LG 50PQ6000 unstable 5v standby

[tom66]

Re: LG 50PQ6000 unstable 5v standby

They would work. Put one across the 4.45V output. Does it stay at 4.45V?

[Baileybum1234]

Re: LG 50PQ6000 unstable 5v standby

They would work. Put one across the 4.45V output. Does it stay at 4.45V?

Drops to 3.9v with the 10 ohm load on the standby 5v line.

[tom66]

Re: LG 50PQ6000 unstable 5v standby

How do two in series (20 ohm) make it drop by?

[Baileybum1234]

Re: LG 50PQ6000 unstable 5v standby

Back up to 4.45V

[tom66]

Re: LG 50PQ6000 unstable 5v standby

Hmm, so maybe that is normal, it might not be designed to support 445mA load. It's probably not bad caps, if it does that. So I really don't know any more. I am out of ideas. I am of course assuming your DMM is correct. Have you checked it against a known good voltage source?

[Baileybum1234]

Re: LG 50PQ6000 unstable 5v standby

DVM is fine. I have 2.

I have tried using an old computer power supply and injecting 5v to see if anything got hot and that voltage dropped too and made me wounder.

Looking in the training manual everything says 5v not 4.45v hummmmm

I hear the relays click and parts of the TV fire for a second then off. Something is up in there somewhere.

[tom66]

Re: LG 50PQ6000 unstable 5v standby

Wait, so when you fed 5V from computer PSU into output of plasma PSU, it dropped??

[Baileybum1234]

Re: LG 50PQ6000 unstable 5v standby

yes. Made me think some thing was deadish.

[Baileybum1234]

Re: LG 50PQ6000 unstable 5v standby

something is creating an ubnormal load?

[tom66]

Re: LG 50PQ6000 unstable 5v standby

But I do not understand how a computer PSU which is completely different would drop down to the same voltage.

[Baileybum1234]

Re: LG 50PQ6000 unstable 5v standby

Im not even sure if it was a propper test in all honesty.

I was still working on the theory that my 5v standby was low. If I have a good 5v standby voltage to start with I would be prepared to move on with different tests and look elsewhere.

I thought I would use one of the 5VDC lines on the computer PSU and just attach it to the standby 5v line to see if it brought the voltage up or maybe caused some heat somewhere for a clue. If I had a test bench power supply I would of tried that. The computer PSU was dragged down I need to retest for the exact results. Maybe there is some mileage in it.

[tom66]

Re: LG 50PQ6000 unstable 5v standby

I would consider that idea, but the fact that you've got 4.45V under no load tells me something else is wrong with the PSU.

[Baileybum1234]

Re: LG 50PQ6000 unstable 5v standby

Yeah agreed.

So I need to trace the 4.45V back and find out where its developed from.

[Baileybum1234]

Re: LG 50PQ6000 unstable 5v standby

Ok IC301 could be something to do with it. Its a str w6252 9531p ssome kind of protection device if it was playing up it would effect the standby 5v.

Also need to work out what tappings on the coil I need to look at and the 3 components strapped to the heat sink what they should measure.

Its hard to see how exactly the 5v standby is made by following the tracks. Does any one have a simular circuit diagram or have they done it before?

Circled in blue.

[Baileybum1234]

Re: LG 50PQ6000 unstable 5v standby

Not given up on this im just building up the insperation for another session.

[Baileybum1234]

Re: LG 50PQ6000 unstable 5v standby

Does any one know of a link that talks through the path and demonstrates how the standby 5v is made?

I have tried to follow the tracks back but they just dissapear.

I feel if I could wok out how the sb5v is produced I could start looking at the components inline and testing them.

Hopefully then working out why I have a low 5vSB might hold a clue. Also maybe I could follow the path and lift a leg on a component to see if the voltage goes upto 5v again indecating a short?

[tom66]

Re: LG 50PQ6000 unstable 5v standby

It just comes from the transformer, goes through a diode, into a capacitor.
There's a feedback loop to regulate it too.

[Baileybum1234]

Re: LG 50PQ6000 unstable 5v standby

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. The STBY5V (standby) is B+ for the Controller (IC701) on the SMPS and output at P813 pins 11 and 23 then sent to the Main board for Microprocessor (IC1) operation.

So it looks like a good place to start. I should measure 160V across F302 in standby and 380v in run.

Please could I just confirm im measuring across the fuse and not to ground and is it DC or AC at this point. Im in two minds about the second point so would like a second opintion.



Thanks

Jarrett

[tom66]

Re: LG 50PQ6000 unstable 5v standby

Your ground ref on the primary is the negative lead of the large caps. Measuring a substantial voltage across a fuse is never good. It indicates the fuse is blown.

[Baileybum1234]

Re: LG 50PQ6000 unstable 5v standby

Your ground ref on the primary is the negative lead of the large caps. Measuring a substantial voltage across a fuse is never good. It indicates the fuse is blown.

Got you! Cheers.