuit from oscillating. Attaching picture of replaced components, hopefully it will help someone.

[Caleb]

Re: Emerson LC320EM2 No power after Kit Replacement

Picked up a couple sets over the weekend, the first I opened was a 2011 magnavox and low and behold this same crappy power supply. q600 shorted, check d640 shorted check, d604 shorted check, seems like deja vu.... Gotta order some more parts. Does anybody have a compatible q601 (2sc2120) transistor number that is still in production? dont really want to wait two weeks for a shipment from china. I wish digikey would fix their search engine so you could actually find something with the drop down filters.

[johnbinsc]

Re: Emerson LC320EM2 No power after Kit Replacement

Caleb,
I am working on my first one of these A17F8MPW power supply inverter boards. I think a good replacement for Q601 is BC33725BU-ND available from digikey. Look it up and let me know what you think. I have not tried one of these yet, I am just going by the specs.
John

[johnbinsc]

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.

Tom,
I really appreciate the analysis you provided, but have a question.

I am thinking that the primary mechanism to turn off Q600 in each oscillation is from the mirror winding (T600 pin 8) through zener D605 (4.3V) and R607 to the base of Q601. I expect that R608 is functioning more as an overcurrent protection, as it would need to be at least 1.2V so that the signal through D607 back to the base of Q601 would be high enough to turn it on and shut off Q600. That would correspond to a current of almost 7 amps, which I think is too high to begin shutdown of Q600 for "normal" operation in this circuit. Similarly, D607 has the dual role of passing this overcurrent shutoff signal, and also providing a clamp on the base of Q601 when the mirror winding pin 8 is driven negative.

Let me know what you think about this!

John

[johnbinsc]

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,
I realize that you already basically said the same thing in your step 6 above that I just repeated in my response to Tom in the previous message.

The jumper from the resistor chain R603, R604, R600 is labelled on the board as "Start". So my feeling is that this path is only needed when you first plug in the TV. Here is the process in detail as I see it:

1) Plug in TV, primary DC charges up across C602, Q600 gate starts to rise through "Start" resistor chain. Q601 is off, IC601 is at 0 V all around, as is the base of Q601.
2) Q600 switches on when gate = Vth, pulls current through winding 3-5, which induces voltage in secondary windings and primary mirror 7-8.
3) Secondary winding currents through D640 (Panel+15.8), D649 (negative half of AL+5V, which is the only full wave rectified secondary circuit), D637,644,646 (P-ON+24V), D634 (Amp+24V) are rectified, beginning buildup of secondary raw DC voltages on those circuits.
4) Primary mirror winding induced voltage (positive on pin 8) does three things. First it further raises Q600 gate voltage via "Drive" path (R605, R606, C605) putting Q600 into saturation. Second, it begins to charge C609 via D609 and R610, thus providing a bias supply for IC601 phototransistor. And third, when it gets above 5V, it turns Q601 on via D605 (4.3V zener) and R607, thus shutting off Q600 and ending the cycle.
5) On Q600 off, primary winding overvoltage on pin 5 is controlled by D613/C613 combination. Secondary windings reverse, and induced currents through D642 (LCD-6V), D643 (positive half of AL+5V), D660 (LCD+24.5V) are rectified.
6) Primary mirror winding induced voltage (negative on pin 8) is clamped to about -1.2V via path D605, R607, D607, R608/D610.
7) At this point, after one cycle, the rectified raw secondary voltages have begun developing towards the "standby" level, which is about half of their "ON" levels. Of these, two are important in standby mode, P-ON+24V and AL+5V. P-ON+24V supplies power to the feedback control for the primary switching regulation through Q634, Q635, Q631 and optocoupler IC601. P-ON+24V is the supply circuit that is directly regulated via the primary, because it is the largest load and the only circuit whose load varies once the power is ON, as it is the main power for the panel backlight inverter. AL+5V is the "always on" standby 5V supply, and it supplies the standby voltage to the mainboard, so that the mainboard can receive control signals from the IR remote and/or the button panel. At the same point in time, on the primary side, the bias of the phototransistor in IC601 on capacitor C609 has begun. I do not know how many cycles it takes to develop full standby voltage and/or bias voltage.
8) As the reverse induced current in the secondary of T600 collapses, it induces a positive voltage in the primary, but more importantly in the primary mirror winding of T600. The mirror winding drives the gate of Q600 back above Vth (via the "drive" path R605, R606 and C605) to begin the next cycle of the oscillation, and we repeat steps 2-6.
9) As the secondary supply voltages and IC601 bias voltage (C609) develop, Q631 drives IC601 to full on, and IC601 drives Q601 full on, such that Q601 discharges C605 and pulls down the gate of Q600 to below Vth as quickly as possible. This reduces the duty cycle on the primary switching regulator to the minimum required to maintain P-ON+24V and AL+5V in standby mode and while providing the full inhibit feedback signal. This is the steady state situation in STANDBY mode, where P-ON-H2 is low.
10) Upon POWER ON, P-ON-H2 goes high, Q631 is turned off, and the feedback signal from IC601 comes under control of Q634 and its zener Vref D654 and voltage divider R645, R647, R651 and R652. Q601 is not driven as hard, and the ON duty cycle of the primary switching regular Q600 increases, thus increasing the secondary circuit voltages towards their full POWER ON values. At steady state in POWER ON, the P-ON+24V is directly regulated via Q634, IC601, Q601 path. The other secondary supplies have additional levels of regulation. The P-ON-H2 signal also turns on the P-ON+3.3V regulator. Two additional POWER ON signals, P-ON-H1, and P-ON-H1-1, turn on the regulators for the other supplies circuits in phases.

