I just spent some time with a ZP90 that would not power on.
Mine did not have the fuse blown, and the PWM IC appeared to be switching but it was bursting on and off.
That is where the onion began, the more I peeled the more I cried.
It has a very simple power supply. It has just a 2 output fly-back with second stage LC filters on each of the 2 outputs (3.3V, 12ishV). Which means there are 2 electrolytic caps on each output and 1 on the housekeeping voltage output. So if you're playing at home, that is a total of 5 output caps. Of my 5, 3 had a huge ESR. Which means they failed.
First i replaced the housekeeping winding electrolytic, and the bursting largely stopped but there was still a weird oscillation happening on both outputs. So after scratching my head for a while I checked the output caps and found that most of them were bad too. What happens with 2 stage filters is that if one of the C value changes in either stage, they can start to act on each other and oscillatory behavior can begin. ON the 12V output the oscillations dipped below 12V and the LDO on the analog board quits and you get no analog output even though the controller works as expected in terms of the Sonos app.