Re: Piezo element?
The clip in the opening post now has a piezo attached and ticking is audible through the earphone when it's clipped to a small wristwatch. The solenoid plate in my waxie-free unit appears to bang in time with this ticking, but only when the gain is increased significantly with the appropriate knob. I guess the piezo has quite a low output, and solenoid firing is intermittent with a new piezo in the microphone unit shown in post #13.
I guess my next step is to try one of the 'vibration sensors' from the RS link, and then perhaps investigate inserting a solid-state preamp stage to the microphone input.
The timing unit is well shielded - hum is heavy with the lower grille removed for access to the circuitry and chassis, but with it in place it's very quiet even at full volume. I'm impressed when it's only a piezo signal going into an ECC83 in the same chassis as the mains supply with plenty of opportunity for interference.
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