Agilent FieldFox N9912A 6 GHz RTC battery swap/quick repair - Page 1
Forum section: electronics / repair

author: TheSteve
author_rank: Unknown
timestamp: April 12, 2018, 05:18:39 am
content: 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%.

author: Samogon
author_rank: Frequent Contributor
timestamp: April 12, 2018, 06:37:11 pm
content: Nice,What tool did you use to weld tabs to the battery?

author: TheSteve
author_rank: Unknown
timestamp: April 12, 2018, 08:10:22 pm
content: Quote from: Samogon on April 12, 2018, 06:37:11 pmHome brew spot welder I put together 10+ years ago. It is basic but does the job. Each time I pull it out I figure I should make some improvements but then it quickly does the welds and I get back to whatever I was already working on.It is powered from a 10-12 volt regulated supply limited to 1 amp of current. When you fire it the power supply is not disconnected so it needs to be current limited. The probes are a pair of cheap soldering iron tips. The foot pedal was made by heating and folding a piece of plastic and installing a game switch.I've done hundreds of battery tabs over the years with excellent results. The tabs done on the CR3032 above was with the power supply set to 10.5 volts. The nickel tabs will generally rip before the welds break off.

author: nikonoid
author_rank: Regular Contributor
timestamp: April 12, 2018, 08:21:06 pm
content: Thanks for sharing fieldfox pictures. It is very cool and probably way too expensive.I like the welder very much. Is the stack of transistors responsible for “on/off” switching? How about transistor on the right? It might be fun to make one.Sent from my iPhone using Tapatalk

author: Samogon
author_rank: Frequent Contributor
timestamp: April 12, 2018, 08:28:21 pm
content: FieldFox can easily get to $10k and more

author: TheSteve
author_rank: Unknown
timestamp: April 12, 2018, 08:46:59 pm
content: Quote from: nikonoid on April 12, 2018, 08:21:06 pmThe stack are SCR's that "fire" when you press the button on the foot pedal. The 3 legged part on the input is just a polarity protection diode.FieldFox's do indeed get expensive - a loaded 50 GHz one could be easily 70K + with options enabled.

author: Zucca
author_rank: Unknown
timestamp: April 12, 2018, 09:02:48 pm
content: Another bullseye fromTheSteve.Those FieldFox are sexy, pretty much nice step in the vodoo of RF black magic.Good for rich RF beginners.

author: KE5FX
author_rank: Super Contributor
timestamp: April 12, 2018, 09:39:47 pm
content: Quote from: Samogon on April 12, 2018, 08:28:21 pmProbably closer to $50K or even higher, now that they have 50 GHz models. I've always hoped someone would do a detailed teardown of one.Drop one of those, and you might as well high-tail it to Mexico in the bucket truck.

author: TheSteve
author_rank: Unknown
timestamp: April 12, 2018, 11:04:29 pm
content: Quote from: KE5FX on April 12, 2018, 09:39:47 pmI won't tear my N9912A down but at some point I will tear down an N9923A RF section. Like the N9912A it is a lightweight compared to the microwave analyzers but should still be cool.