|The disconnected shaft is just to the RHS of the YIG oscillator|
You forget how USELESS 1960s and 1970s designs were in terms of ease of manufacture and repair. To get at the part, it looks like you have to take apart about 1 zillion screws and plastic cog wheels, and all because the designer didn't put the control about 5mm to the left, in which case none of these complicated mechanical arrangements would have been needed! These days, where competition, manufacturing and repair costs matter, the design would not be so unnecessarily complicated. After trying to join the 2 pieces together for nearly an hour, I gave up. In the coming days I'll return the 8640B to its owner and I expect he'll have the manual and the patience to fix it. My eyesight is also not as good as it was and that didn't help.
Elecraft XG3 signal generator (about the size of a pack of cards) would do all I need (and a lot more) and I think will be a good investment. It also weighs about 1/100th the weight of the 8640B. I am not after a signal generator with professional calibration and noise floor performance, just a simple way of checking the performance of simple receivers.
Unless absolutely necessary KISS - keep it simple stupid. This applies as much to professional gear as to amateur gear. Don't make life harder than it need be.