Looking for PS schematic LG 6709900019A

[kevinm34232]

Re: Looking for PS schematic LG 6709900019A

That worked LOL. Why didn't you just say that in the first place! Just joking, thanks.

Im curious though, on the primary side of the optocoupler I had about 20V on those 2 pins, and then 5V on one of the secondary pins which is connected to the 5V standby. What is missing that triggers the other pin to turn on 5V if that makes sense?

[tom66]

Re: Looking for PS schematic LG 6709900019A

I'm not sure of the exact failure cause, but I have a feeling it has something to do with mains transients and the TL431-type ICs failing. I don't generally work on these LG IPM monsters any more so I have no way to validate my theory.

From another post of mine:

The AC detect circuit is there to monitor AC power input and see if it is stable and to make sure the set can operate properly. It needs to work from 100V (160V DC) to 250V AC (360V DC) so has to have a wide range.

So it probably detects any signal beyond around 80-100V DC as being good. As mains surges/transients* and time goes on the reference circuit drifts. Over time maybe it eventually wants 350V DC to work (after it has drifted and got worse and worse.)

Your AC line voltage varies over the day, with load on the neighbourhood, grid demand, local household load etc -- basically appearing random to the average person. Slight variations prevent the set from always working. but in a seemingly random way.

*Transients: around 5~10 per day in excess of 500V AC typically up to 1000V AC. Few if any surge suppressors actually suppress this.

[kevinm34232]

Re: Looking for PS schematic LG 6709900019A

Im wondering if the optocoupler itself could be bad since there was voltage at 3 of 4 pins except the one that obviously needs it. I don't know how exactly they work, but Im assuming that the primary side of it in this case should trigger the pins on the secondary side to 'close', just don't know what it's looking for..

[tom66]

Re: Looking for PS schematic LG 6709900019A

Current flowing through the LED A-K causes the C-E junction to conduct. Reducing current through the LED A-K increases the effective resistance across C-E.

[cooper trace]

Re: Looking for PS schematic LG 6709900019A

You have a schematic -- that is a trace of it. Just locate the components by number.

Why don't you just install the optocoupler bridges that everyone else does? The fix I originally discovered? It has worked for at least 8 people by my count.

Where is this optocoupler bridge post at ? I would like to try it on the one i ma working on now .