Gates M-5693 Modulation Monitor

Discussion in 'Amplitude Modulation - AM Fans' started by W2WDX, Jul 11, 2018.

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  1. W2WDX

    W2WDX Ham Member QRZ Page

    So I'm sitting here looking at a Gates M-5693 modulation monitor I picked up on ePay. I do plan on buying the REA one soon, but I have this Gates which is in very excellent condition. Some of you may know Gates supplied two input transformers for each end of the AM BC band, one for 540-900 and another for 900-1600. I'd like to use this on the amateur bands, so I'm pondering what it would take to add transformers or a multi-tap transformer or if I really even need to worry about it. In the schematic below the tranny in question is "XT2". Anybody have thoughts or any experience?

    Here's a schematic of the RF input:

    gatemodinput.JPG

    It just that unit is in really good shape inside and out, so it would be nice to have some real swinging meters going in the station. I assume any sampler that would provide the correct voltage would work fine as a tap off the transmission line, right?

    Thanks

    John, W2WDX
     

    Attached Files:

    Last edited: Jul 11, 2018
  2. N2DTS

    N2DTS Ham Member QRZ Page

    Maybe just try removing C1 and tune C2.

    I am sure there is a broad band input method...what about a resistive network?
     
  3. N2DTS

    N2DTS Ham Member QRZ Page

    Oh, a long time ago I had a General Radio mod monitor, looked close to your gates but used octal tubes.
    It had an input trans with taps on it you had to change for different bands.
    I later got an RCA (Bellair) solid state one that was broadband.

    The REA is great, you get a scope display also, and good clean audio out.

    One advantage of the old ones is you could set the carrier to 50% and read double the positive modulation, 100% would really be 200%.
    70% would be 140%.
    In the old days, some would try and get very high positive peaks, even though it would distort in most detectors...
    We also tried to get very low frequencies through a transmitter, another silly thing to try.
     
  4. W2WDX

    W2WDX Ham Member QRZ Page

    I'm sure there is too, I just have never seen one and I can't wrap my head around one right now. I have to pull the one inside this thing and see what it is exactly. May be able to tap it somehow, depending on how its constructed. How about simple capacitive coupling? That might be wonky.

    Well I hope someone reads this that has done it already, it's a cool thing and would like to get it working with a minimum of modification. Fortunately, it's a plugin transformer, so mods could be "free hanging", if you know what I mean.
     
  5. AC0OB

    AC0OB Subscriber QRZ Page

    Hi John,

    According to my analysis, the front end system secondary is simply a parallel resonant circuit.

    Now the median capacitance is approx. 86 pf considering (C1 || C2). with a total range of ~ 30 to 164 pF

    Neglecting the inductance of the primary (which is most likely just a few turns anyway) and mutual inductance, you could wind your own coils with the following values assuming once again the median value of capacitance is approx. 86 pf:

    1.8 MHz, L = 90 uH
    3.75 MHz, L = 20 uH
    7.2 MHz, L = 5.5 uH
    14.2 MHz, L = 1.5 uH.
    28 MHz, L = 0.4 uH


    Pheel

     
  6. N1BCG

    N1BCG Ham Member QRZ Page

    Yours IS in great shape. You’ve inspired me to restore one of those that’s been sitting around. I hope I’ve got all the tubes...
     
  7. W7TFO

    W7TFO Subscriber QRZ Page

    BC AM mod monitors of that era were meant to be driven from a tapped coil across the output line.

    Don't even try a cap coupling, they will introduce phase angle problems and it will lie to you.

    73DG
     
  8. AC0OB

    AC0OB Subscriber QRZ Page

    I wouldn't cap couple the front end either.

    What we need to know is what are the turns of the present XT2 transformers, then maybe we could scale it, but it would still need calibrating for each band.


    Pheel
     

    Attached Files:

  9. N1BCG

    N1BCG Ham Member QRZ Page

    One perk of being single is not anticipating the inevitable holler:

    “You’re not cleaning that rusty piece of junk on the clean counter meant for food, are you?!”

    CEB74A88-7848-45D6-8352-1D28CE5587F4.jpeg 41ED1269-E62C-4D5C-939F-97A75E1768D6.jpeg

    Uh, no, dear...
     
  10. W2WDX

    W2WDX Ham Member QRZ Page

    It's hard to tell. The original I have is one of the low band ones 540-900. It's one of those Millen 4-pin coil (45004?) with a tuned slug type winding (coil above coil). It's hard to see inside the Millen housing and it's probably wired with I'd say #20 enameled wire, maybe smaller. Lots of turns. I could take a picture and post it.

    And Pheel, the schmatic you gave is what exactly?
     

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