LG 55LM6200-TA not powering up

[notallbad]

The TV sometimes turns on fine, sometimes it doesnt.

Checked the PS voltages to find that PS_On sometimes reads 1.1v and drops off to around 0.9v which is enough for the tv to turn on, other times theres no voltage reading or around 0.7v which is not enough to switch on, would think this should be around 3.3v.

So disconnected the ps_on wire between main and psu and jumpered a 1k ohm resistor across the ps_on and one of the 3.5v stby pins - the ps board powers up fine and the tv turns on when power button pressed but of course its less than ideal leaving it like this as the power board will remain awake constantly. Whilst its wired up like this the ps_on from the main thats disconnected is now actually reading 3.5v, which is what I would expect it to be.

How can I narrow it down more? On the main theres a MMBT3906 transistor which provides the ps_on signal - is this the likely culprit?

Main : EAX644320B-1.0
PSU : EAX64310801 (1.3)

[notallbad]

From a 'Cold' start (i.e not been powered up for a good few hours), Power up psu standalone with just leds connected and 1kohm resistor between 3m5v stby and power_on, get the following

1 power_on 1.6v
3 24v 24.9v
5 gnd 0v
7 gnd 0v
9 3.5v 3.5v
11 3.5v
13 gnd 0v
15 gnd 0v
17 12v 12.1v
19 12v 12.1v
21 12v 12.1v
23 gnd/P_Dim2 3.5v

2 24v 24.9v
4 24v 24.9v
6 gnd 0v
8 gnd 0v
10 3.5v 3.5v
12 3.5v 3.5v
14 gnd 0v
16 gnd 0v
18 invertor on/off 0v
20 led n.c 0v
22 PWM Dim1 3.5v
24 error out 0v

The psu board fault led begins to flash after about 30secs, 5 flashes.

Also just to add I have already replaced the backlights with a complete brand new set!

With the main and psu connected back normally only get the 3.5v stby voltages, no psu board led flashes.

[notallbad]

Main to PSU disconnected and using 2 1kohm resistors for power_on and invertor on/off from 3.5v stby get the backlights come on and 6 flashes from the ps board led.

With the led connectors removed the ps board flashes 5 times from power up then continually flashes 3 times.

[notallbad]

With everything connected as normal apply power and get 3.5v stby as expected. Power on and get power_on 0.6v but no 12v or 24v, apply heat to the main board in the area below the main-psu connector and 12v and 24v appear and 5 blinks on ps board. Press power button and wham, invertor_on, p_dim and pwm_dim appear - backlights on and tv working.

Not sure if this is just causing a fault condition and resetting the micom which brings the tv back to life or if its a faulty component/solder joint/pcb trace fracture.

But then attempt to reproduce these results but now the power_on drops out.

[petehall347]

bad capacitors can start working when heated .same with dry cracked solder joints

[notallbad]

yh, the reason why I was using heat. But curious that when directing heat at the bipolar transistor, though of course heat can quuckly transfer, the rv powers up from 'cold start'. Believe as its heated a greater current flows.

Might just buy one and swap it over and see! Shall heat before powering on a few more times from 'cold' just to confirm it gives consistent results.

[petehall347]

cooling it when its working is what i would try . only ever had the odd transistor in a crt tv that would work when heated then sometimes last for a few weeks or a few hours after cooling down on its own

[notallbad]

So hopefully just to complete this repair,
continued to heat the transistor before every power on and start up and it worked every time so replaced the transistor and now getting 3.4v on power-on pin and tv turns on from cold start every time!