Viewsonic VX910

[neumannu47]

I have a VX910 that died. The power light flashes, but the monitor will not turn on. When I opened it up, there were several caps with bulging ends. The large cap, which does not normally fail, was not only bulging, one lead fell off when I touched it.

After replacing all of the electrolytic caps with low ESR 105 degree parts, the problem persists. I've started doing some probing on the power supply board, and it seems that the PWM chip is not starting up. The 10uF startup cap has been replaced twice, just for good measure. There is 171 volts across the big cap. The only activity on the board is a spike about once per second, which is probably the kick starter pulse. It's hard to tell where it's coming from, but it would have to come from U320. In any case, I'm confident the problem is on the power supply board.

This is the first LCD monitor I've tackled. If I'd seen one working correctly, I'd have a better idea where to start. Thanks to SMITHWHO, I have the correct schematic. My best guess at this point is the PWM chip, U320. At no time is there a signal on pin 8, which is the gate drive for the MOSFET. Pin 2 is constantly low, and the optoisolator is not shorted. Is there anything else to check before replacing U320, which I'm probably not going to do.

The monitor is not worth fixing, but it's a learning experience for me.

VX910 Schematic

[budm]

Re: Viewsonic VX910

Did you check the rectifier didoes on the secondary side and the power transistors (c5706/5707???) and the tuning caps in the inverter section? Also solder joints in the inverter section. This is very similar circuits used in Dell and Ben Q.
"The only activity on the board is a spike about once per second, which is probably the kick starter pulse." That sounds like the SMPS tries to start up but then shut down and just keep cycling which can be due to shorted circuit as I mention above.

[neumannu47]

Re: Viewsonic VX910

Did you check the rectifier didoes on the secondary side and the power transistors (c5706/5707???) and the tuning caps in the inverter section? Also solder joints in the inverter section.

Yes, I did. Why would the inverter section have an impact on the PWM chip's operation? The inverter section is not dragging the power supply down.

"The only activity on the board is a spike about once per second, which is probably the kick starter pulse." That sounds like the SMPS tries to start up but then shut down and just keep cycling which can be due to shorted circuit as I mention above.

If there is a short, I can't find it.

Pin 2 on U320 is connected to a phototransistor. The voltage on pin 2 is exactly zero - not even a tenth of a volt that you would expect if the phototransistor were all the way on. There is no voltage on the LED on PC330A. The transistor in the phototransistor is not shorted.

[budm]

Re: Viewsonic VX910

"Yes, I did. Why would the inverter section have an impact on the PWM chip's operation? The inverter section is not dragging the power supply down." if the inverter has shorted circuit it will cause the power supply to go into start-stop loop. The power supply is just a 12V single power supply, so if you draw too much current it will go into shutdown, once the output is shutdown, the circuit will try to start up again, it does not use the latch shutdown to keep the power supply off until you remove the AC from the unit. You can disconnect the 12V feeding the inverter sections to see what happen or use external 12V 4A to see if it will work with external supply.
What is the P/N of that SMPS IC U320?

[neumannu47]

Re: Viewsonic VX910

What is the P/N of that SMPS IC U320?

SG6841

[budm]

Re: Viewsonic VX910

If you look at the internal block diagram of the IC, FB PIN2 has internal pull-up resistor to 6V internal power supply, so if you are getting 0V (the photo transistor in the OPTO should be at cutoff (LED is not turned on) the the OPTO is bad or bad IC itself. You can turn off the Opto by disabling the LED so the photo transistor should then be off and see if you get Voltage on pin 2.

[Uniballer]

Re: Viewsonic VX910

I had two VX924's with bad power supplies, with the "flashing green" symptom. After recapping them, one still didn't work. The power supply would go through start-up, then shut down approximately once every second.

I found a power supply schematic out on the web, and after some probing I realized that the feedback circuit was not working. It took a while for me to decide it was the optoisolator. Freeze spray would make it work for a short while. Replacing the opto fixed it. Just saying that sometimes you have to look a little deeper. Not sure if your power supply board is the same, but it is probably similar.

[neumannu47]

Re: Viewsonic VX910

If you look at the internal block diagram of the IC, FB PIN2 has internal pull-up resistor to 6V internal power supply, so if you are getting 0V (the photo transistor in the OPTO should be at cutoff (LED is not turned on) the the OPTO is bad or bad IC itself. You can turn off the Opto by disabling the LED so the photo transistor should then be off and see if you get Voltage on pin 2.

I'm thinking the same as you are. What gives me pause is that while pin 2 does have a pull-up resistor to 6 volts, I have no idea what's inside the "Limited Power Controller". It could have the ability to clamp pin 2 to ground. However, even if it does, there will almost always be at least 0.1 volts on the pin. I measure no voltage on the pin at all.

When I get home tonight, I'm going to remove the phototransistor and test again, but I'm sure it's not shorted.

[neumannu47]

Re: Viewsonic VX910

Just saying that sometimes you have to look a little deeper.

That's the truth!

[neumannu47]

Re: Viewsonic VX910

I've decided to pack it in on this board. The problem is escaping me although it looks a lot like a bad PWM chip. If it's pad, I'm not sure I can see well enough to replace it since it's an SMD. I did order a through-hole version of the PWM chip so that I can play with it outside of the circuit. That will help me with future repairs.

[thunder77]

Re: Viewsonic VX910

Did you check CCFL lamps?

[neumannu47]

Re: Viewsonic VX910

Did you check CCFL lamps?

No, I'm positive the power supply card is the problem. The PWM chip is not oscillating. I'm 75% sure it's the problem. I'm probably not going to fix it.