It is not clear to me at this time if in steady state operation, the shutdown of Q600 in each cycle is caused by Q601 acting alone in discharging C605, or if the mirror winding pin 8 path via D605/R607 also plays a part.

Comments appreciated. This analysis is entirely from studying the schematic and doing some tests on a NON-Working board.

John

[johnbinsc]

Re: Emerson LC320EM2 No power after Kit Replacement

Do you think R608 might be the culprit??

Hi David,

Do you still have a 1.8 ohm resistor substituted for R608? If so, that may very well be the culprit. With a value that high, you may be generating a high enough voltage on R608 that you would shut off Q600 too quickly via the path R608, D607, Q601. Anything above 1.2 V peak on R608 should start to turn Q601 on, thus shutting Q600 off. This would correspond to only 0.67 A peak current through Q600 with your 1.8 ohm resistor. If true, this would explain why your secondary side standby voltages are not developing fully (should be about 12V on the nominal 24V supply line).

It could also explain why you are not getting any bias voltage on C609/IC601 pin 4. Q600 is not turning on hard enough before being shut down, so the induced voltage on the mirror winding (T600 pin 8) is not large enough to apply substantial voltage to C609 via D609.

To test this, I would measure the voltage on the PS-ON+24V secondary (at say the cathode of D646) in your current setup. Then change R608 to a smaller value, say 1 ohm, and measure again. If the standby "24V" level goes up, the you know that R608 was too big. If so, find something closer to the design value of 0.18 ohm.

John

[Caleb]

Re: Emerson LC320EM2 No power after Kit Replacement

I had come across that b337 in my search, only problem is the pinout is different so you would have to insulate the legs and wrap them into the correct positions.

[johnbinsc]

Re: Emerson LC320EM2 No power after Kit Replacement

Thank you for noticing that! I didn't catch it.
I will look again and see if I can find something in the right configuration.

[Caleb]

Re: Emerson LC320EM2 No power after Kit Replacement

It would seem that only toshiba makes transistors with that pinout, I dont know why they had to switch it up from the standard pinout.

[johnbinsc]

Re: Emerson LC320EM2 No power after Kit Replacement

Caleb,
This one looks like it will work.
Digikey p/n 2SC3940ASACT-ND

It appears to be a higher power/current version of the original transistor.
Although the curves are given over a wider range of Ic, they look to be very similar over the lower Ic ranges to the original transistor. And the pinout is right.

[johnbinsc]

Re: Emerson LC320EM2 No power after Kit Replacement

From a small sample, it looks like the transistors with p/n beginning with K have the ECB pinout. The ones beginning with B have the CBE pinout. But it is a very small sample.

[johnbinsc]

Re: Emerson LC320EM2 No power after Kit Replacement

Another one:
KSC2328AYTACT-ND
Again a higher maximum current version, but looks about the same over the current range of the original 2SC2120-Y.

[budm]

Re: Emerson LC320EM2 No power after Kit Replacement

Most of the Japanese 2SA, B (A, and B are PNP), C, D (C and D are NPN) Transistors have ECB pinout, always verify them first, I still have the Japanese transistor spec book that I keep for ref.
http://www.ceitron.com/tech/specbook.html

[Caleb]

Re: Emerson LC320EM2 No power after Kit Replacement

Thanks for the info budm and john- I already have some parts on order, but with the frequency that these things die I will probably have to order more in the future.

[Caleb]

Re: Emerson LC320EM2 No power after Kit Replacement

My parts arrived yesterday, glad I ordered extras as I picked up another Emerson set the day before. The guy I bought the Emerson from had removed the power connector from the board and soldered in 2 wires, still haven't figured out why, he also ran a wire across the fuse, probably why it was in worse shape than normal. On the Emerson I replaced
Q600- TK10A50D mosfet
Q601- 2sc2120y transistor
D604- 27v zener
D610- 36v 1w zener
D609- 4148
D640- rectifier diode
R602- 2.2 ohm 5w
R611- 2.2 ohm 5w
R608- .18 ohm 2w
Fuse
power connector

The Magnavox wasn't in as bad of shape, on it I replaced
Q600- TK10A50D mosfet
Q601- 2sc2120y transistor
D604- 27v zener
D609- 4148
D648- 36v 1w zener
D640- rectifier diode
R608- .18 ohm 2w
Fuse

On a side note, I ordered up a couple of the bc337 transistors and was able to substitute one for the 2sc2120y, I insulated the emitter leg and wrapped it across the back to give the proper pinout. The set works fine with the new transistor. I was pretty happy to be able to save a couple more from the dump.

[David53]

Re: Emerson LC320EM2 No power after Kit Replacement

After much frustration over the repair of these boards I finally took Paulstef advice and replaced R608 with the correct .18 2watt resistor and now all my sets that were dogs have been repaired. The value of this resistor is critical as the resistor I had used to replace it with was 1.8 ohms and did not work. Thanks again for everyones help and especially you Paul for making me see the light.


David

[Herbssat]

Re: Emerson LC320EM2 No power after Kit Replacement

Anyone looking for the original 2sc2120 transistor should try band enterprises in pennsylvania. They had plenty when i ordered mine. Happy thanksgiving to all my fellow techs.