47PFT5609/12 Philips (/LG) TV repair - 12v becomes 8.6v after power on.

[Odd]

This TV appears just black when power is applied.

The 12v PSU output is 12v and 24V- all good...   the TOSLINK led is on. PSU on signal is 3,3v.

A long press, and the TV switches on sometimes displaying "Philips" for less than 1/3second - then dies. 
The 12V output from the main power supply is now 8.6v   and the 24v output is zero.

I have found the service manual:   https://tel-spb.ru/files/philips_chassis_tpm14-1e_la.rar

I tried to start it up with all load 12v + 24v connector disconnected - and the 12v rail is then 8.66v.
When the ON signal is left floating, it's 0v.

How can I know what the PSU is supposed to provide when it has no load?  right now I am not sure whatever a floating PS_ON is the problem - or not..
is the 8.6v an fault, or just stnadby/poweroff voltage?
...and why does it die (apparently because the power_on signal goes low? - or does it go low due to power loss (back to 8.6v?)

 - how to troubleshoot further?

[Harvinder Singh]

The PSU should have a minimum of 12V output (12V nominal) if it's a good quality unit. If it doesn't then that's a sign that something is wrong with your PSU. You may need to replace it.

If it does have a load connected to it then the PSU should supply at least 6-8V under load.

You could try connecting a multimeter between the +ve and -ve terminals of the PSU and measure the voltage across them while the PSU is supplying power to the system. That way you'll know whether the PSU is providing enough power to the system.

If the PSU isn't providing enough power then you might want to consider replacing it.

[fzabkar]

The 12V supply has two possible states depending on the state of the DV5 control signal (page 70 of PDF).

When DV5 is high, Q9341 is switched on and R9345 is shunted in parallel with R9344. The potential divider consisting of R9343, R9344 and R9345 then sets the supply voltage to 12.3V.

When DV5 is low, Q9341 is switched off and the potential divider consisting of R9343 and R9344 then sets the supply voltage to 8.04V.

DV5 is 12V when PS_ON is high, and 0V when PS_ON is low.

Therefore it would appear that 8V is the normal state of the 12V rail in standby mode.