I should really finish off this thread.
My 8640B is now working pretty well. All the switches and things seem to do what their labels say and a test on 120MHz gives a nice clean CW signal.
Not sure about the level of harmonics but it does meet the >30dB down figure.
I took some pictures yesterday and a decent camera lens will be here on Tuesday to replace my manual focus example.
http://www.radiomuseum.co.uk/HP%20Test.html
I'd be interested to hear how these compare with proper non barn-find examples.
Picture here is 2nd harmonic with attenuator set to 0dBm.
I should also raise the possibility that some damaged RF hybrid modules may work as amplifiers with a blown-up detector (and therefore usable). It would depend on exactly how much RF had been fed into the output socket.
Allan