12 Volt short question

[Rocky]

Rewired the boat putting all the accessories to a switch panel, each with its own switch and found that my VHF must have a problem with it. Before it was wired directly to the second battery with an inline fuse (still basically the same just a switch in between now) and I noticed on the underside of the gunnel the antenna wire is burnt where it is going through the gunnel, I have a rubber grommet there and the sheathing is intact, no bare wire. After doing some testing I found out that when powered up the connection on the back side of the radio is hot and Im pretty sure this aint supposed to be so but have never checked before, have had lots of CB's and never thought to check. Am I missing something or anyone have any hints before I go drop a bundle on another radio and antenna. Thanx in advance

[StickFish]

The connection to the backside of the radio is hot? Which connection, power, coax or other? Or are you referring to temperature?

Assuming that hot means you can read positive voltage using a VOM between the heat sink on the radio and the hull - there is something wrong. The back of your radio should not have any positive voltage on it - it should read zero ohms resistance to ground or the negative post on your battery.

Did you install the switch on the Positive line only or on both? What is the amperage rating of the switch?

The burn mark on your coax is very interesting especially since you say that there are no bare wires. How did the burn mark get there and did it happen instantly when you turned on the radio and the switch panel.

I never install a radio that isn't directly connected to the battery and use large gauge wire to limit voltage drop

[IDWITHELD]

Is it possible the grommet wore off on the coax cable and it's not burnt? Maybe friction damage? If the antenna was shorted you could measure the coax connector with an ohm meter. If shorted you'll know.

[Rocky]

Sorry I guess I was not real clear on that. Yes the coax connection to the back of the radio is getting current (which I know now is not right), I dont know if the heat sink was hot or not, didnt think to check it.
The burn mark on the coax happened before I put in the new switch panel and I just noticed it while I was under the dash running wires, only about an inch long and only on the part of the coax the is below the gunnel on the underside of the rubber grommet. I will take the radio (two years old) down to West Marine today and let them deal with it. (Just checked the ground wire and its hot too :depressed Not sure how long its been like this but it has always seemed to work right.

[StickFish]

Is it possible the grommet wore off on the coax cable and it's not burnt? Maybe friction damage? If the antenna was shorted you could measure the coax connector with an ohm meter. If shorted you'll know.

Some antennas are designed to read a DC short.

First indicator of bad antenna - no static with the squelch turned all the way down. Next most people complain that knowbody can hear them - circuit in the final amplifier restricts current to the amplifier when the standing wave ratio is too high (Reflected RF current caused by an antenna problem) which results in little or no transmission.

Best way to check an antenna is using an antenna analyzer, next best is a Standing Wave Ratio meter, or a forward and reverse reading watt meter.

[StickFish]

Sorry I guess I was not real clear on that. Yes the coax connection to the back of the radio is getting current (which I know now is not right), I don't know if the heat sink was hot or not, didn't think to check it.
The burn mark on the coax happened before I put in the new switch panel and I just noticed it while I was under the dash running wires, only about an inch long and only on the part of the coax the is below the Gunnel on the underside of the rubber grommet. I will take the radio (two years old) down to West Marine today and let them deal with it. (Just checked the ground wire and its hot too :depressed Not sure how long its been like this but it has always seemed to work right.

Hot meaning 12 volts DC or Hot meaning temperature? Radios do produce heat when they are working - heck my 2m Handy talkie can get so hot its not comfortable to hold.

When you are measuring the current on the radios coax connector is this when you are transmitting? Or receiving? How many amps and volts are you measuring? I checked my IC-746 in receive there is no voltage between the center coax connector and the shield. I didn't want to transmit to a open connector and you shouldn't either.

If there is something wrong with the radio - best to take it to Rogers Electronics on Marine Dr. West Marine doesnt have any electronics repair facilities that I know of.

Also are you sure it is a burn mark - deteriorating rubber grommets will leave black marks on white Marnie coax.

[IDWITHELD]

... best is a Standing Wave Ratio meter, or a forward and reverse reading watt meter.

....lets see here....hook sharpener, pliars, 4/0 hooks...ah here it is my standing wave radio meter. I knew I packed it in my tackle box!

(kidding) Very informative. Thanks for the tech info.

So, what caused the burned insulation? Internal short in the wire?

[StickFish]

Sorry, I know its a lot of radio stuff mixed in there and I tried to make it as simple as possible. Don't buy a meter - I've got one and will be glad to test radios and antennas.

Internal short, only time I've seen one is due to tampering. Some CBers like to "pin" coax they think its a cool practical joke. Basically take a push pin and push it thought the coax and cut it off flush - pretty hard to find.

Without actually seeing what happened and looking the wiring over - nearly impossible to tell. Even small coax used on marine antennas can take a huge load and not loose its cool.

If it is a burn mark on the coax it would be a first for me. I've seen burn marks inside antenna tuners when they are subject to extreme Standing Wave Ratio miss matches and components inside them will arc to ground. I've never seen coax arc to ground or heard of it - but that doesn't mean it couldn't happen. After all radio is radio its all theory that is what makes it soo much fun. If it is happening it will be very audible like a spark plug cracking off.