Hi
Ok, here's what you are looking at:
At one end you have a neon bulb that's filled with Rb instead of Ne. Rather than fire it up with high voltage they fire it up with RF. (A neon bulb will fire on RF as well). If the RF end of things does not have enough power (or the bulb is shot) you will get no glow. The RF is running a resonant circuit to increase the field intensity. If that is out of whack, no glow.
Once you have a glow, you have a couple of atomic lines being excited. That light goes through another cell that takes out most of what you don't want (nothing is ever perfect). With nothing else in the cell, it would pass through to the photo detector and give you a nice DC voltage on the detector.
Toss in another cell (or monkey with the gasses) and you have a bit of magic. Hit the combo with microwaves at just the right frequency and the light to the photo cell goes way down. You have a notch filter. It's a bit nutty because the notch is RF in and light out. The why has to do with quantum mechanics ... we'll skip that for now. The next trick is how to lock to a notch.
The normal way to lock to the notch is to modulate the microwave signal and look at how the recovered light responds. Tone in and only tone out = tuned right. Tone in and tone + second harmonic out = tuned wrong. With a bit of work you can create a PLL that will keep things centered up.
So what can go wrong:
1) No RF into the bulb or frequency way off from correct (= no field).
2) One or more bulbs broken
3) Photo detector broken or covered in slime
4) SRD (or other multiplier) not putting out microwaves
5) Cavity not tuned to SRD output (again no field, just at a very different frequency)
6) A whole raft of problems in the synthesizer or MCU.
7) A copule of other things relating to the quantum mechanics stuff we skipped (C field levels etc)
To make it even more exciting:
Back in the "good old days" the cells had to be pretty much right on or the (narrow) synthesizers of the day would not lock to the resonance. Along came DSP and DDS and a bunch of FPGA's. Now you do a cheap synthesizer with a much wider range. A greater range of cells can be used (like over 100's of KHz). The magic numbers get stored in an eeprom or flash somewhere and the unit chugs happily on it's way. Does everybody do a Rb that way? Who knows, there certainly are some people who do that.
How all this relates to which test points in the LPFRS, no idea beyond what has been posted here. This is not anything I designed. Many of the fine details of how this or that is done vary between manufacturers. The way Temex does it may not be how FE does it or how Efratom once did it or how Symmetricom does it today.
edit ... ====================
Got a chance to play with 8 of the 10 Rb's today (still need to drill out the other two).
5 of the eight lock up properly and appear to be ok.
1 of the eight locks up, but it is about 180 Hz low
1 of the eight sweeps past 10 MHz without ever locking
1 of them starts low and never sweeps as high as 10 MHz, it also has odd voltages.
None of this is with any cap changes. No pot tweaks. I simply opened them all up, looked around in them, put a DVM on the 9.2 V supply and let them warm up. I watched them on a 53181. I did not bother to hook up a serial port to them.
So here's a question:
There are two versions of the CPU board. That has been mentioned before. There is also a version of the RF board that has a daughter board on it. The daughter board has the crystals attached to it. Anybody else seen one? With the daughter board it seems to warm up more like an AT cut based oscillator. The normal parts are quite strange on warmup (very little shift).
All for now, more fun in the morning.
Bob