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Modulator low level output




 
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N4LTA
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« on: July 20, 2020, 10:44:47 AM »

Is there an easy way to get an accurate low level signal out to monitor a modulator? I know some modulation transformers have a 600 ohm tap but mine is a UTC VM2 and does not have one.

Thanks

Pat
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MikeKE0ZUinkcmo
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« Reply #1 on: July 20, 2020, 09:56:39 PM »

Don't know what kind of Voltages are involved, but a DC isolating capacitor of proper voltage rating ((DC + AC peak)X1.5) connected to the modulation transformer's secondary, followed by a string of resistors to form a voltage divider, is the simplest way to achieve your goal.

If you know the maximum RMS output of the modulator, simple Ohms law calculations will give you your answer.   Keep in mind that each 1/2 Watt resistor should only have something like 350 Volts maximum across them.
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« Reply #2 on: July 20, 2020, 11:33:26 PM »

I had thought about doing something like that but I was thinking about a jack for headphones to monitor the signal. Not sure if I want to trust a capacitor connected to the plate supply of the final and a resistor network.to the headphones. The unmodulated plate supply will be near 1KV with 60 watts of audio at 6000 ohms riding on it.

Not sure exactly what the UTC VM 2 mod transformer windings look like and if there is a possibiliy of stealing some audio from the transformer.

Maybe an off the air audio connection is a better idea.


Thanks

Pat
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K8DI
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« Reply #3 on: July 21, 2020, 08:06:33 AM »

How about a voltage divider, a low power plate to speaker transformer, and unless an SE type, a DC blocking capacitor on the primary?

Ed
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« Reply #4 on: July 21, 2020, 08:11:58 AM »

I asked this same question a few years ago and Stu posted this schematic for me. I've built several and they work great all the way down to 50 watts carrier. There's enough power there to directly drive a pair of 8 ohm headphones. The audio pickup is electrically isolated from everything so there's no danger of electrical shocks.

I use if to monitor my transmissions live-time on the air. The headphones have their own relay. They're switched to the receiver during receive and to this device for transmit.

You can use an FB-43-1020 as the ferrite.

Jon

** After reading your first post again, I guess you're looking to monitor a tube modulator only and not the RF stage too. But, this is another option.

* Envelope Monitor Floating.pdf (67.7 KB - downloaded 43 times.)

* IMG_2533.jpg (1846.49 KB, 4032x3024 - viewed 29 times.)
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Just follow the instructions, more or less.
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N4LTA
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« Reply #5 on: July 21, 2020, 10:17:49 AM »

Both of the above ideas look interesting.

Ed - That was what I was thinking might be possible. Are you suggesting a small SE plate to voice coil transformer  connected to the modulation transformer secondary

This is for the modulator that you gave me the circuit for the balanced input.

I also like the off air monitor.

Thanks to both of you.

Pat
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« Reply #6 on: July 21, 2020, 10:40:31 AM »

Both of the above ideas look interesting.

Ed - That was what I was thinking might be possible. Are you suggesting a small SE plate to voice coil transformer  connected to the modulation transformer secondary

Yes, but with a voltage divider that also limits the current. Just putting it across the modulation transformer would be bad...

Something like a 10K to speaker, across a 10K resistor with 100K to each side of the modulation transformer -- so the load added to the modulator secondary is 200+K and the DC current through that is 5mA at 1kV and then only a small part of the 60+ w audio is coupled out...you only want milliwatts in your headphones anyway...

Ed


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« Reply #7 on: July 21, 2020, 11:28:07 AM »

Hi Pat -

Is the modulator audio monitor just an interim test point before you use an off-air monitor monitoring modulated RF?   If so, disregard the following... :-)

If not, the off-air RF pickup is a much better choice because it shows you the real sound after the high level mixer. (The final and modulator)   The modulator itself can sound great, but all the stuff that goes on afterwards could sound like crap and you would not know it. I like the QIX REA monitor for that as well as a scope and SDR waterfall, etc..

I've heard many guys listening to their audio tap thinking it sounded great but the RF final loading, voltage regulation, IMD and a multitude of other problems were dominant.  It's real common in screen modulated rigs (loading) and flat-topping problems where tuning is critical.

