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Cobra148gtl final&bias

Discussion in 'General CB Services Discussion' started by kaos513, Nov 13, 2017 at 2:26 AM.

  1. kaos513

    kaos513
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    Hello guys im looking to adjust my final and bias on my 148gtl can anyone help out to know what i need to align them ? thanks


     

  2. sonoma

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    you need a volt meter with milliamps on it.
    go down to page 5 of the manual item 4 and the section that says to align the bias. unplug the leads at the test points and hook the meter up to each lead and key the radio to adjust the bias.

    http://downloads.cobra.com/CB/Models/148-SERIES/148alignment.pdf
     
  3. kaos513

    kaos513
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    I did use my meter as you suggested its stays at 13 will not adjust
     
  4. sonoma

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    do you have it on miliamps DC area on the meter.. also check inside the unit most have a fuse that will blow if you go above what it is rated at. mine goes to 200ma and if I go higher it will blow the fuse. not sure what the 13 you have posted.
    to make sure you are doing it right you unplug the wire at the test point then put neg to one contact and the other wire to the other end. then key the mic. one wire is for the driver bias and the other wire is for the final bias. the wire that runs towards the driver is the driver bias ans the wire that goes closer to the final is for the final bias.
     
  5. kaos513

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    k will try another meter
     
  6. Klondike Mike

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    I've had all sorts of difficulties making this measurement with "crap" meters ... until I built my own.

     
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  7. nomadradio

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    Just be sure the radio is set to either sideband mode. Should not matter whether it's USB or LSB. The mike gain must be at zero so that room noises won't affect the reading.

    I will always take BOTH jumper wires loose while making this measurement. Any trace of carrier leakage from the driver transistor can change the current reading on the final transistor.

    This particular measurement gives a lot of folks trouble. More than once we find that the current-range fuse in a meter has blown. Typically because one of the meter's probes came loose and touched exposed grounded metal. Most meters use a fast-blow fuse to protect the "mA" input.

    Got in the habit of using the 10-Amp range of the multimeter. The correct readings will look like "0.060", but the fuse on that range won't blow if a probe briefly gets loose and brushes against the chassis.

    73
     
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  8. kaos513

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    So the last 2 numbers would be the mA reading on 10-amp on the meter?
     
  9. Robb

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    Nomad; you da man!

    Quick viddy for biasing the 148:



    EDIT:
    What happened in that viddy when the meter started beeping was that the driver biasing VR was dirty and wouldn't adjust. So I gave it a spritz of Deoxit cleaner; turned a few times and then adjusted it. Old VR's will have these issues from time to time. Best to give them all a spritz when reconditioning a radio.
     
    #9 Robb, Nov 13, 2017 at 9:32 PM
    Last edited: Nov 14, 2017 at 2:24 AM
  10. kaos513

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    Thanks will try it out and report
     
  11. kaos513

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    My meter is setup the same way as yours all i get is 13 will try another meter out its klien tools meter
     
  12. kaos513

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  13. sonoma

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    maybe that will work for you Kaos513.
     
  14. Robb

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    Yup

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    kaos:

    That meter - sad to say - has a limit of 100mA. Problem is - is that a driver or final VR will change greater than that. What happens if the bias circuit of the radio under test is waay out of adjustment - like @ 350mA? Well; it will peg that needle and possibly burn that meter.

    The real issue to solve your dilemma has already been pointed out by Nomad. There are two fuses inside any meter. One is for the 10A current test and the other is for the mA current test. It is not uncommon for a newbee to to accidentally pop the mA fuse in your meter. Done it a few times myself.

    " . . .This particular measurement gives a lot of folks trouble. More than once we find that the current-range fuse in a meter has blown. Typically because one of the meter's probes came loose and touched exposed grounded metal. Most meters use a fast-blow fuse to protect the "mA" input.

    Got in the habit of using the 10-Amp range of the multimeter. The correct readings will look like "0.060", but the fuse on that range won't blow if a probe briefly gets loose and brushes against the chassis. . ." (Nomad)


    That's sound advice. I suggest that you find a way to test the mA fuse (the smaller of the two fuses) and get on with using your meter. They are best because they have a much greater range than the meter you just bought - sad to say.

    Don't look at all of this as all bad; look at it all and treat it all as a learning experience that will widen your own mental database as you strive to work on radios. We are all pulling for ya to that end. Cut yourself some slack, as making mistakes is usually the best way to learn.

    Robb
     
    #14 Robb, Nov 14, 2017 at 9:48 PM
    Last edited: Nov 15, 2017 at 2:25 AM
  15. kaos513

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    i bought meter i will report my readings here eductional puproses and will post the settings as well
     

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