Triplett 1295 Modulation Monitor

Discussion in 'Amplitude Modulation' started by N1BCG, Jun 20, 2021.

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  1. N1BCG

    N1BCG Ham Member QRZ Page

    Has anyone seen one of these before?

    [​IMG]

    It sold on ebay recently and caught my attention for its simplicity.

    [​IMG]

    [​IMG]

    I almost bid on it but decided to try my hand at making one. How hard could it be? Here's the result:
    BasicModMonitor.jpg

    Aside from not having peak indication, it does show average modulation fairly accurately when calibrated. I used a 0-100uA meter for the carrier level, and when the modulation (or VU) meter was set against a scope, the 10k pot was adjusted for a midscale (50uA) reading. This meter could even be 50uA fs since there was plenty of headroom. The circuit needed just under 5V PtoP for proper modulation indication.

    The 400pF cap value was just enough to prevent RF from affecting the modulation meter, preventing it from zeroing, yet did not affect high frequency readings and was tested out to 20kHz. This was tested in the BC band so a smaller value could be used for 160m and up.

    The carrier level meter only reacts to carrier shift and is quite steady with symmetrical modulation. I was surprised how a simple circuit could separately read RF level and modulation compared to the multi-op amp circuit in my homemade mod monitor!

    I thought I'd post it here in case someone had some meters and parts needing a purpose.
     
    Last edited: Jun 20, 2021
    W2BTK, SM0GLD, AD5HR and 1 other person like this.
  2. W3SLK

    W3SLK Ham Member QRZ Page

    Yep I have one. Its been 'solid-stated' by the previous owner, Ed, N3GWE(SK), the 'Great Watt Emitter'!
     
  3. N1BCG

    N1BCG Ham Member QRZ Page

    I’m super curious... how close did I come to the Triplett schematic?
     
  4. W3SLK

    W3SLK Ham Member QRZ Page

    I'll have to look. Its been a loooooooong time since I last checked out the papers I got with it. But if I can remember correctly, Ed said that he removed the 80 tube(?) and replaced it with a matched pair of crystal diodes. I may have to replace them, mine only the carrier meter works.
     

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