AOC 22LVWk only turns on with 3 lamps connected

[Sparky]

I have a 22" AOC 22LVWk monitor that would turn on, green LED, then right as the backlight would turn on it would shut down and repeat the cycle. Unplugging any one of the 4 CCFL lamps would allow it to stay on.

It had some Hermei caps in it (which by the way have the same vent stamp as Panasonic, shame on them!) and none of them were bulged or venting, but I recapped it anyway with Panasonics and Nichicons. It still has the same issue. On a whim I yanked the large cap (Elite) and stuck a Capxon cap I had in there (just for testing purposes). Still the same issue. Turns on, blinks off, repeat. Unplug any one CCFL and it stays on, so I can't blame it on a bad lamp.

The Nichicon caps I put in there are only VR series, could that be the issue? It was what I had on hand and I thought at least for testing purposes they would work short-term. Maybe not? Panasonics are FM.

Pics are attached.

[retiredcaps]

Re: AOC 22LVWk only turns on with 3 lamps connected

Hmm, that is odd. I don't think I have ever seen this before.

When all 4 ccfls are plugged in, are the voltages on the connector cable stable?

[Rtech]

Re: AOC 22LVWk only turns on with 3 lamps connected

In effect, its working a little in reverse..as normally if you unplugged a lamp, then the others would flash and then shut down,but in your case they stay on and work normally ???..my initial thought would be the control chip for the inverter and associated circuitry.

[Sparky]

Re: AOC 22LVWk only turns on with 3 lamps connected

Just did some more testing. I was a little mistaken on the 3/4 lamps thing.

3 lamps, it turns on, then about 2 seconds later the screen goes dark as the backlight turns off but the monitor is still on.

4 lamps, it turns on, then as soon as the backlight turns on it shuts down and restarts.

The 5V (well, measured at 4.9V) going to the logic board drops to 0 as soon as the backlight makes the monitor restart, as does the 27V to the inverters (the taller caps between that feed and direct ground are 35V rated). The voltage across the large cap remains the same.

*edit* apparently this board is also used in the Viewsonic VS11446 and HannsG HG216D so I'm doing some further searching to see if that gives any more info. I also found a used board on ebay for $35 but what's the fun in doing that? It still has the crappy Hermei caps so it would need recapped anyway.

[budm]

Re: AOC 22LVWk only turns on with 3 lamps connected

Now that makes more sense.
Just follow the guide by retiredcaps.

[Sparky]

Re: AOC 22LVWk only turns on with 3 lamps connected

I have been but haven't found anything yet. 2 seconds to off with a CCFL disconnected seems to make sense.

I'm starting to think that 27V reading is too high.

[budm]

Re: AOC 22LVWk only turns on with 3 lamps connected

"The 5V (well, measured at 4.9V) going to the logic board drops to 0 as soon as the backlight makes the monitor restart" That sounds like the inverter circuits has shorted circuits so the power supply goes into shutdown.
There are two MOSFET's on the heat sink by the inverter transformer. Do the resistance reading on all 3 legs:
1 & 2
1 & 3
2 & 3

[Sparky]

Re: AOC 22LVWk only turns on with 3 lamps connected

Those two transistors at the top each have legs 1 and 3 attached together via the traces. Seems a little odd. So 0 ohms between 1 and 3. I'm getting 159 ohms between 1 & 2 on the right transistor, and a very, very high reading between 1 & 2 on the left transistor.

To the left the two little heatsinks that look like they should each have a transistor do not. Not sure what purpose they serve as the are just sitting there.

[budm]

Re: AOC 22LVWk only turns on with 3 lamps connected

Nothing attached to those two heat sinks? Can I see the bottom side of the board? Closed up area of the transformer also (bottom side).

[Sparky]

Re: AOC 22LVWk only turns on with 3 lamps connected

more pics

[retiredcaps]

Re: AOC 22LVWk only turns on with 3 lamps connected

You have 4 mosfets on the back, Q905, Q906, Q911 and Q912. See if they are shorted.

[Sparky]

Re: AOC 22LVWk only turns on with 3 lamps connected

Nope, no shorts on them.

[retiredcaps]

Re: AOC 22LVWk only turns on with 3 lamps connected

If you disconnect the main board, do you get stable and solid 5V standby on the power board?

[retiredcaps]

Re: AOC 22LVWk only turns on with 3 lamps connected

Those two transistors at the top each have legs 1 and 3 attached together via the traces. Seems a little odd. So 0 ohms between 1 and 3.

What is the part number of the above two transistors?

[Sparky]

Re: AOC 22LVWk only turns on with 3 lamps connected

OK that was weird. I plugged it in with just 3 lamps connected and got a steady 5.1 volts, 0.2 volts higher than when the main board is hooked up. Connected all 4 lamps and as soon as I plugged it in it started its flashing backlight again. But as soon as I put the meter to the 5V line to test all 4 backlights came on and stayed on for 2 seconds, then went out (the line tested 5.1V again). While they were on there was a hisssssss sound from the board somewhere. Subsequent mains connections either have made it turn on and hiss for 2 seconds before the backlight shuts down, or it flashes a few times first then comes on for 2 seconds and hiss.

[Sparky]

Re: AOC 22LVWk only turns on with 3 lamps connected

The transistor on the left is a MOSPEC SRF10200C and the one on the right is 10H68C I think (hard to read the small numbers on it where it is sitting behind the inductor coil).

[retiredcaps]

Re: AOC 22LVWk only turns on with 3 lamps connected

The transistor on the left is a MOSPEC SRF10200C

http://pdf1.alldatasheet.com/datashe.../SRF10200.html

is a schottky rectifier so pins 1-3 being shorted is fine.

and the one on the right is 10H68C I think (hard to read the small numbers on it where it is sitting behind the inductor coil).

I can't find a datasheet for the above.

[retiredcaps]

Re: AOC 22LVWk only turns on with 3 lamps connected

I can't see any markings on the power board, but one of those pins should be PS_ON and around 3V DC. The PS_ON is generated by the main board.

You can try jumping 5V SB with a resistor to PS_ON and fire up the secondary voltages on the power board (obviously the main board is disconncted). If it is working properly, you should get steady stable voltages.

I'm just trying to to see if there is a load problem.

[budm]

Re: AOC 22LVWk only turns on with 3 lamps connected

Are you talking about the two devices attached to the heat sink? Those are Common Cathode rectifier Diode? By the way, what are the P/N of those 4 MOSFET's and how did you check them?

[Sparky]

Re: AOC 22LVWk only turns on with 3 lamps connected

OK now I'm totally crosseyed but I think I got the right part number on the one on the right. It is 10H60C which I believe is a 10A Schottky rectifier.

I'll have to come back to this later and see if I can find out which wire is PS_ON. None are labeled (annoying). I'll test both with and without the lamps connected (I'm assuming the inverter circuit should technically shut down under a zero lamp situation).

I checked the mosfets on the bottom for shorts between pins and found none. The two center ones are 4953P and the two outer ones are STM6920.

Thanks for all the help so far.