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WU2D
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« on: March 01, 2008, 04:21:23 PM »

Well it is time to get back to my Apache Modulator project. Last heard a couple of months ago, I was stunned by the amount of hum that I was getting into the grid of the 12AX7 microphone amplifier and I set the beast aside. I never got past that first stage! GRRRRR; I mean HUMMMM...

So before diving back in, I decided to do some hum housekeeping based on some AM Forum advice:

An unbypassed rectifier diode is a noisemaker. - I have no idea if this is true but I am willing to try it!

The PTT relay supply is run up to the the microphone connector and it could be a big hum highway. Who knows?

As you may remember, I had a large value resistor on the grid of the tube (for a D-104) and had gone so far as to run new shielded cable from the tube directly to the mic jack and route it well away from the front wall wire bundle. I had also tried grounding and ungrounding the shield on each end of the mic cable - all to no avail.

I bypassed all of the diodes in my medium supply module and my high voltage supply module with 0.01 1kV ceramic caps. I also bypassed two bias supply diodes and the bridge diodes on my 24VDC PTT supply. This took 20-some caps!

Halfway to the front of the chassis from the PTT relay, I cut the PTT wire and inserted a terminal strip with a series 470 uH choke and a 0.1 uF to ground on either side.

Any other de-hummifications that anyone can suggest? The B+ and B++ and Fils are clean - no hum on them.

Mike WU2D
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« Reply #1 on: March 01, 2008, 05:58:32 PM »

Simplify.

Take and put a relatively low Z resistor to ground at the grid of the input tube of the speech amp, remove the wire running to the front panel. See if it still hums or not.

Then, work your way back toward the 6CA7s... the gain pot that sits in the middle of a stock Apache audio path can also be used to trouble shoot - set that for no gain and see if there is still hum.

One might make a case for using a D-104 with the FET follower in the base, and going back to lower Z at the input, IF that reduces the hum enough.

I've got one Apache with one very very funky audio section - still have yet to find all the problems... it may turn out that many of the carbon comp resistors have gone south... also it definitely was necessary to increase the value of the two PS filter/decoupling caps on the front end, otherwise I had a LF oscillation. That might sound like "hum" too... That happened as I increased the value of the coupling caps.

Also check the bias supply for the modulator final tubes...

                       _-_-bear

PS. the unbypassed rectifier diode might make HF noise, not hum, imho.
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WU2D
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« Reply #2 on: March 01, 2008, 07:33:28 PM »

Thanks Bear - I will try some of that. Note that I have a stripped out modulator section and am starting at the speech amp.

I thought I would bring her up slow and make sure everything was OK after the cap install and I saw no problems. The CW note was clean and plenty of power out.

Then I decided to check out the one function that I had not played with yet, the Apache crystal control mode. I had a nice 14.700 MHz crystal that would double nicely up to 29.4 MHz. Sure enough it gave even more grid drive than the VFO. The problem was, when I switched back to VFO, I still had output on 29.4 MHz!

So I flipped it on its back and noticed some gloop on top of the XTAL/VFO rotary switch and some "rotten" soldering. It looked like the switch had broken in half and the terminals were all just strapped together. So the Apache is presently capable of Diversity transmit with a crystal installed - OH BOY...

I could not let that be so I started removing shafts and parts until I could get the broken switch out. It was a nice clean break so I cleaned up the switch and epoxyed it - I have no idea if it will be OK to re-install but we will see. This is a horrible place to work in the radio!

Mike WU2D
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« Reply #3 on: March 01, 2008, 11:00:05 PM »

yeah ok Mike,

I'd not put the stock speech amp back in the holes once it is stripped out!

Think I'd do a ground up custom design myself.

Probably I'd go with a cathode follower to drive the grids of the mod tubes...
and in front of that, I think I'd pick a differential input/long-tail pair for the first tube. That makes it really easy to wrap feedback around it, and might make the hum lower since the inverting input sees ground... I'd probably opt for some soylent state parts in that section... a current mirror for the plate loads and a current source instead of a simple 'tail' resistor at the cathode. That enforces balance on both halves and also max's the gain. That's just one way to fly...

You can also do a differential cascode with a FET for the lower device.
That makes it easy to arbitrarily set the input Z any way you want, and to set the gain by switching in/out source resistors on the bottom of the FET (low/high gain).

Another option is to just put the FET follower in the rig!
It won't really care if a regular mic drives it or not.
You could even make the input Z switchable that way... it can be anything you pick.
The FET is dead quiet, and then the input tube sees some current, and the grid resistor can be lower Z, making the noise lower - and maybe the hum too... maybe.

