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c.b no power up

Discussion in 'Troubleshooting and Repair' started by dantheman1972, Jun 29, 2013.

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  1. dantheman1972


    May 29, 2013
    i have a Uniden Grant LT mobile c.b. that won't power up. The previous owner hooked up the psu in reverse polarity, and i have identified two blown (actually broke/burned in half ) diodes which i suspect are protection diodes for the very purpose of the original mistake (reverse +/-)

    Question>>>what kind of diodes would be used to replace these with? like a 1N4007 power diode or a 1N34A R.F. diode? I'm new at this but a Ham op so i know the general theory but I'm by no means an expert. any help?
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    They're probably rectifier diodes, but...

    If they are placed in such a way that they will dissipate lots of power when the power is connected the wrong way, they are designed to blow the fuse.

    If they have been destroyed to such an extent that they are open circuit, this implies that a vastly too high value fuse was used (or no fuse at all).

    This would result in protection until the diode went open circuit (maybe a second or so) followed by reverse voltage to the device which may have done substantial damage.

    If you are exceedingly lucky, the diode may have failed short circuit as the fuse blew and the further damage was done with the power connected the right way around. In this case you would not be reporting the device unable to power up.

    The best advice is to provide us with images of both sides of the circuit board, sharp enough that we can read all the writing on the board and components. If necessary, a closeup of the affected area may be useful. Take these pictures perpendicular to the board in indirect light (i.e. shade or under cloud cover) and without the use of a flash.

    We have has a case where a reverse application of voltage resulted in only the destruction of an on-board regulator, so all is not lost. However, as much honest history of the fault that you can obtain would be useful.

    Just read that you are a Ham. If you can trace the circuit from the power input to those diodes, it would be of great assistance.
  3. davenn

    davenn Moderator

    Sep 5, 2009
    Hi ther
    welcome to the forums :)

    a 1N4007 doesnt have the current rating for that
    No definately not a RF diode :)

    go for something like a 1N5404 or similar, you need something that can handle at least 4 Amps

  4. dantheman1972


    May 29, 2013

    ok, thanks to both of you for the answers i know the owner said the unit worked fine before he screwed it up.. i know what a regulator looks like, this one has 5 or more in various places around the board w/ a heat sink soldered to the side...
    which one would be the actual one most likely to go? I would assume the one nearest the power supply side? idk.. I will; try to replace the diodes as with what Dave has suggested, if this gets me nowhere then I shall start looking into what Steve has said ..
    oh and the fuse was the same kind as original (owner took fuse to radio shack and got a replacement for the original which i assume was correct as it was most likely original to the radio as far as he remembered.)
    If the radio wasn't worth saving i would use it for parts, but its a SSB 40ch unit (im not a cb'er..but...) I traded one i fixed before for 2's worth it to me
    i also have a Saturn Turbo cb/ham/sorta unit that needs a board but more on that later, thanks to both of you..I'll report back in a day or two with results..73, KD2CWM
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