How's the two projects coming?


T
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Use an "AM Courtesy Filter" to limit transmit audio bandwidth  +-4.5 KHz, +-6.0 KHz or +-8.0 KHz when needed. 

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« Reply #8 on: July 21, 2020, 01:38:33 PM »

Thanks Ed. That is what I meant. I have some decent fidelity transformers that should work. This is for the modulator testing. I will also add an off the air monitor.

Thanks everyone for the help.

Pat
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« Reply #9 on: July 21, 2020, 10:24:11 PM »

I have some push pull transformers. They should work since there is only a few mA DC flowing through the core.

Pat
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MikeKE0ZUinkcmo
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« Reply #10 on: July 22, 2020, 11:03:20 AM »

If you're only wanting to monitor the modulator's output, this will work.  The same basic scheme can be used to feed Trapezoid modulation monitors.

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« Reply #11 on: July 27, 2020, 09:43:17 AM »

Mike

thanks for the schematic. I will probably use this circuit as i am running out of space.

Pat
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« Reply #12 on: July 27, 2020, 11:06:03 AM »



Pat,

  What is in the schematic will work fine. Some comments though.

With roughly a 2000:1 divider ratio, is there really a need for the two HV capacitors up top?

Without those caps to block DC, and 2KV plate voltage, there will be ~ +1 volt DC at the bottom. Then AC couple from there with a cheap 50v capacitor.

The diodes could be 2 or 3 silicon each way to prevent clipping, or just removed entirely.

The divider ratio at AC will be impacted by load capacitance. If this is a concern where audio frequencies above 1 Khz might be attenuated, and phase delayed, then perhaps compensate the voltage divider. Think of how it is done in a 100x or 1000x AC compensated scope probe. I remember at my work I used a Red Fluke 1000:1 DC probe on a signal that was 10KV P-P 1Khz Triangle wave. The scope showed a low amplitude sine wave! I then got a Tektronix 1000:1 AC compensated probe (ones with Freon inside), and measured the same signal, and got a near perfect triangle waveform of the proper amplitude.

Jim
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W4RFM
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« Reply #13 on: July 27, 2020, 12:41:29 PM »

RCA Ham Tips had an article on a real modulation monitor for amateur AM way back when meter as well as a sound tap).
  I printed it off, let me find that and I will post the link.  It's on the web somewhere.
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« Reply #14 on: July 27, 2020, 08:28:35 PM »

There is this one, which is a bit simple:

http://n4trb.com/AmateurRadio/RCA_Ham_Tips/issues/rcahamtips0801.pdf

And this one, which is a bit more complex:

http://n4trb.com/AmateurRadio/GE_HamNews/issues/GE%20Ham%20News%20Vol%2011%20No%205.pdf

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« Reply #15 on: July 28, 2020, 09:29:26 PM »

These are off the air monitors which I plan to add to the transmitter. I am looking for a low level monitor for the audio chain itself. The transformer or the resistive divider should work fine.

Thanks

Pat
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MikeKE0ZUinkcmo
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« Reply #16 on: July 28, 2020, 10:05:11 PM »

]Jim said,

 
Quote
... What is in the schematic will work fine. Some comments though.

With roughly a 2000:1 divider ratio, is there really a need for the two HV capacitors up top?

Without those caps to block DC, and 2KV plate voltage, there will be ~ +1 volt DC at the bottom. Then AC couple from there with a cheap 50v capacitor.

The diodes could be 2 or 3 silicon each way to prevent clipping, or just removed entirely...

Don't forget resistors have a maximum voltage rating depending on power rating, 1/2 watt =300-350V, so decouple and we're left with +/- 1KV or so MAX P-P audio, easily handled by 4 half Watt'ers.   Of course, a longer string of lower value resistors is an option as well if the caps are a problem.   And, if there's no PA B+ to contend with, decoupling is a moot point anyway.

For HF compensation perhaps something on the order of 100pF across each 470K resistor would be adequate.

The double decoupling caps, instead of just one, and the diodes across the jack, were to ease the OP's stated fear of shock hazard, and are truly unnecessary, as drawn.

======
CRT modulation monitor is another option if on the air application is the desired mode.   A Heath HO-10 would do the trick.
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