Megmeet MLT666T lcd smps

[jord4231]

Hi All,

I have an old 2006 ish TV that after the last storm we had would fail to come out of standby. We had a few outages on off on off etc

I am only repairing this as a challenge and to improve my skills.

Firstly I would like to mention I am no stranger to working with high voltages and understand the dangers of the high voltages present in SMPS, I have repaired a few before but this one is a little more challenging.

The board is a Megmeet MLT666T REV: 1.2a

The symptoms. TV has standby power, when turned on the power led will go green - no back light / picture or sound.

With the power supply out of the TV and powered up with the PS-ON with the +5VSB connected
I am seeing the 12v and 24v come up for around 500us then then it goes to nearly zero. (as seen in the oscilloscope screen shot) the top trace is the 5v sb power

I am also seeing the Resistors r80 r78 - r84 (around that area) are generating some heat (with a thermal camera) the area is around 40c ,
I have replaced Q7 thinking there may have been something wrong with the feedback circuit (also close to these resistors getting warm)
The opto couplers look ok (with the curve tracer (component tester and cro))

So far I have tested the TO220 style diodes and transistors on the primary and secondary.
I removed and tested the diodes on the secondary. All looks good there.

I have looked at the SMD caps near the drivers on the primary for shorts.

I see no bulging capacitors a bit of a probe around (only with a dmm set to capacitance at this stage shows things (at least in circuit) aren't too bad.

There is a lot of capacitors secondary. I have an LCR meter. Not yet been down that path.

I guess lots still to check however any pointers would be fantastic


Kind regards
Jordan

[R_J]

Re: Megmeet MLT666T lcd smps

It is usually the small electrolytics that go bad, Also check the voltage across the main filter cap, the voltage should be around 300vdc in standby and rise to 385vdc when on if the pfc circuit is working.
You can also check the voltage on Q6,(using hot ground) this transistor switches and also regulates the VCC1 voltage to the 24volt circuit drive ic (the collector is input voltage supplied by the standby circuit, the emitter is the output voltage. Z1 is on the base and sets the regulated voltage, P2 opto, turns the transistor on/off
I would replace EC3 and EC4 to start with.

[jord4231]

Re: Megmeet MLT666T lcd smps

Thank you for the reply R_J I will have a bit more of a look around and report back

[jord4231]

Re: Megmeet MLT666T lcd smps

Hi R_J I managed to get the supply working. Thank you once again for your help.
The issues turned out to be C16 and P4 opto coupler

Kind regards Jordan