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OK, what'd I do?

Discussion in 'General CB Services Discussion' started by KB1TPX, Dec 22, 2009.

  1. KB1TPX

    KB1TPX W9WDX Amateur Radio Club Member

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    I accidentally hooked up my Lafayette 120 mobile to the power supply in the house backwards. (red to black and black to red) for about a minute, before smelling that "hot electronics odor" and realized what I did.:oops:
    The meter tells me that it still receives and transmits, but no sound.
    Any tips as to what I fried would be appreciated.

    Here's the schematic:



    http://www.cbtricks.com/radios/lafayette/telsat_ssb120/graphics/ssb_120_sch.pdf

    Oh, by the way, it had the PA circuit disconnected to accomodate the extra channel switch via the pa switch.
     
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  2. TonyV225

    TonyV225 Supporting Member

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    With it being plugged in backwards for that long you should beable to look inside and see where your troubles are at for a starting point. Start by looking at where the power comes in where the power socket is. Besure to look at both side of the board (foul side and component side) if you look at the component/part side of the board you will see or smell burnt parts resistors, diodes ETC.

    If your lucky when you look at the foil side (flate side with curuit traces and none are lifted like a roller coaster track consider yourself lucky. Im thinking there will be a lifted trace due to the amount of time radio was plugged in backwards without popping a rectifier diode (protection diode.)

    traces can be fixed with jumper wires if doene carefully but you need to pull the case screws and take a visual to start.
     
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  3. KB1TPX

    KB1TPX W9WDX Amateur Radio Club Member

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    Didn't see anything upon first look. But I'll look again with the big light and the magnifying glass.
     
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  4. cominatyalive

    cominatyalive Active Member

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    Hey,

    Don't feel bad. Worse things have happened to the best of us.
    Good luck in your repairs & keep us posted on the outcome please.


    James
     
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  5. Robb

    Robb Yup

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    If/when Mr Fatty and Loosecannon read this thread - I'm sure they will both have plenty of insight and experience to give you with this problem.

    Also - you might prepare for the possible need to take it to a shop if you cannot fix it. Sure helps in the long way of thinking/looking at things...
    My guess - probably a diode.
    Mr F and LC will have the particulars
     
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  6. TonyV225

    TonyV225 Supporting Member

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    I am thinking since no rectifier diode went out and the thing still powers up that the power traveled allover in that thing. Todays radios have the protection diode (rectifier diode) the protects the radio from reverse polarity by the diode popping or blowing out but its a bit scary seeing that this particular radio still fires up telling me that the reversed DC current was able to run and play in that particular radio.

    Ive seen people add jumpers because they had no replacement diodes!! its like putting aluminum foil around a bad fuse because no replacement was available at the present time if someone bypasses a protection diode the next misshap is the radios demise in most cases.
     
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  7. Radioman1

    Radioman1 W9WDX Amateur Radio Club Member

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    Reverse Polarity

    I would look at the voltages on IC-5 TA-7205

    It is your audio output and since you said it appears to transmit

    and receive but has no audio, it would be a good bet, examine it

    closely for a hole burned in it.

    It mounts to the side of the chassis near the back of the radio close to

    a large transformer and close to the output coax connector.

    Let us know.....Mac
     
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  8. MisterFatty

    MisterFatty Poppin Fresh!

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    Radioman may be in the right area, have you checked to see if you still have abnormal heat on the chassis?

    From memory, many years ago I had a similar radio, tried using it on an old Positive ground truck, its been a while, and I was only 13 years old at the time.

    From what I remember it took out the modulation transformer (melted laminations) Radio still worked, but no more TX/RX Audio, ordered and replaced the Mod transformer and it worked again.

    look over the Mod transformer closely for signs of over-heating.
     
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  9. Radio Tech

    Radio Tech Radio Operator

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    Good advise has been given.

    Look and see if D51 is even there. When the radio is hooked up backwards D51 should short and blow the fuse. This blocks reverse current from entering the audio circuit.
    Possibilities is the radio had the incorrect fuse installed. This would blow D51 and open the path accross it. Reverse voltage now feeds into the audio chip (IC 5) and takes it out along with the windings in the transformer (T16).

    Another possiblity is the radio has been hooked up backwards several times in the past and the diode went quick when you hooked it up.
    These radios had some hefty solder traces back then. So the audio circuit takes the brunt of the load. Most radio's today would blow the trace.
     
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  10. Captain Kilowatt

    Captain Kilowatt Professional Amateur Staff Member

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    I have to ask this, what value of fuse were you using or were you using one at all? A properly rated fuse should have blown instantly if the protection diode was good and certainly blown in far less than a minute if the diode was bad or missing. If all else fails hook it back up again and watch for the point of origin of the smoke. :whistle:
     
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  11. KB1TPX

    KB1TPX W9WDX Amateur Radio Club Member

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    Thanks all for the advice. Gonna be pretty busy and out of town for a couple of days. I'll try everything when I get back. Too bad I don't have a mobile rig for traveling:censored:, oh well worse things have happened.
    Merry Christmas everyone!
     
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  12. KB1TPX

    KB1TPX W9WDX Amateur Radio Club Member

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    Shipped it out to Key CB in Warren, IN today. He figures it's the audio chip that fried. ($25.00) Well, whatever it is, he'll fix it.
     
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  13. Peddler

    Peddler SWR (sandwich with rum) Expert

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    [​IMG]

    LOL ... not LOL.
     
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  14. KB1TPX

    KB1TPX W9WDX Amateur Radio Club Member

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    The fuse was definately too big. I had just bought the rig 2nd hand and neglected to look at it. It wasn't until I crossed the wires and damaged it that I checked and found a 30A fuse.:headbang Lesson learned.
     
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