Some meter and/or shutter problems on an old AT-1?

[BHuij]

Calling all experienced photographers who know their way around older cameras!

I recently acquired an old Canon AT-1. It's still in very good shape with no light leaks or anything, and the meter even works.

Naturally, since it's an old camera and I bought it on Craigslist and thus have no idea where it's been, I wanted to do some tests and see where the shutter and meter are at and if they need to be recalibrated.

I shot a roll of 100 ISO film, making sure to use every shutter speed available. I shot 1/3 of the frames using the camera's built-in meter, 1/3 using Sunny 16 as reference, and 1/3 matching exposures with the histogram of my 5D Mk II shooting the same scene.

When I developed the roll, all of the frames came out about 2 or 3 stops overexposed. There were minor differences between the three "tests" I did on the roll, but just about everything was overexposed by more or less the same amount.

I shot another roll of Delta 100 yesterday, using only the camera's built-in meter. This time I shot 1/3 of the roll with the meter set to ISO 100, 1/3 at ISO 200, and 1/3 at ISO 400 in an attempt to compensate for the meter's readings being off. Once again, all of the frames were overexposed. The frames shot at ISO 400 were slightly less overexposed than the rest, but I couldn't even tell a difference between the frames shot at 100 and 200.

So my best guess is that both the meter and the shutter need to be recalibrated - but I'm having difficulty interpreting my results here. Why am I not seeing a bigger difference in the exposure of these frames when I'm shooting them 1 to 2 stops apart?