Hi
As I feared, the root cause of the problem with the non-functional external FM/AM modulation traced back to the firmware.
It turns out that the order of going from one modulation to another is important. The attached drawing shows the permitted sequential steps in green, and the ones that don't work in red.
I can go from Internal AM to Internal FM, but I can't go diagnonally from Internal AM to External FM. So all the hardware symptoms I saw were due to a firmware "feature".
The Wavetek also has an option button for external phase modulation, but this button does nothing I can see and this oiption does not show up in the self test. There is a test that shows the options installed. Also the manual indicates phase modulation requires an additional PCB module, which is not present.
The front panel has a covered hole for an option for pulse modulation of the RF output. It also needs added circuitry. I definitely don't have this option fitted.
So the plan is to soak test the Wavetek 2520 tomorrow to try and flush out any latent defects before I put the covers back on. Then over the coming weeks, I will try and get access to enough equipment to do a basic calibration, focusing on crystal frequency reference and output level.
I think I started this repair about a year ago. I never expected to find so many faults in one piece of equipment. It has been a challenge but I learned electronics at around the period this was designed and built, so it has a familiarity to it. It has definitely been hand built. The circuit boards are dated and signed by real people. No stick-on labels here. Unlike old HP gear, it looks like Wavetek didn't quite finish design and development before it went into production. The optional modules are solder-in not plug-in, and probably needed to be factory fitted, including new EPROMs with updated/modified firmware.