Compur-Rapid: erratic super-fast speeds

[Dan Daniel]

I recently cleaned and oiled a Compur-Rapid shutter, from the late '30s I believe (off an old Zeiss Ikonta 512/16). It has a habit of not going slow, if that makes sense. Every now and then, maybe 1 out of 5 releases, it will simply fire sounding like 1/250 second. This happens most often at 1, 1/10, 1/25, and sometimes at 1/50.

In dry firing, it is an obvious issue: the small time gap between the opening and closing cannot be heard. In shooting, the one time it happened in 2 rolls (120 film, mainly at 1/100 and 1/250), the shutter DID open, but the frame was underexposed, maybe about 2 stops from the 1/50 I wanted.

Any ideas on where to start looking for a problem like this? I've checked all the various cam levers and such and nothing seems off. But I could be missing something.

Thanks for any ideas.

 

[John Koehrer]

Clean it again. Oil only the gear shafts on the slow speed escapement with something like watch oil, it takes just a touch on a pinpoint to work.
Check that the SS escapement works, you can do this with the front cover off. There should be a cam or lever that rubs the cam on the escapement.
If it doesn't strike the cam, it can't give a long exposure.
If you had the escapement out of the shutter. there's enough slack for the assembly to shift slightly when you reassemble it.

 

[Dan Daniel]

John, thanks for the idea. Studying the various cams and such again, I found the part that was holding up. Not sure what to call it, but the pivoting partial gear at the right (main spring) end of the escapement. This is the piece that has a pin on top that rides against the main cam plate. When the shutter was being cocked, this part would sometimes not engage and spring forward to ride on the cam. Not being engaged, it provided no delay on the shutter closing.

I removed the escapement, soaked again, oiled the main pivots, reinstalled. Still hanging up at times. Went and increased the spring tension on this part using the slots provided. No problems any more. Well, at least on the original problem. I don't have a shutter speed tester so I will need to shoot a roll to see if the high speeds are somewhat close.

 

[John Koehrer]

Good that you got there.
When you test with film it's not important that you have the correct speed, just a progression of speeds. Most shutters like this, if fast speeds are close that's as good as it gets.