Once having gone as far as stripping out the speech amp, you might want to consider putting some sweep tubes or other higher current tubes into the modulator holes - it sure as heck works well in the Valiant II that I have here, straps with the 3 diode limiter circuit...

Of course you can just follow the mods that are up in the AMwindow section too... there are a number of speech amps and power boost ideas for the mod tubes...

regardless, if you can drive a lowish Z, there are all sorts of dead quiet mic pre circuits out there - that presupposes the FET in the base of the D-104. Also you can then use a transformer for the input and not have the usual hum issues to deal with... but that's more like a "pro" style input design and not a high-Z ham design.

Oh, and no matter what, the first tube will benefit from extra filtering or even regulation of the B+ on its plate. I have had very good results using a small power transformer (very small, like 1" square!) and two large value but small size switching supply caps as an extra C-L-C filter for an inpoot tube... really knocks the residual PS hum down.

Well... having said all that, mine is still boots up on one of the benches here!  Roll Eyes

              _-_-bear

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« Reply #4 on: March 02, 2008, 08:17:02 AM »



Mike

Hi!

You said:

"Any other de-hummifications that anyone can suggest? The B+ and B++ and Fils are clean - no hum on them."

Please clarify what you mean by no hum on the filaments? Are you saying that the "hum" you are hearing is not 60 cycle hum?

Also... please clarify the method you are using to listen to (to hear) the hum at the point you are making your measurement. [I.e., your post says that you hearing hum getting into the grid of the microphone amplifier. Are you attaching a piece of test equipment to the grid?]

Best regards
Stu


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« Reply #5 on: March 02, 2008, 09:44:50 AM »

OK Bear and Stu,
The hum is 60 Hz and I am using a scope.

This is a ground up modulator design - not stock and I have a working schematic. The circuit uses a 12AX7, first stage preamp then the mic gain pot into the second stage - but it is not quite conventional because it has a switch to take a high level low Z input. It may be the wiring around this lash up that is causing the problems but I have everything tight and am using shielded coax to the switch which is located where the 6AL5 clipped lived. You reach in and flip the switch for Hi-Lo. This circuit works great except for the hum.

That is as far as I have built and tested so far and I am seeing the hum on the plate of the second section of the 12AX7.

Since adding the caps, I have not gone back in yet to see what is going on but I will try some trouble shooting with just resistors as Bear suggested. That is a great idea.

I am heating the fils with DC in the modulator and this is wired on all sockets.

My schematic has a long tail pair phase splitter as the next stage (12AU7) feeding a 6CG7 (which can be configured as a P-P follower) feeding a pair a shoehorned KT-88's into a bigger modulation Xfmr.

Mike WU2D


* ApachePreamp.jpg (96.12 KB, 1204x690 - viewed 380 times.)

* Apachelation.jpg (177.62 KB, 1503x1173 - viewed 429 times.)
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« Reply #6 on: March 02, 2008, 10:24:01 AM »

Mike

Looks like a fine design...

You might check to see if the lead coming from the 1k cathode resistor back to S1A (via the 22uF capcitor) is acting as a hum pickup (magnetic coupling to sources of 60 Hz magnetic fields from transformers, etc). The grid (pin 7) of the 12AX7 would be very vulnerable to hum pickup from this lead ... even with the switch in the high impedance (up) position, where the lead is not directly connected to the grid.

I began to really appreciate how easily a small loop of wire can pick up 60Hz hum when I built my homebrew ribbon microphone. 

First try just grounding the lower terminal on S1A when the switch in the high impedance (up) position to see if the hum is significantly reduced.

You may want to try routing the 22uF capacitor to a separate microphone input jack, and eliminating S1 completely (i.e., just "turn down" the 500k pot when using the low impedance microphone input).

Stu

P.S. I think that if you take into account the phase shift at 60Hz associated with the 0.1uF capacitor (-27k ohms of reactance at 60 Hz) feeding into pin 2 (the grid of the 2nd triode) and the phase shift at 60 Hz associated with the capacitive coupling across the open switch feeding into pin 7, you might not even have negative feedback around the loop (including the leakage across the switch) at 60 Hz... although I haven't checked to see if this is true or not.
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« Reply #7 on: March 02, 2008, 10:46:57 AM »

Well, here we go..................... Just a couple of points to keep in mind. First of all 6550s dont take anything exotic to drive them. They were designed to run AB1 and just need a few volts of audio to tickle the grids. they also have a slight tendency to take off into parasitic oscillations if one is not careful in their implamentation.

