Hi......... The next highest Probability, to me, is the Power Supply within the Unit....
These do fail occasionally.....Internally there is both a +5 volt and -5 volt regulated
supply system ,derived from the 12DC volts input externally......sometimes people get lazy and plug most anything into the jack, and , wallah, its toast.......but one saving grace is this one dies without allowing massive over-voltage to the sensitive logic circuits that need no more than 5 volts.....usually......cant know for sure until the supply is fixed, then you get to discover if the 'next layer' survived......These Power supplies are a bit difficult to repair... known as "switchers" .....pretty tricky......some guys I know just gut it out and
replace with external supply, regulated +/- 5 volts.......naturally you need the schematics
for all this....... let me know !
Cheers
Dan