[Putershark]

Yep Brother,
Forgot all about your battery system (my bad) and the 3 switches with its smart isolated circuits.., I would still look at the "back" of each switch, and make sure the wire connections are still all "tight" and make sure that each switch is not only in it's proper position, but that the switches do not have any slop in them that could allow a temp loss of power supply / drop. Now you may not be able to "feel" such by hand when moving them, but many switches have a nice locking (indent) click feel and even then, the actual switch can have an internal break down (wave hopping) that is not readily apparent as has happened to me with some of your standard "Perko" offerings in the past

In reading your article our terminology is a little different (but has the same use) and I consider all battery banks NOT used for trolling motors as "Ship Service" units (Has to do with doing a lot of push ups years ago for using the "wrong" terms ) since they can supply power to the ship (boat) for both the engine & the running of all console gear. I do though understand what you are trying to convey but being able to "mentally" trace out your system this late is a lost cause and I would really need to see it all again

Now some people have dedicated batteries for starting others for running batteries, yet I was thinking in the terms of the "Factory Rigging" which is 2 batteries for everything (minus the trolling motor) and you just run on either A or B (1 or 2) battery alone all day long.... Now I have never seen a real strong use for the "both" position unless both batteries are weak and you have a slow cranking issue and may need to barrow a few AMPS Even then, many times that just drops both batteries even lower and can make the problem even worse... But you can't divide zero you know and can always try both! It sure beats floating out to Sea on a dead engine and sometimes I think the simple act of switching back and forth fixes such NOT because the battery was really that low, but because the switch was acting "funky" (non-official technical term) and then the engine started when you landed on the "Both" position

So your system is pretty unique and has a lot of fail safe features for sure! But as the saying goes "The more plumbing more possibility for leaks" you have a lot of connections (potential points of failure) that I would pull a once over "just" to make sure that over time you have not developed a loose connection or ground, from banging over a few waves in the past

I am also wondering about power draw / spikes with the Radar & Sonar running and is it (they) on their own circuit and more even more, important grounds? Here again, the point being that the VHF also has some smart circuits installed on the PCB and a voltage drop (even for a nano second) could scramble things that "may" kill your ability to Xmit. A simple power cycle of the radio (all the way off and back on again) though "may" correct such an issue just remember, that many PCB's will hold current for a while... So make sure you allow them time to discharge (at least 60 seconds) before turning power back onto the VHF.

I would also start looking for common / combo related issues such as.... When I key up the Mic the Radar screen or Sonar screen blinks, and then nobody can hear me Not to say that the issue is "not" inside the VHF or its own PCB at all, but troubleshooting of this type can "sometimes" show related effects (though may be hard to find) if one just keeps their eyes open the "instant" it happens

Though not related other than the fact it was also 12VDC system, but a great story on digging for the "Real cause" for sure... About 13 or 14 years ago (from what I recall) I had a customer that owned either a Audi 90 or a Porsche 944 (can't remember which right now) that kept blowing a headlight for no (at first) logical cause... One of those real mind benders if you know what I mean

So after a few trips to the dealership my Assistant Manager came to get me go over the issue again. Now I kept telling him and the customer that we need to really "duplicate the condition" so I went down "my" asking of questions trying in vain to "nail down" the real cause... One of my standard T/S questions was "Were you alone or was somebody riding with you when such happens?" This goes along the lines of "Was the A/C on? Or were you making a turn and had the signal on" etc,etc,.... Trust me, I had long mental of list of questions pending the issue, with electrical problems being one of my longest list! Ha-Ha...

Well come to find that it happened WHEN and I could make it do myself (Hot Dog) his wife was ALSO in the car and... After going down that T/S path further of what "she" was doing COME-TO FIND-OUT that when you pulled down the R/S Visor and then opened up the cover for the vanity light to put on her lipstick, it blew out the one headlight from a shorting ground! What a gas that was to finally to find So we put in a new bulb (again) disconnected the wire to old visor until the new visor came in on special order and could get swapped out... Problem finally diagnosed and fixed

I say such to help you "boost" your SA /Situational Awareness (think that is spelled correct?) to look and see what else may (or may not) be happening at the same time when you loose the Xmit function on VHF

Enjoy and let me know your final fix and if needed, call me and we can crawl all over it together at your house...