John E. wrote:
Palindrome sez:

I'd go for the continuous - every time where component and circuit board
burning were evident.

The reason this machine drew attention in the first place was because the
solenoid valve was gummed up and sticking. I wouldn't think that this would
cause problems with the drive circuit. Au contraire, it would result in no
back-emf.

I would think that a partially shorted zener would keep the
solenoid energized, giving a "gummed up" symptom. If the
board allows space for the modification, I would replace the
47 volt zener with a series combination of a 4.7 or 5.1 volt
zener in series with a 1N400X or similar small rectifier
diode, connected directly across the coil, instead of across
the fet. Such a low voltage zener will be a lot more rugged
(dissipating only a small fraction of the power dumped
into the 47 volt zener, since it discharges only the
solenoid energy, rather than that energy plus lots more from
the supply). The energy dump per solenoid discharge is so
much lower you may get by with a .75 or .5 watt zener and a
1N4148 diode, if the solenoid current is less than about .1
amp.

The rectifier cathode connects toward the positive supply
end of the solenoid, but the zener cathode points toward the
fet drain.

Can you find a place to put those two components?