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Palomar 300a low receive on operate.

Discussion in 'Amplifiers' started by Jimbo165, May 18, 2018.

  1. Jimbo165

    Jimbo165 Active Member

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    20180517_214113_resized.jpg 20180517_214113_resized.jpg 20180517_214332_resized.jpg I have a Palomar 300a a older gentleman brought to me to try and fix for him. If you turn power on amp and amp on standby radio TX and RX good. Turn amp switch to operate and amp works fine as far as power out put but receive on radio drops down drastically . Put operate to standby and radio receive normal again. Board has three relays on it. First one switches operate , second is preamp and third tx rx standby I believe. The third switch I think should not engage until radio keyed and amp on operate but engages moment I use standby to operate. I have replaced diodes and transistors and am stumped any ideas????


     

  2. nomadradio

    nomadradio Analog Retentive

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    When this amplifier is on standby, you should still hear a "click" when the radio is keyed. Yeah, this sounds wrong, but it's the way this model is built. That "click" on standby serves to let you use the preamp while the transmit side is still on standby. This is a feature most other amplifiers don't have. And the reason that those amplifiers have only two relays. The third relay lets you use the preamp while the transmit amplifier is still on standby.

    To do this, that third relay has to go "CLICK" when you key the radio even if the switch is set to standby.

    I'll guess that you don't hear that "click" on standby when you key the radio.

    The other job this relay does is to activate the transmit relay when the switch is moved from standby to operate. A 300A that's working right will have TWO relays operating when the amplifier is set to operate and the radio is keyed. (AND the preamp is off).

    The transistor at the lower left of your picture is the same one that's at the lower-left corner of the schematic you included. This one senses the radio's RF power.

    Remove this transistor from the board. Put the board back in place, fasten the ground screw and see if the radio now has receive with the switch set to operate.

    If the radio receives now, the problem lies with that transistor. Maybe you turned it around the wrong way. The 2N2905 transistor the factory used was round and made of metal. Your transistor is plastic. Should work, if it's the right type. ***AND*** it's turned around the right way.

    Small detail: If it got turned around wrong, this can damage that type transistor.

    For that matter, what type did you use to replace the original metal transistors?

    But that's the quick test. Pull it out and see if you can receive with the switch set to Operate now.

    73
     
    #2 nomadradio, May 20, 2018
    Last edited: May 20, 2018
    Jimbo165 likes this.
  3. Jimbo165

    Jimbo165 Active Member

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    Thank you nomadradio it is fixed now. I made a mistake on that transistor when I replaced it . I had mismarked board as for orientation and had that transistor wrong. When I took it out to test like you told me I found out what I had done. I am thankful for your help and the older gentleman LOL cause I am 68 myself is grateful . I just enjoy learning about these old amps. Thank You again.
     
  4. nomadradio

    nomadradio Analog Retentive

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    Rock on!

    Good to hear you got it straightened out.

    73
     
    Jimbo165 likes this.

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