The 08660-90070 manual (HP 8660A/B/C service) has been helpful/friendly so far.
There are separate "big" operating/service manuals for 8660A, B and C; the 08660-90070 manual sends technicians to one of the "big" manuals at some point - I've not reached that yet.
The unstable 20-30 MHz output seen on HP 11707A test plug-in (typically been 200-300 kHz below specified frequency with considerable "wandering around" in that range) led me to another section in the 08660-90070 manual, which describes that frequency being driven by three synthesizers (called N1, N2 and N3). The three synthesizer outputs can be measured from the bottom side of the motherboard at pins shown in the same manual. After a little learning curve and oscilloscope adjustment, it turns out that those three signals appear to be correct (and stable) - the match tables in Appendix A.
N1, N2 and N3 feed two more synthesizers called SL1 and SL2 (Sl1 appears to be presented at the 11707A output.)
That's the next step.
