Help Please.....Colour Video Sounder repairs

Hi....Anyone suggest where/who might be able to repair malfunctioning video
sounders? I have 2 that have now failed on me for no apparent reason - a
Koden 101 and a Furuno 266. Both relatively modern and regarded as high
quality devices used by professionals.
The Koden has full colour display on screen, with other functions and modes
like range
shifting, VRM etc all appearing to work OK, but no useful seabed image. I've
tried replacing the transducer, but no improvement.
The Furuno was working well until I switched it on recently and the screen
display appeared compressed into a 4cm wide band, rather than in the full
screen width of 12cm. It was still displaying good colour and seabed detail
and all other range and mode functionality was working fine. I tried to trim
the display back into line with the edge potentiometer on the video pcb for
the horizontal gain, but it then wobbled and now presents with a totally
blank screen! Nothing, zip, no relay click when switched on.
Any suggestions, apart from spending another £500 on a replacement?

Regards.....Keith

I think I would look very carefully into the power supply. 12/24 volt
supply can deterioate rapidly in salt air/water situations. Resistance
from corrosion can reduce voltage yet still allow the system to appear
functional. Checking with a voltmeter may show full voltage although
full load voltage would be lowered and the meter may not show that. I
would consider trying a tottaly seperate hot feed (and new earth lead)
directly from the battery before I condemmed the unit.