How to disable the backlight on the XLT PCB plug without opening the display panel?

[Polymer]

My backlight on my XLT-e series causes much interference now and flickers.
I am worried that might kick back to the PCB and make it kaputt,
since the backlight always turns on for a while when powering up the XLT.

The DC/DC converter for the backlight must be controlled or powered via the 16-pin
connector on the main PCB, or not?
I found the following partial schematic for the eagle spectrum which i assume is the same as the xlt e-series, but am none the wiser

It just says "light" on the bottom left where the matrix is shown.



Any elegant ideas on this ? Or do I have to get the hacksaw and cut that &%")*' backlight out of the display

Thanks for any insights.

[Carl-NC]

The "LIGHT" on P3' connects to P3 which is the output of a step-up transformer driven by Q1. I'm pretty sure the XLT has a similar design, and the step-up driver is almost certainly in the pod, not the main box. It's easy to open (just snaps together) and the backlight should be easy to unplug.

[Polymer]

Thank You for your reply Carl.

Whoops, I totally mis-expressed myself in the original question. The August heat/humidity makes for good brain mush ... a zombies delight

I meant that I want to disconnect the power to the step-up converter for the backlight.
That part has me worried about "backfiring" and frying the main PCB.

I've taken the pod apart, it's relatively easy as you said.
The hardest part was keeping fingerprints off the LCD

Here are top/bottom pics for reference:





The lcd pcb from whites is copyright 1994 as compared to the detector pcb which is copyright 1999.
This leads me to believe that the the e-series model uses the same display control as the eagle spectrum.

I had a closer look at the schematic to find Q1 you mentioned and found U-ACU at pin 4 of the 16 pin connector (P5) which
is the power supply line to X1 (which I'm sure is the ac step-up source for the backlight).

As far as I understand now, all I have to do is snip off pin 4 of P5 and the step up converter for the backlight is permanently powerless.

Do you see this as a viable solution? Do you see any possible catches? Did i miss something?

[Carl-NC]

Sorry, but without a schematic I'm reluctant to agree. '94 was the release date for the XLT, so I'm not convinced it has the same pod circuit as the Spectrum. And P5 looks like the keypad connector.

In any case, I recall the XLT has an adjustable backlight, so I doubt there is a single drive line to disconnect. You could remove the transformer (the big honkin' black cube). Speaking of, it looks like it has solder issues, which is probably why it having problems.

[homefire]

Yep, Solder didn't even Flow. That one looks like it could be shorting. Corrosion it looks like.

[Polymer]

Thank You Carl!

I tried measuring my circuit against the schematic and got nowhere. So i just unsoldered the "honkin black cubey".

The solder joints were not the problem.
The white residue on the inverter & pinheader joints are probably leftovers from the manual soldering of these parts.
It was easily removed with alcohol.

@ homefire - Thanks for your input