I bought an Agilent N9912A 6 GHz FieldFox a while back. I got a great deal on it because the date reset itself each time the power was cycled. Sounded like a typical dead RTC battery to me. Oddly there are no other reports of a dead RTC battery that google could find.
The service guide suggests the battery is a BR3032 soldered to the PCB.
It also states you need to replace seals/screws anytime you open the case. I suppose that is to assure it meets its dust/water specs.
The N9912A is cable/antenna analyzer, spectrum analyzer, network analyzer and vector voltmeter that works to either 4 or 6 GHz. From factory it only does cable/antenna analysis, everything else is a licensed option. Thankfully this one has nearly every option licensed.
I couldn't find a BR3032 locally so I bought a CR3032(this is not a very common size)
BR = Lithium/Carbon Monofluoride Li-(CF)x - less current capability but great long term usage.
CR = Lithium Manganese Dioxide (LiMn02) - can do more current but has a higher self discharge rate.
I opened the unit up which was easy. The battery was front and center, and very much glued to the PCB. I had to be careful breaking the glue when I snapped it off the PCB(after desoldering the legs).
Before doing any rework though I noticed an 0603 sized resistor that was not placed correctly. I am pretty sure it was lightly touched by the iron when someone soldered in the RTC battery at the factory(It would have been hand soldered in after reflow). I suspect this resistor has something to do with the dead RTC battery. Chances are this FieldFox has been faulty since day 1. Soldering the resistor back into place was a quick fix.
I spot welded some tabs on the replacement battery, added some double sided tape for mounting and kapton tape for insulation and then soldered it in place.
I then angered the Keysight gods and reused the original screws instead of buying the crazy priced reseal kit. Opening the case and changing the battery required no messing with any of the RF goodness.
It has been a few months now and the FieldFox continues to work 100%.