A Sticky Shutter at 1/25
The folding medium-format camera that came out of a relative’s loft has a leaf shutter with speeds from one second to 1/250, and the slow speeds were sticking. At 1/25 and slower the blades would open, hesitate, and close somewhere between half a second and never. The fast speeds sounded fine, which is typical of old lubricant that has gone gummy in the slow-speed escapement.
Before touching anything I measured what the shutter was actually doing. The method is to put a small microphone right next to the lens, record the click on a laptop, and look at the waveform: the opening and closing make two distinct spikes and the time between them is the exposure. At the marked 1/100 the measured time was about 1/70, which is a third of a stop slow and perfectly usable. At 1/25 it ranged from 1/8 to over a second between firings. At 1/10 it sometimes did not close at all.
The repair was less heroic than I feared. With the front lens group unscrewed and the shutter cocked, I could see the escapement, a little toothed wheel and pallet that governs the slow speeds, and it was coated in a brown varnish of old oil. I flooded it with a few drops of lighter fluid from a syringe, worked the shutter thirty or forty times at each slow speed while the fluid was wet, and blotted away what came out with the corner of a lens tissue. Three rounds of this, letting it dry completely between rounds, and the hesitation was gone.
Measured again afterwards: 1/25 now reads about 1/20, consistent across ten firings, and the one-second speed reads 1.3 seconds, also consistent. Slightly slow across the board but repeatable, which is all a negative film needs. I have written the real speeds on a strip of tape inside the back door.
The thing I was warned about, and respected, was not to add any new oil. The temptation is enormous because a lubricated mechanism sounds better, but a shutter escapement is designed to run dry or nearly so, and oil creeps onto the blades and glues them shut within a year. The first roll through it since the repair came back with even exposures across all speeds, including a thirty-second tripod exposure of the kitchen at night that I am quite pleased with.