Further question: Does the enlarger ever turn on without being first turned on
by the timer?
There are two scenarios:
1) The timer's contacts are being 'welded' shut by the inrush current
of the enlarger. If the timer is controlling a relay in the enlarger's
power supply this is probably not the cause. However, if the timer
is controlling the lamp current directly then contact welding may
be the culprit. Does the enlarger turn off if you give the timer a
2) More likely: The timer's output relay may have a snubber circuit across the contacts [or SSR output triac] to prevent arcing with
cold-light heads and this snubber circuit passes enough current to
activate the enlarger's power supply.
A quick fix is to put a small resistive load at the timer output so the
timer turns on both the load and the enlarger. Try a 7/15 watt light
bulb for a test - plug an outlet strip into the timer and plug the light
bulb and enlarger into the strip - and see if this fixes the problem.
If solves the problem then putting the light bulb in a black box would
be the easiest solution. A 1 watt 22K resistor wired accross the
enlarger's relay coil/SSR LED would be a more permanent fix.