Analog and digital playback go through completely different paths and have different heads for playback, so the fact that analog works doesn’t prove much of anything. (Well, aside from the fact that the tape transport actually works, anyway.) Even the track layout is different:
So, the digital tracks for Side A on a DCC tape are on the upper half of the tape, while the analog tracks for Side A on a standard analog cassette are on the lower half, and vice-versa. The analog tracks aren’t included in the service-mode procedure I described above; only the digital tracks are checked for errors.
If recapping the boards doesn’t help, then you’ll probably have to enlist the help of the guys at the DCC museum – I don’t know of any easy (and more importantly, risk-free) way to test the actual heads on one of these units without some specialized knowledge and equipment.