Posting for a buddy:
The HP 8640B comes standard with a 400/1000 Hz AM modulator. The entire assembly, including front panel knob, is mounted on a plugin board.
The Option 01 replaces the fixed frequency modulator with a variable frequency unit. It, too is mounted completely on one plugin board, including front panel control.
When he attempts to swap the 01 variable frequency modulator into a unit which was equipped with the fixed frequency modulator, it doesn't work, and the FM modulation, which is located in another part of the instrument, fails as well.
These results are constant over multiple units.
Why?