LG50 PC1 -oscillating RL-0n & AC-det voltage

[Cornboats]

I'd given up with trying to sort this board out once but as I can't get a replacement I'm going for another attempt at finding the fault.

The set came in with relays clicking. I found 11 bulged capactors on primary side which have been replaced.
As the fault remained I removed board for bench testing and took flyleads from standby, rl-on, ac-det & earth so I could monitor the voltages. I also put a load of 2 x 60Watts incandescents across VS + earth.
On start up with 2 x 100watt light bulbs in series with mains incoming, psu clicks on and off. Cycle repeats. During this time Standby voltage is a constant 5v. When relays click on AC-det voltage and RL-on voltage goes up to about 3v When relays go off both RL-on & AC det go down to 0v.
I thought the RL-on signal came from the main board so why is this present? The relays shouldn't be trying to start anyway- should they??
If AC-det is supposed to be 5V maybe this is the problem. If its developed from the same place as the 5V standby voltage then something is pulling it down.
I would appreciate any help on this one.
Photos uploaded taken from camera in VGA mode 16:9 1920x1080

[tom66]

Re: LG50 PC1 -oscillating RL-0n & AC-det voltage

Have you tried the optocoupler bridge "hack"? A failure in the AC detect circuit can cause issues, which mainfests itself as an oscillating on-off relay clicking sound. Search Badcaps for my posts on this matter.

[Cornboats]

Re: LG50 PC1 -oscillating RL-0n & AC-det voltage

Tom thanks for your response. I did search on optocoupler bridge hack which only came back to my post.
You posted a thread "LG 42PC1D plasma - clicking repeatedly" which refers to bridging ac-detetection optocouplers but no specific information. I've got a faulty board so I could just swop the couplers. Do you remember which ones they were? If not how would I trace the ones on my board? I'll try it if I can find them but theres about 9 on the board and some are under transformers.

[tom66]

Re: LG50 PC1 -oscillating RL-0n & AC-det voltage

The optos aren't what go bad AFAIK - the circuit driving them does. Bridging them hides the problem with the AC detect circuit, although minor issues do exist as a result of this hack.

[Cornboats]

Re: LG50 PC1 -oscillating RL-0n & AC-det voltage

Ok so can you elaborate a bit. What is AFAIK? How do I bridge the circuit?

[tom66]

Re: LG50 PC1 -oscillating RL-0n & AC-det voltage

AFAIK = As far as I know
The method is desribed in that thread - bridge the SECONDARY SIDE of the two back-to-front optocouplers.

[Cornboats]

Re: LG50 PC1 -oscillating RL-0n & AC-det voltage

I can't find the optocouplers but found a reference in another reply you gave to bridging out ac-det circuit.
https://www.badcaps.net/forum/attach...9&d=1371933868
This looks like the bottom of a transformer to me. Do you think this is standby transformer T501?

[tom66]

Re: LG50 PC1 -oscillating RL-0n & AC-det voltage

They will vary in position but there are two optos next to each other which are back to front, i.e. most send a signal from SECONDARY to PRIMARY (feedback) but these do the opposite from PRIMARY to SECONDARY (ac detect, monitoring etc.)

[Cornboats]

Re: LG50 PC1 -oscillating RL-0n & AC-det voltage

Tom thanks for taking the trouble to reply. I'll do some investigating. No one else has responded.

[Cornboats]

Re: LG50 PC1 -oscillating RL-0n & AC-det voltage

Tom. I believe I found them. PC501 & PC502. Just out of interest I took the voltages before I bridged;
PC501 pin1=upto 0.32V- primary side
pin 2=upto 0.32V-primary side
pin 3=upto 4.48V-secondary side
pin 4= 5.03V steady-secondary side

PC502 pin1=upto 0.32V-primary side
pin 2=upto 0.6V-primary side
pin 3=upto 0.77V-secondary side
pin 4= 5.03V steady-secondary side

I soldered a bridge across pins 3 & 4 on both opto's. Switched on and exactly the same is happened. Fast clicking relays. Standy voltage is the only 1 thats steady at 5v. AC-det peaks at about 3v then dies in step the the relays. RL-on voltage is a copy of ac-det. Something I've been scratching my head over is the RL-on voltage. Doesn't this come from the main board?. I've got the board on the bench with only mains connected for testing( through a couple of incandescent lamps in series) so how come I've got voltage on the RL-on pin of connector P800?

Can you think of anything else I could try?