Viewsonic VA902b dim lights, then black

[sceva]

This was showing 2 seconds to black. I opened it up and found 3 bulging caps. (the three with black marks on top in the inverter side.) I replaced them with good caps... no change. I then removed the rest of the caps and tested with ESR meter. Found two of the smaller caps near the main transformer reading high, so I replaced them also. Still get 2 sec to black.

I then did some tests with the power on:
Primary side of inverter transformers show 14.4V before the ccfl's turn on, and then drop to 7.35V when the lights turn on, then back up to 14.4V when they shut back off.
I plugged in some good ccfl's and when they come on, all of them only light up about 60% of the tube.
Setting my meter to 150V AC I tested the ccfl connectors. I put each probe on one of the pins by the ccfl connector. CN501 and CN502 show 75V when they light up. CN503 shows 60V, and CN504 shows 50V.

I checked the inverter transformers with a ring tester (while still on the board) and the secondary side shows 1 yellow, which should be ok?

It appears that it isn't getting enough power, but I don't know where to look next...

[PlainBill]

Re: Viewsonic VA902b dim lights, then black

This was showing 2 seconds to black. I opened it up and found 3 bulging caps. (the three with black marks on top in the inverter side.) I replaced them with good caps... no change. I then removed the rest of the caps and tested with ESR meter. Found two of the smaller caps near the main transformer reading high, so I replaced them also. Still get 2 sec to black.

I then did some tests with the power on:
Primary side of inverter transformers show 14.4V before the ccfl's turn on, and then drop to 7.35V when the lights turn on, then back up to 14.4V when they shut back off.
I plugged in some good ccfl's and when they come on, all of them only light up about 60% of the tube.
Setting my meter to 150V AC I tested the ccfl connectors. I put each probe on one of the pins by the ccfl connector. CN501 and CN502 show 75V when they light up. CN503 shows 60V, and CN504 shows 50V.

I checked the inverter transformers with a ring tester (while still on the board) and the secondary side shows 1 yellow, which should be ok?

It appears that it isn't getting enough power, but I don't know where to look next...

Have you checked the outputs of the power supply, as well as the voltage across the big cap? The normal SMPS design regulates the 5V supply, and lets the 12V supply 'float'. The result the 12V supply will tend to vary under load. 14.4V isn't unusually high for an unloaded 12V supply, but 7.35V is very low for the supply under load.

PlainBill

[sceva]

Re: Viewsonic VA902b dim lights, then black

Have you checked the outputs of the power supply, as well as the voltage across the big cap? The normal SMPS design regulates the 5V supply, and lets the 12V supply 'float'. The result the 12V supply will tend to vary under load. 14.4V isn't unusually high for an unloaded 12V supply, but 7.35V is very low for the supply under load. PlainBill

I had seen the 12V 'floating' before, but did not know it was by design.

The big cap shows163V and drops to 160V when the ccfl's turn on.
At the two schottky diodes the center pin of one shows 5.1 volts and the other shows 15.1V and drops to 14.76 when the ccfl's turn on.

The on/off pin from the main board is 0 when off, and 5V when on, and stays at 5V until the monitor goes to standby.
The brightness pin shows 3.3V all the time. Doesn't that indicate it is dimming?

[PlainBill]

Re: Viewsonic VA902b dim lights, then black

I had seen the 12V 'floating' before, but did not know it was by design.

The big cap shows163V and drops to 160V when the ccfl's turn on.
At the two schottky diodes the center pin of one shows 5.1 volts and the other shows 15.1V and drops to 14.76 when the ccfl's turn on.

The on/off pin from the main board is 0 when off, and 5V when on, and stays at 5V until the monitor goes to standby.
The brightness pin shows 3.3V all the time. Doesn't that indicate it is dimming?

I can never remember the voltages for bright and dim displays, but you have a point to check there.
Also check the voltage on J808 when inverter is on, then off.

PlainBill

[sceva]

Re: Viewsonic VA902b dim lights, then black

I can never remember the voltages for bright and dim displays, but you have a point to check there.
Also check the voltage on J808 when inverter is on, then off.PlainBill

Removed the dimming wire and got same bad results...

