My two units: 848-00131    Replaced Rb cavity all together with fresh electromechanics from -00107 unit.Also replaced the AC Amplifier with later (improved) version copied from HP manual. 836-00107    Scrapped since someone robbed it of the Quarz oscillator.The Rb cavity was used in -00131 above. The cavity was of a newer batchand had the light brown-greyish color of later generation HP instruments.Will use whats left of this unit for parts. When salvaging parts I noticed that the RX cell SRD driver module did nothave any output and that someone had tried and failed to open the unit. I experienced the same thing. My own "first" unit failed after having beenin operation for a week. I replaced the final (TO-5) transistor with a 2N5109and one of the other FET's with a 2N4416. This was exactly the samefaults in the scrapped unit's driver module. Possibly why that one was cannibalised anddecomissioned. Ulf Kylenfall SM6GXV