I take care of the teaching darkroom for a university. I have been having the enlarger bulbs blowing on me right and left on one of my banks of Beseler 23C enlargers for the past couple years. Sometimes several a day, then a long break. I was buying the bulbs 24 at a time several times a year. I had our electrician to put a voltage recorder on the curcuit, etc.
But I think I discovered the problem this morning. I checked all the Time-O-Lite timers (Masters and Pros) and found one that throws about 400 to 500 volts back into the supply line just as it shuts off. So if someone on the same curcuit has their enlarger light on while this beast shuts off, it might be blowing the bulb with the power spike.
Now the timer is engraved HSC and then a inventory number. The university changed its name from HSC to HSU in 1973...so this beast is at least 35 years old. I suppose it is amazing that it still works at all after 35 to 40 years of constant use (I have others of the same vintage that still work fine, and the back-up timer I put in its place is of the same vintage).
Any electricians out there that might confirm my suspecions about the timer? The way I measure it was to put the timer on a two out-let extension cord. Plugged in the timer to one of the out-lets and put the meter's probes in the other...and watched the needle jump from 110V to about 500V as the timer shut off.
PS...the next question is if it is worthwhile to send it back to the factory to be repaired ($75 to $100 is the average repair bill, as per the company).