[SOLVED] LG home theater amplifier power board fail

[eagle1109]

Hi,

I got this LG home theater amplifier that isn't working at all.

I opened the case, and did some inspection tests on some devices.

The fuse is ok, rectifier, main capacitor is charging and discharging ok, main power mosfet isn't shorted, no shorts on power output interface pins.

I'm suspecting the standby circuit since there's no power at all.

But how this circuit works ? and how does it affect the operation of the power circuit ?

 

[betwixt]

Highly unlikely it's that part. Far more chance it's one of the smaller black electrolytic capacitors, one of the two on the left side of the " ...no signs of shorts" box and the one near the top right corner of your "I'm suspecting this part box".

Smaller capacitors like that tend to go open circuit an simply stop the otherwise good supply from starting up. However, I think there are probably other parts underneath the board (on the solder side) that should be checked.

Brian.

 

[Relayer]

Hello eagle1109,
I have to agree with betwixt in regards to the chopper transformer being faulty.
You need to check the two small electrolytic's in the primary of the power supply for
high ESR or lowered capacitance.
Also measure any high value resistors in the primary, that perhaps are used for start up.
Have to tried to measure for any output voltages, even if they are incorrect?
Please let us know how you get on.
Regards,
Relayer

 

[eagle1109]

Yes, there are SMD components on the back.

I found this cap to be shorted.

On the back, the diode on parallel is shorted too.

If I wanted to repair this amplifier and replace the cap and the diode, I would use the lamp limiter, because both are in the hot side, in case the repair might fail, I shouldn't get any explosion or more damage to the board.

 

[Relayer]

Hello eagle1109,
You'll probably find that the capacitor in question is not shorted.
It seems to be in the same circuit as the shorted diode. Check the capacitor
out of circuit or lift out one leg, then measure it.
That diode might be the only culprit that's preventing the unit from working.
Regards,
Relayer

 

[eagle1109]

I just did another extensive test on the rest of the SMD diodes and I've found the following:
1. 3 shorted diodes --> marked with red circles
2. other measures on both directions with diode mode and 125k ohm with resistance mode --> marked with yellow circles
3. 2 diodes works fine, measures 125k ohm on forward bias and open circuit on reverse bias --> marked with blue circles