how do you know that the 78 rpms are accurate?Do you have mains stabilization?
As I understand, the 60-cycle frequency of our mains in the states are guaranteed to a very high level of precision... A friend of mine works at a power company and told me how the breakers kick out any circuit that deviates even slightly in frequency. It's the voltage that fluctuates freely around 120 nominal and I wouldn't be surprised if it varies 5 volts.
The old record player I used in the test has a synchronous motor, and its speed is governed by the mains. Now I know not to trust an old record player speed to be engineered at "exactly" 78 RPM. And it's not. I calibrated it (the old one runs at 79.2 RPM) to a German-engineered Dual CS-5000.
That particular turntable has a unique design. It's belt-driven with a computer controlled speed regulator. A small bar on the platter passes between a light source and a photo-detector on the base. Once per revolution the turntable speed is checked by a quartz-controlled clock. Any deviation from the selected speed and the needle lifts.
p.s. Shots from the camera are pleasingly well exposed, since I calibrated the shutter speeds. It is rewarding to get properly exposed negatives from a vintage camera. Now I see a need for hand-holding skills and scale focus skills, because many of the shots suffer camera shake and focus mistakes. But that's something I can work on.
I'll recommend performing this procedure (calibrating vintage shutter speeds) on particularly robust cameras and lens/shutters. For example, this is where you can appreciate the value of a high quality-build camera, because you can trust that tests like this remain valid for long enough to be worth the investment in time.