MX-P50M Problems

Discussion in 'Amateur Radio Amplifiers' started by WS4JM, Apr 14, 2019.

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  1. WS4JM

    WS4JM Ham Member QRZ Page

    I bought one of these at my local hamfest. I worked well for a few days then I had trouble with it with bad audio dropout. After doing more checking the bias was low and I could only get about 40mA at around 2.3V. Replaced the MRF-186, set the bias around 400ma @ 4.5v and the amp worked fine for a while. Tried it again and I am now back where I started with bad audio and dropout and low bias.

    What have I done to this poor amp and how can I prevent this in the future? Any help would be greatly appreciated, using a MicroBitx at 4 watts as the exciter.

    Jay
     
  2. BG6GLL

    BG6GLL QRZ Member

  3. WS4JM

    WS4JM Ham Member QRZ Page

  4. WS4JM

    WS4JM Ham Member QRZ Page

    Update, I replaced the blown MRF-186, set the bias at 250mA (~3.75V). No dropouts, about 40 Watts output with 3 Watts drive. I think this will suffice.
     
  5. K1BAA

    K1BAA Ham Member QRZ Page

    Where are you measuring the current and voltage? I am needing to replace my MRF 186 as well.
    73 K1BAA
     
  6. WS4JM

    WS4JM Ham Member QRZ Page

    I put my ammeter in line with the power connections. Measure the voltage at the top of R4 or R6

    https://www.qsl.net/wb4kdi/BG4IGX/mx-p50m.html
     
  7. KI7AJA

    KI7AJA Ham Member QRZ Page

    How do you change the biases? Is it the little potentiometer labeled “W3”?
     
  8. WS4JM

    WS4JM Ham Member QRZ Page

    Exactly.
     
  9. KI7AJA

    KI7AJA Ham Member QRZ Page

    Ok thanks
     

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