Viewsonic VX510, no backlight

[jarl]

Hello again

As soon as I mentioned to my wife I had fixed the 17" Dell monitor of my previous thread, she says "great! I'm needing one for my office"...

So... next in line is this Viewsonic VX510. When I got it last year the backlight wasn't working at all (I think I had posted something, but I can't find it). I opened it up and found some dubious-looking capacitors, so I just replaced them. As far as I remember there were no changes in behavior, so it went back to the stack of "to do".

Well... I put my hands on it yesterday... The monitor turns on but there isn't any backlight, except for a very quick, light flash when you turn it on or off. There are no obvious burned components on the PCBs, and capacitors look fine -of course-. I measured the volts at the connector and they are correct -except I don't know what I should be seeing on the "dimming" pin-. Is there anything obvious I should be looking next?

I can't find the picture I took when I replaced the capacitors originally, but here are the ones I took yesterday, as well as the schematic for the power PCB (I have highlighted the caps I replaced)

TIA...

[Rtech]

Re: Viewsonic VX510, no backlight

You can ignore the 'dimming' pin for now.Check Q107,108,109 and 110 for short circuits,also Q105 and Q113.The other suspects are the ccfl's thenselves,and these older Viewonics,do have inverter transformer problems,therefore measure the secondary resistance of PT101 and 102, looks like between pins 7 and 8 of each and you are looking for a resistance of between 400-1000 ohms, and the readings MUST be within a very small tolerance compared to each of the readings of about 3-5%.

[jarl]

Re: Viewsonic VX510, no backlight

Oh my... exactly one month later I'm back. Isn't it wonderful what replacing a car's engine makes for the forum's continuity?

Anyway...
I tested the transistors in-circuit, and none of them shows a short-circuit. There's something weird with one of them (Q113), though: If you test it after probing Q105, it will make my DVM beep for half a second. It won't happen again until you test Q105 again. Q001 shows a similar behavior.

I measured the resistence of the secondary on PT101 and PT102 (again in-circuit). There seems to be a typo on the schematic, though, as it shows pins 7&8 on one transformer and 7&10 on the other one (pins 7-8 are connected together on both transformers). The readings I got between 7&10 are ~291 ohms on both transformers.

I guess there isn't an easy way to test the CCFLs, right? Is there anything else you can think of?

Thanks for the patience!

[alexanna]

Re: Viewsonic VX510, no backlight

The easiest way to test backlights is to borrow some from a working monitor, and with substitution see if you can find a combination where the back lights will stay on.
There is no need to remove the backlights from the LCD panel, Just position things where you can plug the leads into the inverter