Bought a DCC730 which would either output no sound or have distorted sound when playing back DCC or ACC. Symptoms were caused by an incomplete or only partial turning off of the mute transistors during playback. This was in turn due to a sagging 5V analog power supply to the Digital Board from 100 mA 5 V reg L78M05 7921 (which was red hot to the touch) dragging down the NMUTER line. Easy, I thought, replace the voltage regulator and its fixed. Did not have a 100 mA part in stock so fitted a 1A part instead. You guessed it, the fuse for the +10VA supply feeding the regulator went. Motto: look for the man with the matches not the fire!! It seems the 100 mA part had been also acting as a current regulator!! Proceeded to measure the input impedance of the 5VA supply to the Digital Board; 15 Ohm, oops!! Now to lift legs/desolder everything hanging off that power line until low impedance goes away. Turned out to be (2780) 47u/10 V SMT electrolytic on Digital Board, it had dropped its guts neatly under its own footprint which had then dried to form a perfect surface mount 15 ohm resistor. Once desoldered the cap measured open circuit and so did its footprint. Proceeded to recap all the SMT electrolytics in the whole unit - a process I don’t really like as its blind shotgunning and I like to fault find to component level. Out of 25 or so caps found 4 faulty ones so… You can always tell by the smell as you desolder whether a cap has leaked or not but I digress…
Previously the unit had been able to record OK but playback on another machine proved erratic and plagued by drop outs but only on side A. Now I could hear the same tape on the home machine there were fewer drop outs (probably because of the new caps on the read/write board) but still too many. Transport was making a rhythmical whining sound and on inspection the otherwise perfect looking pinch roller on side A was jumping once per revolution. There was a hardened area on the pinch roller and replacing this with a pattern part has cured the issue.
Hope this helps you if you have a similar problem. I am now able to enjoy digital compact cassette, a concept which was “simply years ahead” 