J808 is 14.8V when inverter on, 15.0V when inverter is off...

[alexanna]

Re: Viewsonic VA902b dim lights, then black

Every monitor is different but somewhat similar .I was just working on a 20” monitor and it was only lighting ½ of the ccfl tubes.I replaced every capacitor on the board with good low ESR caps and nothing changed. The capacitors in the inverter area were 35v caps and checking voltage when back lights were off was 23v as the back lights turned on the voltage dropped to 18v.checking the primary side of the inverter coil when the lamps were on I was reading 9vDc on both pins. I removed the inverter coil and re soldered it and also re soldered the ceramic caps in the inverter area, still no change.
Next I re soldered all of the jumpers in the inverter area ,and then the very last thing I did was re soldered 2 sot23 diodes that were in the inverter area .well I got lucky and have a bright screen now.
Finding a poor solder joint can be very hard sometimes, But reflowing the solder can be done very quickly and it may fix your problem.
Al.

[sceva]

Re: Viewsonic VA902b dim lights, then black

Finding a poor solder joint can be very hard sometimes, But reflowing the solder can be done very quickly and it may fix your problem.
Al.

I tried resoldering the obvious places, no luck, then I began doing all the joints, a section at a time, and still no luck... Thanks for the idea though. I sometimes forget the obvious, although I usually reflow the transformers and larger transistors on all the boards.

[lucky13]

Re: Viewsonic VA902b dim lights, then black

U501 area looked overheated...

Do you only have this one monitor to fix?

[sceva]

Re: Viewsonic VA902b dim lights, then black

U501 does have a dark area around it. The voltages at U501 and U502 appear to be the same, and nothing is below U501 to get hot... I guess either the U501 or the transformer could have gotten hot.

This is the only VA902b. A local recycler passes his broken lcd's to me and lets me try and fix them and then he resells them.

[lucky13]

Re: Viewsonic VA902b dim lights, then black

I checked my notes...

I had also previously replaced C812 1000uF/10V (it was domed).

If you have 1000/16v, you can lay it down.

[lucky13]

Re: Viewsonic VA902b dim lights, then black

This might help... PlainBill might chime in and help you here (please).

[shovenose]

Re: Viewsonic VA902b dim lights, then black

Sceva-what store/recycler?

[smason]

Re: Viewsonic VA902b dim lights, then black

U501 does have a dark area around it. The voltages at U501 and U502 appear to be the same, and nothing is below U501 to get hot... I guess either the U501 or the transformer could have gotten hot.

This is the only VA902b. A local recycler passes his broken lcd's to me and lets me try and fix them and then he resells them.

Wait a sec. YOU fix 'em then HE re-sells them? What's wrong with that picture...

[sceva]

Re: Viewsonic VA902b dim lights, then black

Wait a sec. YOU fix 'em then HE re-sells them? What's wrong with that picture...

He pays me for fixing... And I am a lousy salesman.

[PlainBill]

Re: Viewsonic VA902b dim lights, then black

This sounds like the inverter isn't putting out enough voltage. There is definitely signs of overheating in the region of U501. Assuming something has gone awry, causing low output from the CCFLs is logical.

I'd pull T502 and look for any signs of overheating at the end nearest to U501. If that looks good, try the ring tester with it out of circuit.

Next point to test is D513, D514. Then U501 and U502. It might be a good idea to just replace U501 and U502.

PlainBill

[smason]

Re: Viewsonic VA902b dim lights, then black

He pays me for fixing... And I am a lousy salesman.

Ahh that's ok then. "He lets me try and fix them" didn't sound like a paid gig.

[sceva]

Re: Viewsonic VA902b dim lights, then black

I'd pull T502 and look for any signs of overheating at the end nearest to U501. If that looks good, try the ring tester with it out of circuit.

Pulled T502 and T501 and they both ring with green on the primary and yellow on the secondary.

Next point to test is D513, D514.

