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On Sun, 14 Oct 2018 10:26:55 +0100, Jeff wrote:
I inspected the electrical system and found that fuse on the negative lead had blown. Why manufacturers persist in providing a negative wire fuse will remain a mystery as there are very few positive ground vehicles still in service and even marine radios with floating grounds are scarce. I have no idea where the radio was getting its ground return for reasons that will soon be obvious. I replaced the fuse and continued looking for problems. The negative fuse is nothing to do with positive ground vehicles (and a radio with the case connected to negative would not work in a positive ground vehicle anyway without additional isolation). The negative fuse is there to stop high currents, such as the starter motor, being drawn through the radio wiring and coax should the battery to chassis connection be high resistance or open circuit, and prevent a possible fire. This is at greatest risk if the radio negative is wired directly to the battery. Jeff That seems reasonable. However, I've never seen that happen. More common was blowing or removing the negative power cable fuse to the radio. That makes the DC ground return for the radio go through either the car frame, which will produce alternator noise on the transmit signal, or through the coax cable, which will produce a smoking coax cable in transmit. I've seen both about 5 times each in the last 50 years. I would consider these faults to be a greater risk than a disconnected battery to chassis (or engine) ground cable. "Wiring and Grounding" http://www.k0bg.com/wiring.html And as shown, the negative lead fuse should not be removed. The reason is, if the grounding point should lose its integrity, excessive current could flow through the transceiver's negative lead. It also prevents a minor ground loop between the leads. Most vehicles have a ground strap between the engine block and frame and another ground cable between the frame and negative battery terminal. I sometimes see a third cable from battery to engine block. In this arrangement, any one of the three wires could be disconnected and one would still have a tolerable grounding system. Fiberglass body automobiles have duplicate ground wiring since there is no frame ground. "How To Properly Ground An Automotive Electrical System" https://www.hotrodwires.com/how-to-ground-automotive-electrical-system.html "Is it necessary to have two ground wires (1 to engine and 1 to" car-frame)? https://mechanics.stackexchange.com/questions/5903/is-it-necessary-to-have-two-ground-wires-1-to-engine-and-1-to-car-frame -- Jeff Liebermann 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558 |
#2
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#3
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On Sun, 14 Oct 2018 13:39:38 -0400, Ralph Mowery
wrote: I don't know anything about the tractor trailer wiring, but around 1972 I picked up extra money repairing the CB radios. Several truckers brought in rigs that were blown up when switching from the car to the truck. None of them had a fuse in the negative power lead. By blown up, most had a diode across the poer wires inside the transceiver and that diode had shorted and blew the fuse. I seem to remember then they talked about the tractor haveing a positive ground. Probably ran on 24 volts also. I installed a few radios in White Freightliner tractors in the 1960's. They were all positive ground 12V. I think they switch to negative ground in about 1975. Many other older tractors were positive ground but switched to negative ground in about 1954-56. I vaguely recall conversion kits being sold at the time. I've seen a few 24V electrical systems, but those were all in military vehicles. I have only ran FM ham rigs in a car. I used to ground the rig to the frame and run the positive to a relay that comes on when the car is started. Never had any alternator whine or problems. For about the last 10 or more years I just plug into the lighter or accessory socket in the car. My Toyota power on those sockets only come on when the car is started. You might want to put a voltmeter across the power connector going to your radio and across the battery, and compare voltages in transmit. Methinks you'll find a rather substantial voltage drop through the cigarette igniter jack. Also, that connector was never designed to handle a plug and jack connector arrangement. It's the only connector that I know of that has a spring which pushed the plug OUT of the jack and lacks a retention system. -- Jeff Liebermann 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558 |
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