As far as an audio preamp / driver circuit goes, if you are not capable of designing your own, the one on Steve's "AM Window" website is just about as good as any! I did up 5 apaches a few years ago for other people and one for myself. I used that circuit with a few slight variations and never had a problem!!

First of all cut out and throw away the speech clipper circuit!! Eliminate the extra stage of audio amplification, you dont need it!! (this also leaves you 2 empty tube sockets, perfect place for some VR tubes to regulate the modder screens) Now, eliminate the second gain pot behind the key jack and move the modder biass pot to that location. (Make sure your modder biass is well filtered) Now bulldoze out the rest of the mic amp / speech amp / driver circuit, eliminating that nasty little circuit board on the side of the chassis. I like to use a 12AU7 with both sections in parallel instead of the triode connected 12BY7 for the driver. (I friggin hate 12BY7s) Build the rest of the circuit as drawn on Steve's site. Use the front panel level control for the mic gain.  Be sure to put some type of RF trap on the input grid circuit. I usually use a .05 cap from the grid of the 1st speech amp into a Pi circuit consisting of a 4.7k in series to the mike connector with 2 100-200Pf caps to ground on each side of the resistor. This keeps the dc grid biass from the 12AX7 off of the mic element and any rf picked up in the mic cord off of the grid of the tube. Also put a very small bypass cap (100-200Pf) from the grid of each speech amp / mic amp / audio driver tube to ground, this will help keep them from picking up any stray RF and /or going into ultrasonic oscillations. And the reactance of those caps at audio frequencies is low enough to have little or no affect on your highs! Last but not least is the PTT line!! (if you have installed PTT) Be sure to use a DC relay, and to have the supply to that relay WELL FILTERED!! If you are using a lower voltage relay, it can draw just enough current where the leads run parallel in the mic cable to induce some ripple into the mic input line and produce a hard to find hum.

I have done 6 of them this way and never had a problem!!

                                                                                              The Slab Bacon  
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N3DRB The Derb
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« Reply #8 on: March 02, 2008, 11:03:21 AM »

Quote
/or going into ultrasonic oscillations.

doin the Derb-100 dance, it's not the dance to do.... Tongue
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Dave KA2J
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« Reply #9 on: March 02, 2008, 08:37:41 PM »

Hi Mike,
Just for the heck of it, try a different 12AX7 or 12AU7 and see if the hum goes away.  Also, double check the wiring on the tube socket.  Just a hunch...
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« Reply #10 on: March 02, 2008, 08:59:30 PM »

And do all of the audio grounds return to a single common point very close to the audio input connector? I'm just wondering about the possibility of a ground loop here. A 60 hz hum can frequently spell a ground loop. When looking at the hum pattern on a scope, if it is not a clean 60 hz sine wave, this is a good indication of a ground loop. If you have multiple grounds within the audio section of this rig, you are asking for trouble. All of the grounds must return to a single common ground point, using the popular star-ground topology; you can't take the expedient path of using the grounding lugs on a tube socket to ground the components for that particular stage.

73,

Bruce
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« Reply #11 on: March 03, 2008, 08:37:36 PM »

I have not taken heroic measures on the grounding, but it is not sloppy. Lifting ground at the mic jack does not effect the hum one way or the other. Soldering or jacking in a 1K resistor to ground at the mic jack does not reduce the hum either. I have two lugs on the screws which mount the tube and the input resistors go directly to these as do the shields for the mic input cable and the pot. I have connected a buss wire to the center post of the socket. The mic input cable hot goes directly to the grid pin. Everything is short. Putting the shield on and off does nothing.

But turning the set off utterly kills the hum and the thing is silent as a lamb for over 30 seconds until the tube fill goes out (the DC on the tube stays up above 100V for almost a minute. So the pickup is related to AC being on.
Just for fun, I tested my D-104 microphone with some whistling and talking and I get around 10 mV RMS with normal speech and 40 mV RMS when I whistle. This is measured into my Leader MV-181A RMS voltmeter.

I removed the switch from the circuit and stuck a 330K on the grid of the first stage with the shielded cable right to the mic jack. I grounded the 22 uF cap on the cathode of the second stage and set the mic gain to full.

With a 1000 Hz 10 mVRMS directly into into the 12AX7, I get 6 VRMS out on the plate of the second tubes plate through a 0.1 uF cap. This is a gain of 600 or 55 dB.

With the input removed under the same conditions I have a static 150 mV RMS of 60 cycle hum on the output of the second stage.

This implies a 32 dB signal to noise ratio at full gain. Two other 12AX7's read the same.

I replaced the tube with a 5814 (a 12AU7 type).

