Viewsonic VA912b brightness no response

[spintop]

I am not sure if this is hardware related to the CCFL inverter or software.

Issue: The screen brightness has no response when the button is pressed. However, can see that the OSD brightness scale is moving up/down from 0-100. Contrast control is working fine, and everything else is fine.

Attached the power supply photo.
(U1- MP1038EY)

Advice needed. Thanks.

[PlainBill]

Re: Viewsonic VA912b brightness no response

I am not sure if this is hardware related to the CCFL inverter or software.

Issue: The screen brightness has no response when the button is pressed. However, can see that the OSD brightness scale is moving up/down from 0-100. Contrast control is working fine, and everything else is fine.

Attached the power supply photo.
(U1- MP1038EY)

Advice needed. Thanks.

First, give yourself a big pat on the back. The pictures were good, and the datasheet for the inverter controller was a big help.

On the top view of the power supply / inverter the bottom two leads are for the inverter on/off control and the brightness. The brightness signal goes to either pin 11 or 12 of the MP1038EY, probably pin 12.

I'd measure the voltage on the two pins, the On/Off should be about 3.3V with the backlights on; the brightness should be between 0 and 1.2 volts, and vary as you adjust the brightness from the menu. If the brightness voltage doesn't vary, look at the signal card. If the brightness does vary, look at pin 12 of the MP1038EY; there is some circuitry between the connector and the IC.

PlainBill

[spintop]

Re: Viewsonic VA912b brightness no response

First, give yourself a big pat on the back. The pictures were good, and the datasheet for the inverter controller was a big help.

On the top view of the power supply / inverter the bottom two leads are for the inverter on/off control and the brightness. The brightness signal goes to either pin 11 or 12 of the MP1038EY, probably pin 12.

I'd measure the voltage on the two pins, the On/Off should be about 3.3V with the backlights on; the brightness should be between 0 and 1.2 volts, and vary as you adjust the brightness from the menu. If the brightness voltage doesn't vary, look at the signal card. If the brightness does vary, look at pin 12 of the MP1038EY; there is some circuitry between the connector and the IC.

PlainBill

Thanks PlainBill for the advice.
Measured brightness voltage not varying when adjustment from the OSD. Looks like issue is in the logic board. Without schematice diagram, will be hard to troubleshoot. I think i can live without brightness adjustment for now.

attached is the logic board area around the connector.

[PlainBill]

Re: Viewsonic VA912b brightness no response

Thanks PlainBill for the advice.
Measured brightness voltage not varying when adjustment from the OSD. Looks like issue is in the logic board. Without schematice diagram, will be hard to troubleshoot. I think i can live without brightness adjustment for now.

attached is the logic board area around the connector.

In this case, Q3 and Q4 would be suspects. One drives the inverter On / Off line, the other is the D-A converter.

PlainBill

[Scenic]

Re: Viewsonic VA912b brightness no response

only thing left to check would be the 2 tiny transistors and maybe the voltage regulators (bottom left corner)
one of them (the lower one) seems to be adjustable, as there's no -33 or -18 marking for 3.3V or 1.8V..
can't tell what's written on the upper one though..

edit: i've seen a cheapo monitor once which used an adjustable v-reg for the brightness control signal..

[spintop]

Re: Viewsonic VA912b brightness no response

Thanks alot guys for the advice. Found the faulty component and resolved the brightness issue.

With PlainBill & Scenic advice, focused on Logic board. Initially tried swapping the transistor (Q4) next to the connector with the transistor controlling volume control, as i found out they are using the same transistor.
Doesn't solve the problem, so eliminated the idea of faulty transistor.

Found out that the adjus signal is always 0V. Swapped out C13 next to the leg of the connnector, and it worked !!! A faulty 1uF ceramic capacitor, grounding the adjus signal to gnd.

Once again thanks guys !!!

[PlainBill]

Re: Viewsonic VA912b brightness no response

Good job. We've had a number of people report problems due to a shorted ceramic cap.

PlainBill