These tested ok both on and off the board. I think it was about .2 one way with the diode setting.

Then U501 and U502. It might be a good idea to just replace U501 and U502.

I pulled both of these and using the diode test they seemed to be ok, one side like a P channel and the other like a N channel. I had a couple off another board so I went ahead and replaced them... the replacements tested the same, and I get the same bad results. I also replaced the 3 other caps in the inverter area even though they tested ok and no change in the results.

I tried to narrow down the problem by unplugging one or more of the ccfls and turning it on and recording the results of the ccfl outputs. It appears the problem is connected with ccfls # 1 and #2 as they have generally lower outputs (note that the connectors are numbered 1 & 3 at the bottom and 2 & 4 at the top):

All 4 cclf's plugged in - all 4 are about 50% lit
#1 removed - 2 @ 80%, 3 @ 5%, 4 @ 100%
#1 & 3 removed - 2 @ 80%, 4 @ 100%
#3 removed - 1 @ 0%, 2 @ 60%, 4 @ 80%
#2 removed - 1 @ 60%, 3 @ 80%, 4 @ 0%
#2 & 4 removed - 1 @ 80%, 3 @ 100%
#4 removed - 1 @ 70%, 3 @ 80%
#1 & 2 removed - 3 & 4 @ 50%
#3 & 4 removed - 1 & 2 @ 0%
#2, 3, & 4 removed - 1 @ 0%
#1, 2, & 4 removed - 3 @ 50%
#1, 3, & 4 removed - 2 @ 0%
#1, 2, & 3 removed - 4 @ 50%

Ahh that's ok then. "He lets me try and fix them" didn't sound like a paid gig.

About a year ago I went part time with my regular job and this helps fill in the gap. I have always enjoyed fixing things, but had never done much with soldering and replacing components, so I have really learned a lot - still have a lot to learn!

[PlainBill]

Re: Viewsonic VA902b dim lights, then black

OK, this is hard to admit, but I was getting confused between two monitors with different problems. Lucky13, thanks for the schematic; it makes things a LOT easier. I STILL don't have a good idea what is causing the problem, but here are a few things to test.

Jumper the following points one at a time with power off, plug in the monitor, and report the results. A very short length of small gauge wire soldered across the component is best.

1. R505
If you still get 'two seconds to black', there is no point in going further.

One or more of the following may block the 'two seconds to black' problem. This is NOT a fix, this is a test to identify the problem.

2. R502

3. R504

4. C523

5. C530

PlainBill

[alexanna]

Re: Viewsonic VA902b dim lights, then black

Every monitor is different but somewhat similar .I was just working on a 20” monitor and it was only lighting ½ of the ccfl tubes.I replaced every capacitor on the board with good low ESR caps and nothing changed. The capacitors in the inverter area were 35v caps and checking voltage when back lights were off was 23v as the back lights turned on the voltage dropped to 18v.checking the primary side of the inverter coil when the lamps were on I was reading 9vDc on both pins. I removed the inverter coil and re soldered it and also re soldered the ceramic caps in the inverter area, still no change.
Next I re soldered all of the jumpers in the inverter area ,and then the very last thing I did was re soldered 2 sot23 diodes that were in the inverter area .well I got lucky and have a bright screen now.
Finding a poor solder joint can be very hard sometimes, But reflowing the solder can be done very quickly and it may fix your problem.
Al.

I spoke to soon on my monitor the minute, I screwed the power supply back in its carrier, the half light bulb problem reoccurred. With everything hooked up except the 30 pin ribbon cable, I was probing around checking voltages and the back lights would sometimes come to full brightness, I have found placing pressure on certain points of the inverter board with my voltmeter leads will cause the backlights to behave normally. I am flexing the PCB in the area of the coils and the FETS.I most likely have a problem with the PCB. But it is something you may what to try to help locate a problem area.

Al.

[sceva]

Re: Viewsonic VA902b dim lights, then black

When I jumper R505 the lights come on really bright (slight buzzing sound on the board also) and then off after 2 seconds... Per PlainBill's instructions I did not jumper the other items.