Now the output with 10 mV RMS in was 410 mV for a gain of 41 or 32 dB.

With the input removed the output falls to 19 mV RMS for a signal to noise of 26 dB.

Mike WU2D
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« Reply #12 on: March 03, 2008, 09:56:13 PM »

Mike,

Since you providing filtered DC to the heaters of the small-signal tubes in the audio section, another thing worth trying is to also make sure that the negative side of the heaters are all connected to the single point ground.

I had built a line-level audio preamp a number of years ago, and I used filtered DC on the heaters. I had a low level of residual hum that I could not eliminate. Grounding the negative side of the heater supply completely eliminated the hum.

73,

Bruce
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« Reply #13 on: March 04, 2008, 09:45:44 AM »

Mike

I agree with Bruce that the wire(s) bringing DC to the filament might be picking up the 60 Hz hum, particularly if there is a loop formed between the + and -. E.g., if the + side is a wire and the - side is the chassis, then the associated loop will pick up a lot of differential mode hum from the magnetic flux passing through it (from nearby sources of 60 Hz magnetic fields).

It's hard to filter out differential mode hum, because the impedance level is so low (filament voltage / filament current ~ 10 ohms), and the frequency is so low. Therefore it is a good idea to use a two-wire twisted pair to deliver the DC from the DC supply to the filaments. This will significantly reduce any differential mode 60Hz across the filament.

If you don't have the filament grounded on one side, or in the middle, then the common mode hum on both wires may be coupling from the filament to the cathodes of the tube (since the cathodes are not directly grounded to the chassis).

Therefore, making sure that the filament (center or one side, as appropriate) is grounded at the tube socket is a good idea.

Stu

 

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« Reply #14 on: March 05, 2008, 09:35:15 PM »

OK Stu and Bruce,

Tonight I attacked the filaments and did a single point ground connection with only the negative side of the DC filament supply connected to ground right at the 12AX7.
All components which get grounded go directly to this one solder lug. The DC supply leads were replaced with a twisted pair. I also checked the wiring on the tube socket.

I did change the input resistor to a 1M and changed the second stage cathode resistor to a bypassed 4.7K. I used a 1K resistor soldered directly to the input connector to measure hum. The gain of the stage changed only slightly. There was no change in the hum or signal to noise. I still have 250 mV RMS of 60 cycles at full MIKE gain. Ungrounding the fil supply had no effect and even the extreme of putting a hum balance pot across the floating DC supply to ground ( which probably has not been done except by audioPHOOLS) had no effect. It is cold. Only turning off power reduced the hum to around 20 mV.

Next idea?

Mike WU2D


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« Reply #15 on: March 05, 2008, 10:09:37 PM »

Mike,
The audio gain pot has long leads to the front panel and very high impedance.
I would think they should be shielded. This looks like a good pick up for noise.
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« Reply #16 on: March 05, 2008, 10:18:53 PM »

Mike

The thing is haunted by the ghost of George Westinghouse! [Who hates DC, and tries to add some 60Hz AC where ever he can sneak it in]

Does the level of hum you measure at the output of the second tube vary as you adjust the potentiometer? [Maybe you already answered this in one of your earlier posts]

Have you measured the voltage at pins 4 and 5 to see if there is any hum superimposed on the DC?

Have you measured the hum on the B+ (just before the plate load resistor of the first tube, and just before the plate load resistor of the second tube), using a capacitor to AC-couple your scope to the B+... to see if the hum is coming from there? You have some pretty hefty filtering on the B+, so I would expect the residual hum at those points to be well below the levels that would cause the problem (and also at 120 Hz).


Which brings up another issue: are both halves of the full wave rectifier working properly?

Stu
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« Reply #17 on: March 06, 2008, 05:46:41 AM »

Mike

By measuring* the B+ on the other side of the plate load resistor, you also will verify that the probe itself is not picking up the hum. I.e. high impedance probe + the loop formed by tip of probe and probe ground lead. Or, even worse: the probe ground lead isn't making good contact... creating a huge loop for picking up hum.

*Using the same sized series AC-coupling capacitor, 0.1uF, that couples the output of the second stage to the scope when you are measuring that output.

Stu
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« Reply #18 on: March 06, 2008, 07:35:08 AM »

Mike

2 cents (may be all it's worth)

Consider lifting the mic cable center conductor at pin 7 to see if that changes the problem. This should show whether the problem is related to the mic input side or not. If the problem doesn't go away leave it disconnected and chase the problem from there.

It doesn't appear that you really have a single point ground since the shell of the mic connector is connected to chassis ground at the connector end of the shielded cable. If the hum is related to the mic input connector you might try floating the shell above ground and letting the shield of the cable connect to the single point ground. Easy stuff to try, anyway...

