VX910 Backlight Inverter Issue

[ryanjvans]

This is my first post on this forum, so here it goes. I have a VX910 monitor that has an issue with the first backlight inverter circuit. When connected via DVI and turned on, the screen lights up with a good picture for ~2 seconds and then goes black. I have verified using a flashlight on the screen that the logic board is working. The power light remains a steady green, and the screen does not reappear if the monitor is left on.

After reading several threads regarding this issue with the VX910, I proceeded to replace all of the electrolytic capacitors (including the large 400V cap and small 10uf, 50v cap), blue high voltage caps, red 250V caps, and the Q6-Q9 transistors. After reassembling, the monitor is having the same issue. The good news is I didn't break anything during the repair, but I still don't seem to have located the issue.

I downloaded the Belinea 101910 service manual and ran a couple of tests as outlined below.

Test 1:
plug in 1 ccfl connector at a time and power up
Inverter circuit 1
CN1: very dim flash in upper right corner
CN2: very dim flash in upper right corner
Inverter circuit 2
CN3: full screen is lit, dimmer than normal
CN4: full screen is lit, dimmer than normal

Test 2: Power supply voltage
with power on:
12.45VDC at R18,20,22,23
1.554VDC at on/off pin. This remains after backlight turns off.
0.267VDC at adj pin, remains.

during brief ccfl lightup:
L1: voltage drops to 0VDC, returns to 12.45VDC when lights turn off
L2: voltage drops to 2.4VDC, returns to 12.45VDC when lights turn off
R18,22: drops to 0VDC
R20,23: drops to 1.7VDC
IC U2-Out1:6.71 VDC
IC U2-Out2:8.23 VDC

Zener diodes D9 and D10 both read approximately 170 in the correct direction when checked out of the board.

Since the identical problem exists after replacing most of the inverter components, I removed both transformers T1 and T2, and tested the resistance at each pin.

T1 & T2 - 0.2ohms between pins 2 & 4, 0.2 ohms between pins 3 & 5

On the secondary side I noticed a difference between the pins. T1 read 283 ohms between pins 2 & 4, while T2 read 426 ohms. While this doesn't appear that the transformer is shorted or open, would it be sufficient to shut down the inverter circuits if the power supply detects a difference between the two sides?

Could there be a different capacitor that I may not have checked, or do these signs point to a bad transformer?

Thanks in advance for any advice!

The power supply board is an FSP035-1PIO1, p/n 3BS006431C

Photos uploaded: These photos were taken prior to any components being replaced.

[retiredcaps]

Re: VX910 Backlight Inverter Issue

I proceeded to replace all of the electrolytic capacitors (including the large 400V cap and small 10uf, 50v cap), blue high voltage caps, red 250V caps, and the Q6-Q9 transistors.

First, excellent clear focused photos!

It is rare that the blue and red high voltage caps are bad. If it is a cap problem, 99% of the time it is the electrolytic caps that are bad.

On the secondary side I noticed a difference between the pins. T1 read 283 ohms between pins 2 & 4, while T2 read 426 ohms. While this doesn't appear that the transformer is shorted or open, would it be sufficient to shut down the inverter circuits if the power supply detects a difference between the two sides?

Yes that difference is enough to shutdown the backlights. The transformer readings should be within 3% of each other.

According to this previous thread,

https://www.badcaps.net/forum/showpo...9&postcount=53

it looks like 426 ohms is the correct reading.

PlainBill explains the 3% variation at

https://www.badcaps.net/forum/showpo...7&postcount=18

If there are any markings on the transformer, you may be able to find a low cost replacement on ebay or try

http://lcdparts.net/T.aspx

[ryanjvans]

Re: VX910 Backlight Inverter Issue

Thank you Retiredcaps! I failed to mention in my first post that at least two of the electrolytic caps were bulged.

I will look for a replacement transformer to see if that fixes the problem.

[ryanjvans]

Re: VX910 Backlight Inverter Issue

I have searched for a replacement transformer, but the closest I can find to the required SPI 8TJ00029 is an SPI 8T00013 at http://lcdparts.net/TransformerDetai...ProductID=3283, unfortunately the pin schematic is different. I'll keep an eye out for the correct model, but I may end up giving in for now and buying a cheap board off of eBay so I can at least use the monitor.

How close do the resistance values need to be if I replace both transformers with the same model? The 8T00013 spec showed 0.3 ohms on the primary side and 315 ohms on the secondary. This is close to the 0.2 ohm primary and 426 ohm secondary, although I would expect a lower output voltage. If I could find a transformer with the correct pin assignments, but with slightly different resistance values, would it be worth a try?

Thanks

[retiredcaps]

Re: VX910 Backlight Inverter Issue

How close do the resistance values need to be if I replace both transformers with the same model? The 8T00013 spec showed 0.3 ohms on the primary side and 315 ohms on the secondary. This is close to the 0.2 ohm primary and 426 ohm secondary, although I would expect a lower output voltage.

315 ohms will be too low. It has to be 426 +/- %3.

I don't think we have seen much success when the transformers are not the exact part number.

[ryanjvans]

Re: VX910 Backlight Inverter Issue

I received the new power supply this afternoon from Hong Kong. It is a used board which looked like it had gotten hot around the T1 inverter transformer and none of the caps appeared to have been been replaced, some were even rated at 85°C. I installed the board into the monitor for a comparison and the backlights came on and remained, now I know that the CCFL lamps are ok.

For the next test, I installed the good T2 transformer from my original board into the T1 socket where the shorted transformer had been. I left the T2 socket empty and powered up the monitor and original board. The back lights came on for 2 seconds, showing that the T1 side circuit appears to be functioning.

Finally, I removed the best looking transformer (T2) from the replacement board, checked the primary and secondary resistance (which were very close to my good T2 values) and installed it into the T2 socket on my original board and reassembled the monitor. Success!

With all new caps, transistors, and two good transformers, I hope to see many more years out of this monitor.

Thanks Retiredcaps for all of your help!

[retiredcaps]

Re: VX910 Backlight Inverter Issue

I received the new power supply this afternoon from Hong Kong.

Wow that is pretty quick shipping time.

For the next test, I installed the good T2 transformer from my original board into the T1 socket where the shorted transformer had been. I left the T2 socket empty and powered up the monitor and original board. The back lights came on for 2 seconds, showing that the T1 side circuit appears to be functioning.

I wonder if the glue turned black on C24 had anything to do with T1 going bad?

You might also consider removing the brown/yellow glue at the other red cap at T2, C320, PC33D, and HS100.

Good job on the repair.

[donatdonlo]

Re: VX910 Backlight Inverter Issue

I'm guessing I must have a similar issue. I replaced all of the Crapxons and the Polypropylene Cap near Transformer T1 ( it was buldged with a reddish brown material leaking out of it ), and the unit powers on and blinks and I can see a screen view in the background but it doesn't stay on. I have the MBR10100CT transistor that appears to be in the area where the board looks cooked. I'll compare the readings.