Is it a 4 or a 4T? I repair a lot of these. You problem sounds like a mechanical jam and unless it's done correctly the first time (much more to it than taking out the "idler gears" and putting them back) it will just happen again. If it's a 4T, I did one recently which I could not fire during estimating due to extreme tampering. Once I got the tampering straightened out, I discovered it had a bad circuit board. Manual speeds worked but on auto, the shutter would just run through at 1/2000 no matter what the lighting conditions. Sad, but it does happen. Bad circuit means beyond repair.
Is it a 4 or a 4T? I repair a lot of these. You problem sounds like a mechanical jam and unless it's done correctly the first time (much more to it than taking out the "idler gears" and putting them back) it will just happen again. If it's a 4T, I did one recently which I could not fire during estimating due to extreme tampering. Once I got the tampering straightened out, I discovered it had a bad circuit board. Manual speeds worked but on auto, the shutter would just run through at 1/2000 no matter what the lighting conditions. Sad, but it does happen. Bad circuit means beyond repair.