   
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« Reply #19 on: March 06, 2008, 04:48:02 PM »

OK Stu and Bruce,

Tonight I attacked the filaments and did a single point ground connection with only the negative side of the DC filament supply connected to ground right at the 12AX7.
All components which get grounded go directly to this one solder lug. The DC supply leads were replaced with a twisted pair. I also checked the wiring on the tube socket.

I did change the input resistor to a 1M and changed the second stage cathode resistor to a bypassed 4.7K. I used a 1K resistor soldered directly to the input connector to measure hum. The gain of the stage changed only slightly. There was no change in the hum or signal to noise. I still have 250 mV RMS of 60 cycles at full MIKE gain. Ungrounding the fil supply had no effect and even the extreme of putting a hum balance pot across the floating DC supply to ground ( which probably has not been done except by audioPHOOLS) had no effect. It is cold. Only turning off power reduced the hum to around 20 mV.

Next idea?

Mike WU2D

I think this is the big clue (turning off the power and listening during fil cool down and PS cap discharge results in immediate loss of hum).  You have confirmed it is 60 cps (not 120 of a badly filtered PS).  Do you have a big enough ISO XFMR to isolate rig from the mains.  What about bypass caps directly from the line to ground coming in from the mains?  Are they OK?  My 0.5 cents worth
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« Reply #20 on: March 06, 2008, 05:10:42 PM »

Had a similar problem in an Apache one time.  Turned out to be the mic connector at the panel was intermittent to ground.  The hole for the connector in the chassis would block proper seating of the connector at the panel. 

Does the panel look like it may have some corrosion near the mic connector?  If it is above ground, then it may be entering on the shielded lead to the mic amp input cap.
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« Reply #21 on: March 07, 2008, 09:10:20 AM »

This is sure looking like a simple job that has gone sour. The scratchie patchie just isnt that complicated of a rig. Like I said before I did 5 or 6 of them, one after another a few years back abd had no real complications ofher than de-hammy hamboning them.

Is the hum coming from the low level audio stages or the modders themselves??

Check the modder bias supply carefully, as IIRC it only has a half wave rectumfreyer! If it is shorted or the biass filter caps are bad, it will produce a 60 cycle hum and not a 120.

Next start pulling out the low level tubes one at a time, starting with the driver tube. If the hum still persists with the driver tube pulled, the problem is in the modders. If it goes away, it is in the stages before.

Next, pull the 12AX7 and see if it goes away. If it doesnt then it is something in the driver. If it does go away, look at your mic / speech amp circuit carefully.

Next take a short clip lead and short the grid of each side of the AX7 to ground and see if that makes it go away If it does, look at your wiring very carefully.

Also, be sure that your filament ground connectors dont have an oxidized, high resistance connection to the chassis. This could have an effect as well.

Next, try firing the transmitter with the front panel switch instead of the PTT on the microphone. This will help eliminate an inductive coupling to the ptt line from the mic cable.

If all else fails, bulldoze out the audio section and redo it from scratch!

                                                                    The Slab Baon
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« Reply #22 on: March 10, 2008, 10:59:40 AM »

I got pulled out of the shack and am on a work trip - when I get back I will try some of the suggestions. The half wave rectifier theory is sounding interesting.

Thanks guys,

Mike WU2D
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« Reply #23 on: March 10, 2008, 04:53:09 PM »

Mike,

If the rectifier was operating in the half-wave mode, instead of the desired full-wave mode, would'nt the ripple/hum frequency be 30 hz, and not 60 hz???

73,

Bruce
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« Reply #24 on: March 11, 2008, 01:09:32 AM »

Bruce,

A full wave rectifier produces two conductions per cycle meaning it rectifies both positive and negative excursions of the sine wave. In this way, it is a frequency doubler.

So if you start with 60 Hz and you half wave rectify, you will get one positive half cycle for each full cycle at the same rate (60 Hz). You will get two positive half cycles for each cycle with a full wave rectifier thus 120 Hz. 120 Hz is easier to filter. Half wave operation producing 60 Hz ripple may not get filtered as well with components made to filter 120 Hz and the half wave supply would have to produce twice the current again stressing the filtering.

Another possibility is that one bank of diodes has a shorted diode causing a high current condition (shorted xfmr winding) every cycle at a 60 Hz rate but I would think something would blow if that happenned.

MIke WU2D
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AMfone - Dedicated to Amplitude Modulation on the Amateur Radio Bands
 AMfone 2001-2015
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