Gates M-5693 Modulation Monitor

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

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  1. AC0OB

    AC0OB Platinum Subscriber Platinum Subscriber QRZ Page

    DTS: "Phase distortion?
    That would show up as what?"

    Did you place a Distortion Meter on the detected output to determine how much distortion was present? That is how it is normally done in the broadcast industry.

    DTS:"The new REA mod monitor sounds the same, so I assume NO phase distortion."

    Looking at the circuitry of the REA detector, it samples the RF signal directly via a dropping resistor before the diode detector from the transmitter via an external "T" connector, and then goes on to the interface unit.

    DTS: "As far as % of modulation, I set it using a scope, the mod monitor matched the scope, changing bands did not change anything."

    That is one way to calibrate a Mod Monitor but not the most accurate.

    Attached Files:

    Last edited: Jul 12, 2018
  2. AC0OB

    AC0OB Platinum Subscriber Platinum Subscriber QRZ Page

    You could try it and see if it is able to detect those low level signals but according to Bird:

    "4273 Series, 1.5-35 MHz, THRULINE® Variable RF Signal Samplers are low frequency couplers for the 1.5 to 35 MHz band and offers a sample that ranges form [from] 40 to 70 below the main power level..."

    Last edited: Jul 12, 2018
  3. K5UJ

    K5UJ Ham Member QRZ Page

    I have a homebrew pickup and network, bandswitched for 10 to 160 m. that was built to work with a vintage RCA mod. monitor, probably very similar to the Gates. It's a box with UHF jacks and a copper line running between the jack centers. A toroid a little over 1/2 inch diameter circles the line between the jacks. The toroid has around 4 or 5 close spaced turns on it and one end of the turns goes to a BNC jack mounted on the box. That's the jack for coax to the mod. monitor. The other end of the coil goes to a bandswitched series of fixed and variable caps, series and paralleled to ground. The variable cap is a two section receiving cap, like what you'd find as a loading cap on a novice rig pi network. There are only a couple of RF padding caps. Looks the the variable does most of the tuning, maybe all of it above 40 m.

    You can build the box with the jacks and the pickup coil then run RF through it to a DL from a novice CW rig and on each band of interest use clip leads and caps to ground to find the capacitance value that peaks the RF available on the BNC, which can be connected to some sort of RF indicator like a neon lamp. Spread it all out on a plastic board and try different values of C. Once you have peak illumination on each band you want to operate on, measure the capacitance and wire it all in with some kind of band switch arrangement. If you have a 3 or 4 section small air variable that may be all you need, adding sections as necessary.
  4. W2WDX

    W2WDX Subscriber QRZ Page

    I do understand what DTS is talking about, a shift in phase angle could cause nulls giving a false reading. However, not sure how that would happen unless the signal is mixed with itself. Otherwise it's amplitude whether phase shifted or not should measure the same. So which is it boys?
  5. N2DTS

    N2DTS Ham Member QRZ Page

    I never had any problems with phase shift in any mod monitor I ever used.
    They sounded good and matched a scope display after calibration.

    If you want broadcast spec's and accuracy, you might need special stuff...
    You don't want a fine from the FCC for over modulation!

  6. W2WDX

    W2WDX Subscriber QRZ Page

    @N2DTS Ha! Well like I said I am going to get Steve QIX's REA monitor, which I am sure is accurate. I just got the Gates since it was in such nice shape, so I don't want to put to much effort into getting it working. It's more for fun and looks, but I do want it to be fairly accurate, at least match the scope or the REA. Ya know, maybe it was Dennis TFO that mentioned the phase issues. Whatever.

    BTW I also have this which I got off that "other" AM site years ago. Would this work? I mean I would bet a simple toroid around a center insulator of some 9913 or 58 or such would work fine. Would be nice if I don't have to build right now. Got so many other things to right now with the new station build. LOL

    Last edited: Jul 12, 2018
  7. AC0OB

    AC0OB Platinum Subscriber Platinum Subscriber QRZ Page

    Diodes are inherently non-linear and one of the things they can do is to mix various signals such as baseband signals with phase-shifted signals.

  8. AC0OB

    AC0OB Platinum Subscriber Platinum Subscriber QRZ Page

    My belief is that with an upgraded M-5693 (good tubes, new caps, and replacement of off-tolerance components), and with a proper pickup coil (as Rob, K5UJ suggested), I think this instrument can do a great job of monitoring modulation, without the need of a computer.

  9. W2WDX

    W2WDX Subscriber QRZ Page

    The tubes are all good; and it's simple enough so going thru all the components and replacing any out of value, replacing all the caps should only take me a day or two. So I will do this, as I do anyway with just about everything I have. I do like Rob's suggestion and have been scribbling away on a design. Hey ... if it works well, maybe Steve will have one less sale.

    Nah ... I like Steve. I'll give him the business regardless. And I have a big RF-proofed workstation computer in the shack already. That's what I'm using right now.

    In addition to everything else, I'll post images of the final sampler once I do finish it up and test it.
    Last edited: Jul 12, 2018
  10. W2NBC

    W2NBC Ham Member QRZ Page

    Your Monitor looks great!
    Here's another idea that works well with my Gates M6659 (produced later): RF to Mod Monitor.jpg

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