Question about Cobra 29 LTD CB Radio

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Keep blowing fuses

I keep blowing fuses when I plug in power cord.

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  • Anonymous Aug 04, 2008

    i used 18 watt speaker wire and wired my cb radio to the batterie with it and it keeps blowing the fuses.

  • powerwagon78 Aug 06, 2008

    i plug the power cord in and i instantly blow a fuse



3 Answers

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More than likely the power plug was turned around the wrong way when plugged in, and it burned ou the protection diode. This is a very common thing to happen and very easy to fix.

If you have and can use an ohm meter, any multimeter than can check resistance works, pu it on the Rx1 scale and put the black test lead to the black power lead and the red test lead to the red power lead. If you blew the diode, it will show up as a short, which is about 0 ohms, or continuity. Then reverse the red and black ohmmeter test leads, red to balck and black to red. It should still show a short.

Note: On a few CBs, the diode comes after the On/Off switch in the circuit, so you may not see the short until you turn the power switch "On".

If this is burned, reply here or you can send me an email at and I will walk you through the changing of the diode. To change it, you need a rectifier diode you can pick up at Radio shack, I believe any one between 1N4001 through 1N4007 will work. I believe the radio shack down the road from me carries the 1N4005. If you don't have a radio shack, go to and order one. I oder from them all the time, they are a very reliable company. They carry the 1N4007 for $0.09 each.

You also need to have a desoldering iron, or some thing to remove solder from a cuircuit board, a soldering iron, solder, and a pair or needle nose pliers or something to hold the diode.

Once you are ready, like I said, let me know and I will tell you which number it is in the radio, how to locate it, and what to do to replace it.

Dean Pitts
Rollin CBs
Mt. Vernon, TX

Posted on May 12, 2008

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You may have a shorted power amp... check for ic's that may be defective. desolder the suspect ic and then test the power again.

Posted on May 12, 2008

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Amp keeps blowing fuses

Blowing fuses, was caused by many thing.
Short of rectifier,resistor,elco,tube heater,tube pin, and also trafo itself (primary or secondary winding). It must check step by step.

The simple steps (see picture) are:
  1. Disconnect all connection from secondary of power trafo (PLV,PDCH,PACH and PHV).
    Power ON the amp. If fuse is not blowing,trafo is good.
  2. Power OFF the amp. Unplug the cord.
    Connect PLV1-PLV2-PLV3 (this are Bias voltage,+/-15VDC and Backlight PCB voltage) to power trafo.Power ON the amp. If fuse is not blowing,PLV circuit is good.If fuse blowing,there are parts is damage (dioda,elco,7815/7915,resistor) in this PLV circuit.
  3. If step 2 is good,go to next step.
    Power OFF the amp. Unplug the cord.
    With PLV circuit stay connected,connect PDCH 1-2 and PACH 1-2 to power trafo. Power ON the amp.
    If fuse is not blowing,PDCH and PACH circuit is good and all tubes heater/filaments are lights after a few seconds.
    If fuse blowing,there are parts is damage (dioda,elco,tube heater pins) in this PDCH/PACH circuit.
  4. If step 3 is good,go to next step.
    Power OFF the amp. Unplug the cord.
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    If fuse blowing,there are parts is damage (dioda,elco,tube pins) in this PHV circuit.
  • Steps had to be done sequentially from 1 to 4, not upside down.
  • If tube/heater pin is short,it can't be repair. The tube must be replacing.
  • PDCH circuit is voltage for V1-2-3-4 heater/filaments.
  • PACH circuit is voltage for V5-6-7 heater/filaments.
  • Always replace fuse with same type.


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