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Gates M-5693 Modulation Monitor

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

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  1. K4KYV

    K4KYV Subscriber QRZ Page

    Or if you have a grid dip meter, find the right inductance to hit resonance by trial and error. I have also done that by feeding an RF sample JS'ing a coupling link to a low-level stage in my exciter, and finding the resonant frequency on the coil using a neon lamp.
  2. W2WDX

    W2WDX Subscriber QRZ Page

    Been kinda busy with other matters here. However, I finally got around to measuring the inductance of the stock transformer in this thing. Leaving the tuning slug untouched, it seems the primary is about 114uH and secondary is about 820uH. So that seems to be about a 1:8 ratio rounding off a bit for measured error. Hmm ... would that also be a 50 Ohm to 400 ohm transformation at the frequencies it was designed for? (.54 to .98) Or am I seeing this wrong?
  3. W2WDX

    W2WDX Subscriber QRZ Page

    I know it's been a while since this thread was floating around, but I thought you all would like to know what I ended up doing. I decided to simply use the original device and carefully remove some windings until I got it to look about right on the dip meter. It seems to have worked, quite well in fact. There is plenty of voltage, and all the waveforms seem to be the way they should be internally in the mod monitor compared to the source signal. So I think I have this. Seems to reflect what I see on a scope on the modulation reading on the Gates.

    As far as pick-up, I went with something similar to what W2NBC suggested. I made the toroid large enough and with the best number of turns to get as much voltage possible, so the Gates is seeing about 10V @ somewhere around 100 ohms impedance. Like I said, I'm not looking for ultra-precision; just a good indication. I'll get QIX's monitor for any more precise measurements. However, I do think this Gates is fairly close. I only use it on 75M, so all the worry about multi-band was for naught. If I need to go elsewhere, I'll just build up different plug-in coils. I found the forms and plugin parts, so I can fabricate coils like the original design for different bands if I ever felt the need.

    I just want to thank all of you for your suggestions and discussion on this since it was helpful for me to think this through. I'm pretty good at some things and other things I am totally in the dark on, so hearing suggestions from others helps me learn. I am pleased that I was able to do this without kluging it; the Gates looks unmodified (something I always like to adhere to with historically significant items) since even the original plug-in coil is still intact and in use, just re-valued.
    Last edited: May 26, 2019
    W2VW and AD5HR like this.
  4. KE0ZU

    KE0ZU Ham Member QRZ Page

    REALLY late to the party here, but, I bought a Gates/Harris 6659 some years ago and it's input was simply a resistive input. I used a typical toroid transformer pick off, on the feed line as a source, dropping the original 75 Ohm input load.

    This pic is the original input circuit schematic.


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