Thanks for all the info, folks, and to Jaymen for his explanation of the reference generator / comparator. I'm not sure that's the whole story, because there are some other very strange things happening around that area, but it did convince me to have another look at the reference generator.
As an aside, the fail-safe suddenly started working again! Maybe the electrolytic reformed, now its been switched on for a while again! Whatever, that convinced me that the fault preceded the failsafe circuitry in the servo, because the servo failsafe'd perfectly in the absence of a signal. Therefore the drive circuits must be OK.
Opening up another servo that works - something I hesitate to do because of all those ball bearings (!) - I noticed that the reference pulse in the broken servo is much shorter than in the working one. This is pointing me towards the reference generator. My next step will be to have a look at D1 and D2 (possibly pulling the volts on the C3 charging circuit down due to leakage) - or maybe C3 itself.
Haven't had much time to work on it this week due to higher priority domestic issues - and also preparing for the single-channel and retro meet in Pontefract in a week's time! It would be nice to get the Digimite working for that!
As another question, the output transistors are GC4066s. I know these were specially made for Bonner - basically a standard Texas transistor with an added heat sink. Presumably the heatsink is needed because the transistors are working in an "analog" mode, ie: not fully on or off. I don't know what the original transistors that these are based on were. I know they are PNP, and suspect that they may be a germanium type. Can anyone confirm this? I ask because I have some small silicon power PNP types that are rated at 1W dissipation that would probably make good replacements if and when the time comes, but if the originals are germanium, that would presumably upset the working points.
Onwards and upwards......!
--
Pete