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help with amp

Discussion in 'General CB Services Discussion' started by 1iwilly, May 4, 2021.

  1. 1iwilly

    1iwilly Well-Known Member

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    need help to find what could be the problem i have a solid-state 200 base amp with 2 2879
    i already check all the connections and the jumpers all check good. the issue is only when using my 29 classic with the amp on the AWI comes on the radio while talking even when i have a dummy load connected just to eliminate my antenna


     

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  2. Big Kahuna

    Big Kahuna Sr. Member

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    So did you check your SWR with and without amp inline and with amp on? How far from the radio is the amp placed (right next to it or 6 feet away)?
     
  3. nomadradio

    nomadradio Analog Retentive

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    The input circuit of the amplifier is where the radio is feeding power into when the amplifier is keyed. The radio is only attached to the antenna when the amplifier is off.

    The amplifier's input circuit should fool the radio into thinking that it's connected to an antenna with a low SWR. The input circuit in this amplifier is throwing the radio a high SWR, triggering the AWI light.

    That amplifier model typically has multiple problems. Unless you just want to make a project out of this one, I'd sell it and try your luck again.

    It will bring more money before it blows up than it will after.

    First problem we're used to seeing from a Gray 200 is the power supply. Your pics don't show it, but it was built with an unregulated power supply. It puts over 20 Volts onto the two expensive 12-Volt RF transistors. Usually causes them to fail prematurely.

    There's a bonus. The power transformer it was built with is too small and also tends to overheat and fail if the transistors manage to last for any amount of time.

    Maybe yours has an updated power supply. That part of it is not visible in your pics. Can't tell.

    But wait, there's more.

    Fixing the high input-side SWR that's triggering your warning light will be a project in itself. Requires some rewiring and adding some components.

    Unless you bought this amplifier to be a project, I'd recommend finding it a new home.

    73
     
    BJ radionut likes this.
  4. Beetle

    Beetle Sr. Member

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    "6 feet away"? Social distancing for radios...
     
    S&W357 likes this.
  5. Big Kahuna

    Big Kahuna Sr. Member

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    :) And you need to put a mask on them.

    I asked because I do remember a while back where I had a radio sitting on top of an cheap CB amp using a short jumper and had issues until it was moved further away. I couldn't remember if it was the SWR light but I remember something wacky which is why this triggered that memory.

    Sounds like Nomad is pretty familiar with the issues with this amp. Note to self never to buy this particular model :)
     
  6. 1iwilly

    1iwilly Well-Known Member

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    no i bought it the way it is meaning everything inside is still original except for the 2 2879 swr are 1.3 with amp on or off and i don't use it but for 5 or 10 minutes when there is skip so i guess i'll start looking for a 200 to 300 tube amp don't need more than that thanks for the reply



     
  7. 1iwilly

    1iwilly Well-Known Member

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    here is the rest of the inside pics
     

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  8. nomadradio

    nomadradio Analog Retentive

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    Yep, power supply is original.

    The "28 Volt" output rating is really two 14-Volt windings, each one feeding one rectifier diode.

    Input side is marked "117 Volts". Considering that the typical residential outlet is supposed be between 120 and 125 Volts these days, the actual AC output will be over 15 Volts. Means the DC output will be around 21 Volts.

    Like I said, before it blows up.

    73
     
    ExitThirteen likes this.
  9. ExitThirteen

    ExitThirteen Grumpy and Cranky

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    Those 2879's don't belong in there, that's partly why your SWR is so wacky. The input/output tune will be way off, and the wraps on both the input and output transformers will also be incorrect, as those amps originally came with MRF455's (or similar transistor). The non-regulated supply will also be the death of those 2879's, since it floats at 21V+ as Nomad mentions. Also, the 2879's will draw too much current than what the stock supply can handle. That amp as it is, unfortunately, is a ticking time bomb.


    ~Cheers~
     
  10. Beetle

    Beetle Sr. Member

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    Back in the 1960s, the "normal" home AC supply was specified as 117 volts.
     
  11. 1iwilly

    1iwilly Well-Known Member

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    ok thanks for the advice i'll be putting it for sale for what i paid $150.00 like i said i only use it for about 5 to minutes when i hear skip so it never even gets warm
     
  12. boatman

    boatman New Member

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    just came up with a black cat base amp, two small tubes, one with no markings, the other with 7189a on it. It has a tuner on the back and the meter on the front works. I am getting about 30 watts. Should it have different tube(s) or what?
     
  13. BJ radionut

    BJ radionut Supporting Member and 6m addict

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    The Blackcat JB-12 was from around 1975 or so.

    It was listed as a Base Modulator with 25-40 watts.

    Some people got more power out of them depending on drive but 30 watts is in the ball park for power.

    Black Cat JB 12 amp.jpg


    Very LOW Drive Amp about 3-4 watts PEP MAX!!!!
     
  14. boatman

    boatman New Member

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  15. boatman

    boatman New Member

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    Really appreciate it. Thanks again, Art
     

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