[ICOM] IC775DSP display backlight problem

[Martin Sole]

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