Discussion in 'Amplitude Modulation - AM Fans' started by W2WDX, Jul 11, 2018.
That was a redrawn circuit of the RF "front end" of the M-5693 up to L1 and L3.
Oh ... of course, d'oh.
Looking at some old Gates BC transmitters and I see the tapped inductor for the monitor feed on the outpoot. How to emulate this for multiband amateur? Multiple taps? I could just set this up for 75m and be done with it. I could really use some help on this. Just not familiar with the BC way of doing things.
Let me ask this, I have a Bird RF Sampler variable (not a directional coupler) #4273. I have never cracked one of these open to see what it is, but would this work I wonder?
What transmitter are you using, and what kind of output network does it have?
The input voltage range as given by the M-5693's manual (Pages 4,5) is about 10 volts with an input impedance of ~ 97 ohms.
The pickup coil could be something as simple as a ferrite core with the antenna wire going through it as shown below:
We just need to think about what we are doing.
The input circuit is providing an RF sample to the rest of the circuit.
Why does it need to be a tuned circuit?
The only RF it is going to see is the transmitter RF, by far.
I suspect all the modern mod monitors have the same basic design and do not need a tuned circuit.
My RCA had no tuned circuit and worked fine up to 20 meters which was the highest I tested it.
To feed that mod monitor, I had a high voltage air variable cap that gave an rf sample off the T/R contactor.
About 50 pf at 4000 volts (to be safe).
I adjusted the sensitivity control (pot) on the mod monitor to the most sensitive and adjusted the sample knob (air variable cap) to center the carrier meter.
Worked fine 160 to 20 meters, 20 watts and up to 700 watts carrier.
I would just experiment with simple coils, or resistor networks, toroids, etc.
I don't think its critical, its just an RF sample.
Verify it works ok with a scope, broadcast may have had to do it the way the FCC said for some reason that may not matter to you.
Maybe look at the RCA/Bellair BW50 circuit and see what they did on the input.
They had the same diode setup....
OK ... most of the transmitters and amplifiers I have are typical pi-net outpoots. Like my small Gates or my Hallicrafters HT-37 or HT-41 amp. Now I'm fine looking at modulation before any amplification. So typical Pi-Net. I would like a calibrated measurement if at all possible, and if that means using this just on one transmitter, like the small Gates M-5078 I have, that's fine. Would a 50W input transmitter be able to deliver the 10V @ ~97 ohms? Should be no problem, right? Here's the tank for the Gates M-5078:
Boy I'm getting lazy in my old age!!! LOL Thanks for your continued help on this.
I was just thinking. I use a Johnson KW Matchbox. A tap off the coupling coil? Or just a separate loosely coupled coil or loop?
DTS: "...The input circuit is providing an RF sample to the rest of the circuit.
Why does it need to be a tuned circuit?..."
In the case of the M-5693, it was designed that way so the detector could provide a high level of detection signal in order process it later on.
DTS: "I suspect all the modern mod monitors have the same basic design and do not need a tuned circuit."
No they don't. Older monitors based on vacuum tubes used a variety of narrow band front end systems. even newer monitors have different designs.
Later monitors based on IC amplifiers and discrete transistor circuitry were able to develop enough gain and to self calibrate to the point they could be widebanded.
DTS: "To feed that mod monitor, I had a high voltage air variable cap that gave an rf sample off the T/R contactor. About 50 pf at 4000 volts (to be safe)."
And how much phase distortion resulted?
DTS: "My RCA had no tuned circuit and worked fine up to 20 meters which was the highest I tested it."
DTS: "I would just experiment with simple coils, or resistor networks, toroids, etc.
I don't think its critical, its just an RF sample."
It is critical if you want a signal that is accurate wrt modulation percentage.
DTS: "Maybe look at the RCA/Bellair BW50 circuit and see what they did on the input.
They had the same diode setup."
No, Belar did not use the same front-end diode setup nor the same circuitry as the M-5693.
That would show up as what?
The audio output sounded great, and better then any other method of monitoring the signal.
The new REA mod monitor sounds the same, so I assume NO phase distortion.
As far as % of modulation, I set it using a scope, the mod monitor matched the scope, changing bands did not change anything.
Many of these were fed from a tapped coil at the antenna feed. The inductor was placed between the LowZ RF output and ground with the mod monitor tap near the cold end.
What you want to see is the full modulation envelope as it exits the transmitter's final RF output port.
One reason the M-5693 has a potentiometer on the input is to reduce voltages.
Wrt JKW Matchbox, tapping off the coil would not only detune it, but where would you tap off? There are nodes on the Matchbox coil that could yield very high and damaging voltages.
My recommendation would be to pick up the signal between the transmitter's output and the Matchbox.
@AC0OB I have that Bird 4273 RF sampler, which I use on my scopes and spec analyzer. The signal is un-rectified and variable. So you're saying this should work? Should I just adjust it up to 10V when connected to the outpoot of the transmitter? I could try it and use the calibration method in the manual for the M-5693 monitor.