[ICOM] IC775DSP display backlight problem

[Juan Pablo Mercé LU4DX]

I have a problem related to the display in my IC-775DSP.
This one works well and without intermittences but I cannot change the 
intensity of the display, try to change the value to LO and HI into the menu 
but nothing happens like the preset to change the low intensity.
The level seems to be in the high position.
The previous owner said to me that the display backlight was intermittent 
and he resoldered the lamp's power transformer secondary contacts to the 
printed circuit board.
Does someone know where the problem can be not to be able to change the 
intensity of the display?

Tnx in advance,

Paul LU4DX

----- Original Message ----- 
From: "Martin Sole" <msole at loxinfo.co.th>
To: <icom at mailman.qth.net>
Sent: Saturday, June 17, 2006 5:12 AM
Subject: [ICOM] IC775DSP display backlight problem


> Dear Icom friends,
>
> I have a problem with an IC775DSP display that I would appreciate some 
> assistance with. The owner noticed the backlight flicker a couple of times 
> then go out. This was accompanied by a burning smell and some smoke. 
> Investigation shows that both Q17 & Q18 in the backlight driver circuit 
> have disintegrated.
>
> I would first like to know what kind of voltage the backlight is meant to 
> be driven with? The schematic shows it to be simply an LED driven from a 
> rather complicated AC source. I therefore assume that the backlight needs 
> more than just a few volts at a few mA.
>
> My second question concerns the voltage source itself. This appears to be 
> some sort of oscillator driven from the 14 volt line but I am unfamiliar 
> with the nature of its design. I did replace the damaged transistors with 
> similar items albeit 100 volt rated rather than 120 volt rated but they 
> immediately self destructed so I assume I need to look elsewhere for the 
> cause.
>
>
> I can email scans of the circuit area if anybody would like to see before 
> passing comment.
>
> Thanks in advance for any help.
>
> Martin HS0ZED
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