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Receive transistor in amp - same/same?

Discussion in 'CB Radio Modifications' started by Robb, Oct 1, 2016.

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  1. Robb

    Robb Yup

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    20161001_134528.jpg



    Working on a local op's 'King Bee 150/250' amp. The receive amp doesn't work; receive goes dead when switched on. What transistor is used? What substitutes can be used? Schematic for this amp available/link? Possible biasing mods?

    Don't usually work on CB amps, just radios. Turned down the AM carrier to 2w to keep it from splattering. ANY/ALL info appreciated and welcome, as I can't seem to find much info at all about it.

    Thanks.
     
    #1 Robb, Oct 1, 2016
    Last edited: Oct 1, 2016

  2. 222DBFL

    222DBFL Sr. Member

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    Let me post a pic of the inside of my Palomar amp that looks similar. The switches are different, but the case looks identical to the one I have. Might help. Not sure. But here are some pics to compare. Again, not sure this will help, but here goes.
     

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    #2 222DBFL, Oct 2, 2016
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  3. 222DBFL

    222DBFL Sr. Member

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    If you need more pics let me kmow. I tried to get as many as I could.
     

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    #3 222DBFL, Oct 2, 2016
  4. 222DBFL

    222DBFL Sr. Member

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    Couple more, might be some repeats. Sorry if it doesn't help Robb. I tried man.
     

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    #4 222DBFL, Oct 2, 2016
  5. 222DBFL

    222DBFL Sr. Member

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    Some more pics
     

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    #5 222DBFL, Oct 2, 2016
  6. 222DBFL

    222DBFL Sr. Member

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    Last ones. Hope they might help.
     

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    #6 222DBFL, Oct 2, 2016
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  7. ExitThirteen

    ExitThirteen Grumpy and Cranky

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    Usually, a 2N2905 or a 2N2222 is used for the preamp transistor in an amp of that era.
    You can substitute a 2N2907 in for the 2N2905. All are cheap and readily available.


    ~Cheers~
     
    #7 ExitThirteen, Oct 2, 2016
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  8. 2RT307

    2RT307 Sr. Member

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    Hey Robb, just curious... what's in the thing? MRF 455's?

    73,
    Brett
     
    #8 2RT307, Oct 2, 2016
  9. Robb

    Robb Yup

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    Yup.

    Correction:
    MRF454's
     
    #9 Robb, Oct 2, 2016
    Last edited: Oct 2, 2016
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  10. Robb

    Robb Yup

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    Don't know much at all about amps; so bear with me.

    When the amp is off, I am seeing a 1 1/2 watt carrier loss thru it into the dummy load. IOW, 3w in to the amp, and 1 1/2w out of the amp when turned off. I'm sure this can't be correct.

    BTW - what is the proper input power to mrf454's?
    5w or 10w/each?

    Took out the mrf454's and tested them. They tested OK with both the diode test and resistance test. Also not seeing anywhere near 200w out of it, so I am at a bit of a loss to figure out why this amp isn't working as it should.

    What could be causing this? Bad relay? Bad coupling caps?
    Again; working w/o a schematic has been difficult.

    Also the original receive amp transistor is marked:
    "034
    MPS
    5172"
    So I have NO idea what part is in this thing.
    Any ideas?
    2N5172- maybe?
    Translates to equivalent 2N2904.
    Perhaps a suggestion for a replacement trans that has low noise/clean output?
    Like a 2N5088?

    blue_thunder_200_inside.jpg

    BTW 2222DBFL, this pic is similar to the inside of this amp and your amp; could you take a closeup pic of the small transistor near the very front of your box (circled in red in the pic), and can you read the numbers on the face of it and its orientation? That may help me figure out what transistor I need to get this amp fixed and off my bench. Much appreciated!

    Thanks
     
    #10 Robb, Oct 3, 2016
    Last edited: Oct 4, 2016
  11. bob85

    bob85 Supporting Member

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    10w drive should make about 200w output with a pair of 454's on 13.8v,

    i have come across amps that have high vswr and attenuate your power in bypass mode, cp163's do it,
    sometimes a trimmer cap from the bypass side of the relay to ground will tune out most of the mismatch.
     
    #11 bob85, Oct 3, 2016
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  12. 222DBFL

    222DBFL Sr. Member

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    Robb here is that part I think. The numbers on it are as seen on the pic below.
    Part #
    MPS2
    222A
    M594
    This is the part number I see. I hope this is what you are speaking of. I am no amp expert by any means either!! Again, hope I got the correct part for you.
     

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    #12 222DBFL, Oct 3, 2016
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  13. Robb

    Robb Yup

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    Seeing about 1/2 that - about 100w on AM.
    Transistors are out now and tested OK.
     
    #13 Robb, Oct 3, 2016
  14. 222DBFL

    222DBFL Sr. Member

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    That bottom row of numbers may be different than what I posted. Hard to tell. Here is another close up pic of it.
     

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    #14 222DBFL, Oct 3, 2016
  15. 222DBFL

    222DBFL Sr. Member

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    I'll see if I can find my magnifying glass and look closer at it as well. Give me a few as I am headed to the store now.
     
    #15 222DBFL, Oct 3, 2016
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