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Dave,

First check your motherboard model:

The E4400-60124 can only use display with Negative contrast (VLDC)
The E4400-60035 can use pos and neg VLDC

I know, its for contrast, but DAC configuration is the same: part A of U604 for brightness, part B for contrast.

Check voltage swing on pin 7 of U605 varying the contrast, should be 0 to -10V.

Same for pin 1 of U604, 0 to -10V varying the brightness.

If this OK, problem is after DAC.

Milan

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On Sun, Nov 10, 2024 at 10:43?AM Dave B via <davebullockmbe=[email protected]> wrote:

Hi Milan,

Thanks, yes I had spotted the DAC is an AD7528 and that it seems to be powered only from +5V suggesting that the output can only be a varying positive voltage?

?

Looking at this simplified block diagram the 'inverting' amplifier following the DAC would then suggest a negative varying voltage at TP601?

This does seem to agree with the voltage I am measuring on my instrument i.e. (minus) 0.24V to (minus) 1.33V. (

Though why such a limited range?)

I have scoured all the 'service' information that I have managed to find on the web, and despite TP601 being a relevant test point to aid fault finding around the DAC area

there are NO "fault finding" flow diagrams or Test point voltage tables to be found anywhere!

It's very frustrating.

If my backlight control voltage is correct (and it now seems to agree with the schematic?) I am still bemused how it controls the CCFL inverter which is powered from +5V, meaning

the control voltage is operating outside the rails of the inverter.

Very intriguing?

Thanks for all your help.

Dave

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