Viewsonic VA912 logic board issues

[sylvester328]

I have two broken VA912 with same symptom of black screen.

I replaced the caps on the inverter board for the first one and it then worked right away.

However, when I did the same on the second one, symptoms remains. I swapped the parts with the first monitor. I can conclude that it is due to the main logic board.

I can see the image when I shine light on the LCD panel. It has to be the backlight not coming on.

I examined the board and could not find amy bulged caps. Any pointer where I should look or caps that I should change?

[retiredcaps]

Originally Posted by sylvester328

I can see the image when I shine light on the LCD panel. It has to be the backlight not coming on.

1) Start by posting clear focused pictures using the manage attachments button below the text area. Do NOT post pictures inline or offsite.

We want picture as close to 2000x2000 and like this.

http://www.badcaps.net/forum/showpos...94&postcount=1

2) Do you have a multimeter? If yes, there might be a blown fuse in the inverter section of the board. Or a shorted mosfet or transistor. Also, you can measure the backlight off/on pin to see if the voltage changes when you press the on/off button.

[sylvester328]

The inverter board and the logic board are in the picture.

The inverter board has no problem as it works when coupled with a good logic board.


I found the schematics around the connector between the two boards (attached). Pins 1,2 are 12Vcc, Pins 3,4 are GND and Pins 5,6 are 5DVCC and all appear normal. I therefore tried to measure pin 7 and pin 8 in the connector. I doesn't seem to get a proper reading for both pins no matter I tried a good or bad logic board (zero readings). But I am pretty sure the good logic board works as I can hear the soft high pitch noise from the inverter while the bad logic board doesn't. What should I expect with those pins?

[Rtech]

[QUOTE=sylvester328;164181]I have two broken VA912 with same symptom of black screen.

However, when I did the same on the second one, symptoms remains. I swapped the parts with the first monitor. I can conclude that it is due to the main logic board.

I can see the image when I shine light on the LCD panel. It has to be the backlight not coming on.

QUOTE]

You may be getting confused between Logic and combined PSU/Inverter !!!
If you can see an Image then the LOGIC board( the one on the right in your picture) is NOT faulty.The problem is with the PSU/Inverter.

[Rtech]

Measure the resistance of the secondary windings on the Inverter Transformer,the ones at each end.

[sylvester328]

Originally Posted by Rtech

You may be getting confused between Logic and combined PSU/Inverter !!!
If you can see an Image then the LOGIC board( the one on the right in your picture) is NOT faulty.The problem is with the PSU/Inverter.

I am NOT confused. The Inverter board is the one with the main voltage input. The one on the right in my picture (the logic board) is the one being faulty. I can guarantee that. As I said in my original post, I have two identical monitors. One is now working and the second one is not. I have been swapping parts between the two and pinpointed that the logic board is the culprit. With the good logic board, the monitor works with any one of the inverter boards. However, with the bad logic board, the monitor does not work with either of the inverter board.

I am now suspecting the 5DVCC to 3DVCC regulator circuit on the logic board. Will test it tomorrow.

EDIT: Hmm.. No. It can't be 5DVCC to 3DVCC either. The RTD2523 takes 3.3VDC and 2.5VDC as well. That parts seems working (as I can see the image).

[PlainBill]

Originally Posted by sylvester328

I am NOT confused. The Inverter board is the one with the main voltage input. The one on the right in my picture (the logic board) is the one being faulty. I can guarantee that. As I said in my original post, I have two identical monitors. One is now working and the second one is not. I have been swapping parts between the two and pinpointed that the logic board is the culprit. With the good logic board, the monitor works with any one of the inverter boards. However, with the bad logic board, the monitor does not work with either of the inverter board.

I am now suspecting the 5DVCC to 3DVCC regulator circuit on the logic board. Will test it tomorrow.

EDIT: Hmm.. No. It can't be 5DVCC to 3DVCC either. The RTD2523 takes 3.3VDC and 2.5VDC as well. That parts seems working (as I can see the image).

Aha!!! The light dawns!! Let's call them signal (logic) card 'G' (for good) and 'B' for bad. With 'G' the backlights are on and you can see the display. With 'B' the backlights are off, but you can see a dim display.

The area you are interested in is pins 7 and 8 of CN1, and the signal lines VRMT and VBRI. Those control respectively the On/Off and Brightness signals to the inverter. And what is more, there is often a third line from the processor chip which switches power to the LCD panel, and often a fourth which controls the power LED. Since you can see a dim display on the 'B' card it is likely all of those outputs are correct, it's the support components that must be checked.

PlainBill