Looking for PS schematic LG 6709900019A

[kevinm34232]

Used in a Vizio VP42HDTV

I had orange standby light only, no response from power button or remote. Tried another main board same problem.

Found a thread on a different model but same board, where it seems Im missing voltage on the AC-DET pin on PS board in standby. When 5V is applied to that pin, the standby light goes green when power button is pushed, but TV still does not come on. There is also a 5V Monitor pin that Im not sure about.

[tom66]

Re: Looking for PS schematic LG 6709900019A

You can't just force up the AC DETECT on these boards because the microprocessor also looks for the AC DETECT signal and you are only forcing up its output not its input. You have to bridge it at its source, that is, the two optocouplers which generate the signal. Generally these are the only two optocouplers that are the other way around (signal from primary to secondary rather than secondary to primary.)

[kevinm34232]

Re: Looking for PS schematic LG 6709900019A

I was trying to determine whether the main board was good, it looked like a usual Vizio main board failure at first.

What is the AC-DET circuit used for and is it part of the standby circuit?

[RON_CET]

Re: Looking for PS schematic LG 6709900019A

Kevin, this is schematic of the power supply from a LG model set using the same part number,hope helps

[tom66]

Re: Looking for PS schematic LG 6709900019A

Ron, any chance you could send me a copy of that to upload to my service manual's website?

[RON_CET]

Re: Looking for PS schematic LG 6709900019A

Tom, would be glad to,where would you like copy to be sent?

[tom66]

Re: Looking for PS schematic LG 6709900019A

Tom, would be glad to,where would you like copy to be sent?

Sent a PM.

[kevinm34232]

Re: Looking for PS schematic LG 6709900019A

Thanks Ron.

Can you let me know when it's up there Tom because the .jpg's are hard to make out..

[tom66]

Re: Looking for PS schematic LG 6709900019A

http://www.tgohome.com/ServiceManual...&Search=Submit

I wish I had this manual when working on that LG plasma a long time ago with AC DETECT issues.

[kevinm34232]

Re: Looking for PS schematic LG 6709900019A

I started looking at this one again, it looks like AC-DETECT comes from (pin20?) on U701 on page 3, but what exactly is AC-DETECT? Im not sure where to be looking for a problem. Is it a bad IC or something bad in hot side?

[kevinm34232]

Re: Looking for PS schematic LG 6709900019A

You can't just force up the AC DETECT on these boards because the microprocessor also looks for the AC DETECT signal and you are only forcing up its output not its input. You have to bridge it at its source, that is, the two optocouplers which generate the signal. Generally these are the only two optocouplers that are the other way around (signal from primary to secondary rather than secondary to primary.)

Do you mean a microprocessor on the PSU then? Because I was applying the 5V at the pin connecting to the main board.

Searching for these optocouplers you mentioned now..

[kevinm34232]

Re: Looking for PS schematic LG 6709900019A

Would these be the 2 then? If so, I have 5V on pin 4 of both.

[tom66]

Re: Looking for PS schematic LG 6709900019A

Those are the AC detect optos.

AC detect essentially tells the TV the AC power cord and AC power is still present. If it is missing, the TV will shut down in expectation for a complete power failure.

[kevinm34232]

Re: Looking for PS schematic LG 6709900019A

So how do I check these?

I see pins 4 on both opto isolators are tied together (A1) where I have 5V, pins 3 run out seperately which seem to go straight to pins 28 and 29 of U701 if Im following the schematic correctly.

[tom66]

Re: Looking for PS schematic LG 6709900019A

Checking the AC-DET pin is usually enough to find that some part of the AC detect circuit is at fault.

[kevinm34232]

Re: Looking for PS schematic LG 6709900019A

Im wondering if there is just something else wrong with the PS. Is AC-DET usually there in standby? I hear no clicking of the relays when plugging in.

[tom66]

Re: Looking for PS schematic LG 6709900019A

AC DET should be continuously present.

[kevinm34232]

Re: Looking for PS schematic LG 6709900019A

Can you help me trace this circuit or point to it to see what Im looking for?

[tom66]

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.

[kevinm34232]

Re: Looking for PS schematic LG 6709900019A

Do you have a link I have never seen that fix..

OK I found one of the threads, just bridge the 2 pins of each optocoupler on the secondary side right?

Sorry early on in the thread you said they needed to be bridged at the source but I didn't know that's what it meant!