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Author Topic: Modulation Indicator  (Read 12543 times)
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W1RKW
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« on: March 22, 2005, 05:00:58 PM »

Anyone know of a simple modulation indicator circuit for monitoring both negative and positive modulation levels?  I'd like to have something that can be very responsive, something like a simple LED indicator for negative peak modulation.  And maybe a two or three of LED's indicating 100% or greater on the positive side.  

I've seen simple circuits for driving a meter but I'd like something that will respond virtually instantaneously to a extremely short peak.

Bob
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W1RKW
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« Reply #1 on: March 22, 2005, 05:53:17 PM »

Boy Bob, I don't want to seem like a smart ass but how about a scope? It meets all of the qualifications you set down and they are so cheap!

--Larry W8ER
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kc2ifr
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« Reply #2 on: March 22, 2005, 06:15:48 PM »

Ya know Lerry......U are a smart ass....but on the other hand your rite!!!! :roll:
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K6IC
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« Reply #3 on: March 22, 2005, 06:51:51 PM »

And,   Bob,  am sure that you have seen WA1QIX,  Steve's complete Mod Monitor on his site.   Not too complicated.  You could replace meters with  level comparator and LEDs for the positive level indication.

Modulation Monitor:    

http://www.classeradio.com/mod_monitor.gif

73  Vic  KF6RIP
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k4kyv
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Don
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« Reply #4 on: March 22, 2005, 07:00:16 PM »

For years I used an 866A as neg peak overmodulation flasher.  Using a 2.5 volt filament transformer insulated for high voltage, run the midtap of the filament winding to the modulated HV lead to the final.  Ground the 866A plate.  As long as the plate of the final remains at positive potential with respect to ground, or zero volts, the 866 will not conduct.  As soon as the plate is driven negative (the condition with negative peak overmodulation) the 866 conducts with the characteristic blue glow.

I put mine inside a box with the interior painted flat black, with a window for viewing the plate.  It made a very effective overmod indicator.

You can try biasing the plate of the 866A positive to get it to flash at less than 100% modulation.  For example, your final runs 2000 volts on the plate.  Bias the 866A 200 volts positive, and it will flash at 90% negative modulation.

Another variation is to place a power resistor equal to your modulating impedance in series with the 866A, preferable from the filament winding to modulated HV lead, and you have modulation transformer protection for negative peaks.  Normally, whenever a plate modulated rig is overmodulated in the negative direction, the modulation transformer operates without a load for the duration of the overmodulated peak, a sometimes fatal condition for the modulation transformer.  With the resistor and diode, the modulation transformer sees normal load impedance while the final is being driven negative.  Since this peak occurs over a very small percentage of the audio cycle, a small resistor, 50 watts or less, is sufficient for a full kilowatt plate modulated.  Of course, the 866A still flashes, allowing the tube to serve double duty as overmodulation protection and indicator.
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W8ER
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« Reply #5 on: March 22, 2005, 07:26:07 PM »

Quote from: kc2ifr
Ya know Lerry......U are a smart ass....but on the other hand your rite!!!! :roll:

Bill, you never surprise me with your absolute grasp of things!

Don,

That is outstanding! Very clever. Couldn't the same sort of thing be done with a divider string and a transistor and an LED? Seems simple enough for negative peaks.

The Eico 730 had a similar circuit with an "eye" tube.

--Larry ...  highly intelligent resident smart ass  ( | )
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W1RKW
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« Reply #6 on: March 22, 2005, 07:46:49 PM »

Hi Larry,
You're not being a smart ass at all.  The thought of using  a scope or even some sort of metering device with a diode circuit on the modulator has been contemplated and that's not a problem at all but I would like not to use any outboard equipment on the newly built 813 modulator.  I want to make the big rig as lean and clean as possible and have something built in if it's possible.  Though if necessary I suppose I could spare one of the four scopes I have for the rig.

I'm looking for something simple but effective that I can put right into the face plate of the unit.  I have enough room for a small circuit as I like flashing lights.

And thanks Vic for QIX's website link.  I forgot about Steve's circuits.
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Bob
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« Reply #7 on: March 22, 2005, 08:04:39 PM »

Bob, thanks for you vote of confidence but I like Bill and he's right!  :lol:

You may look around for an old broadcast mod monitor. While it cannot be made a part of the unit directly it will have remote flasher circuits usually broght out to a terminal strip or connector.

I have a Harris AM90 that has settable flashers for both positive and negative peaks. If I wanted to put two lamps on a modulator panel and run them to a terminal strip on the back of the modulator, I could connect the two and have exactly what you are looking for, but it would involve a mod monitor.

I like the idea of monitoring the plate voltage for going negative peaks. That can't be a tough one.

--Larry
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w3jn
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« Reply #8 on: March 22, 2005, 08:11:53 PM »

Millen made some cute little 1" panel 'scopes that take up less room than a meter.  I have one and someday it'll get installed in a HB xmitter.  Look on eBay, they turn up from time to time.  BTW some manufacturer (can't remember who) made a leenyar, black wrinkled, with one of these for mod monitoring.

Tektronix made some cute little 3" portable scopes that could be removed from the cabinet and mounted on a front panel.

Or you could go with one a them seeing eye toobs - a 1629 would be just the ticket - if you want a buzzardly look.
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« Reply #9 on: March 22, 2005, 08:19:38 PM »

My 100V has one built in:


W0YVA photo
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N3WWL
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« Reply #10 on: March 23, 2005, 07:31:04 AM »

...always thought those 100V's were very cool.
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W1RKW
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« Reply #11 on: March 23, 2005, 03:19:30 PM »

I was thinking about what you guys posted and a monitor scope might not be such a bad idea after all.  I figured I'm going to have some rack space left open when all is said and done and a home brew looking monitor scope to fit right in that open rack space will fill it up that space. And it should look pretty cool I think and it will do the job I want. Plus it can be setup to do other types of measurements at the flick of a switch.   I think I'm going to go that route even though it strays away from my simple blinking light monitor.  I have an old monitor scope I can gut and use for just this purpose.  Thanks for bringing my head out of the clouds.
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Bob
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