LG 50PQ6000 unstable 5v standby

[Baileybum1234]

Re: LG 50PQ6000 unstable 5v standby

ok.

I cant test the PSU board in the TV at the moment since I have had to put it the TV back on the wall while people visit.

I have had a little time with the board stand alone.

When I plug it in the relays make a sound like a galloping horse. Even after I remove the AC the relays keep clicking for about 40 seconds as the capasitors discharge.

F302 is 328VDC
Across the large capasitors is a stable 320V

By my calculations these are Low they should be 382V

stand by 5v is a very unstable 3-4.5VDC

[tom66]

Re: LG 50PQ6000 unstable 5v standby

The standby is your problem; but I am not sure why.

[Baileybum1234]

Re: LG 50PQ6000 unstable 5v standby

In my mind it could be one of three things.

1. the supply where the standby 5v circuit gets fed is bad.

2. the circuit that makes the 5v standby voltage is bad.

3. Some thing is bad after the 5v standby circuit (load) causing problems.

I copied this from the LG50PQ30 manual that is slightly simular to my PSU.

AC Voltage is supplied to the SMPS Board at Connector SC101 from the AC Input assembly. Standby 5V is developed from 160V source supply (which during run measures 380V measured fromthe primary fuse F302). This supply is also used to generate all other voltages on the SMPS.

I think IC701 is detecting a fault and switching the relays on/off on/off maybe becuase its not connected to the TV.

If this voltage is low could it effect the stand by5V?

I have ordered some new 10 ohm 5W resistors (I lost the last 2) and have a new PP3 battery in the DVM.

[Baileybum1234]

Re: LG 50PQ6000 unstable 5v standby

Hummmmmm well put a 5W 100v load across the stand by 5v pins.

With the board unplugged from tv I get some strange readings between hot ground and F320. What are the chances that the big smoothing capacitors are dead?

One thing I did notice was and im not sure if its a coincidence:

If I short the auto/gnd pin to ground and add 230VAC to the board. I don’t even get a voltage across the big 450v 220uf caps. Even after removing the auto/gnd link the board takes a while to get clicking and the voltages to return again.

[tom66]

Re: LG 50PQ6000 unstable 5v standby

What do you mean, strange readings?
Where is F320?
My guess is AUTO is for some factory self test.

[Baileybum1234]

Re: LG 50PQ6000 unstable 5v standby

Well the voltage between hot ground and F302 is 325VDC. Im clipping one of the probes of the DVM to the heatsink since its attached to the capasitor legs.

The document said it should be STBY 123V and RUN 382V so its neither. OR maybe im doing something wrong. Also sometimes the board has no voltage here at all.

[Baileybum1234]

Re: LG 50PQ6000 unstable 5v standby

The reason I was shorting auto ground (PIN 22) P813 is that it was the first stage in the power supply static test on page 40 of the 50PQ30 training manual.

I have to admit I wouldnt expect it to knock all the voltages off.

[Baileybum1234]

Re: LG 50PQ6000 unstable 5v standby

So F302 is the primary supply I have just read.

[tom66]

Re: LG 50PQ6000 unstable 5v standby

We have 230VAC in the UK, so you should see 325VDC in standby and approx 380~400VDC when running.

Do not trust the exact values.

[Baileybum1234]

Re: LG 50PQ6000 unstable 5v standby

Well thats aukward if all the test voltages are out out because of the difference in voltage over here.

I am seeing exactly 325VDC so I can hazard a guess im ok to this point. So the big capasitors are working fine.

[tom66]

Re: LG 50PQ6000 unstable 5v standby

So, possible causes:
- Bad startup cap
- Bad diode as before
- Bad controller IC
- Bad feedack
- Bad output capacitors

[Baileybum1234]

Re: LG 50PQ6000 unstable 5v standby

ok TBC...........

[Baileybum1234]

Re: LG 50PQ6000 unstable 5v standby

Ok been looking at the W6252 today the PWM Switching Regulator. I guess I would need a scope to test this?

[tom66]

Re: LG 50PQ6000 unstable 5v standby

Probably; though you can check the Vcc yourself, and with power off for any short circuits.

[Baileybum1234]

Re: LG 50PQ6000 unstable 5v standby

Vcc should be 15.5 vdc ish from what I understand.

Actual voltage is 11.5Vdc.

PIN 4 VCC
PIN 5 Gnd

No shorts between pin 4 and 5.

Maybe a little clue.

[tom66]

Re: LG 50PQ6000 unstable 5v standby

The startup cap can be bad if Vcc is low.

[Baileybum1234]

Re: LG 50PQ6000 unstable 5v standby

Am I right in guessing the start up capasitor is connected to the VCC PIN 4 of the W6252? or is it connected to the start up terminal PIN 1?

[tom66]

Re: LG 50PQ6000 unstable 5v standby

I don't know, not without a schematic or the board itself in front of me, though more than likely it is exactly as described in the datasheet.

[Baileybum1234]

Re: LG 50PQ6000 unstable 5v standby

Well all the capacitors around the W6252 seam to check out ok. One was slightly low so swapped it out. I had a spare. Using capacitor test on DVM.

There is a resistor attached to PIN 3 (orange, white, silver(maybe grey), gold, black) Im guessing its a 0.39 ohm resistor im not great with 5 banded ones. Its measuring 0.6 ohms. Probes are 0 ohms shorted.

[tom66]

Re: LG 50PQ6000 unstable 5v standby

You can't check the small caps on capacitance or esr; leakage is what kills them.