Thanks for all the tips, guys; really helpful.
I've found a very readable and comprehensible description of the various functional blocks concerned and how they inter-relate to one another in the Analog Troubleshooting section of the diagnostics manual. I'll be getting to grips with that over the next few days as there are clearly many missing parts to my understanding of the circuitry used in this analyzer. If I have any problems I'll check back with you all.
I'm not sure yet if the REF unlock message was just a result of a cold oven; will have to go back over that aspect when time permits, but thanks for pointing out the possibility!
Jinxie