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Author Topic: What exactly could cause a modulation transformer to ocscillate?  (Read 3018 times)

Offline RadRoadrunner

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Today I've been assembling a transmitter and once I had gotten everything connected properly from the amplifier to the transmitter I turned it on. I then proceeded to find out that the mod transformer was oscillating to whatever I sent down in the input. In essence, my transformer has now become a speaker, and is not putting any output to my antenna or transmitter. Any suggestions on how to fix this? Thanks in advance.
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Offline redhat

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A transformer is a passive device and as such cannot oscillate on its own.  You may have the low level electronics too close to the transformer and mutual coupling could be causing some problems.

I have seen audio tube amplifiers oscillate without a load connected, sometimes supersonically which gave me my first RF burn.  A scope should help you find the trouble...pictures are helpful too.

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Offline Pigmeat

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I remember my first encounter with the phenomena about a year after getting my Grenade. I had the transmitter sitting on the box I used for packing it and my gel-cells into my tx sites. I got the audio level I wanted and turned off the receiver to save battery life. I was literally in the middle of nowhere, it was dead quiet. Damned if I didn't I start hearing familiar music? I thought it was RF getting into the "off" receiver, so I picked it up to give a listen. That wasn't it, it was coming from the box. The thing was acting like a resonator for the modulation xformer.

Offline RadRoadrunner

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I remember my first encounter with the phenomena about a year after getting my Grenade. I had the transmitter sitting on the box I used for packing it and my gel-cells into my tx sites. I got the audio level I wanted and turned off the receiver to save battery life. I was literally in the middle of nowhere, it was dead quiet. Damned if I didn't I start hearing familiar music? I thought it was RF getting into the "off" receiver, so I picked it up to give a listen. That wasn't it, it was coming from the box. The thing was acting like a resonator for the modulation xformer.

That's it, that's exactly what I'm referring to. Did you happen to find a solution or do you have any suggestions about what might apply to my situation? (Btw, it's one of Stretchy's LULU's, v2.5)
Please QSL At:  sansswl@gmail.com

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Offline Pigmeat

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Try cutting your audio level back & see if that helps. Overdriving the audio too much is a sure way to make mod transformers "sing".

Offline Stretchyman

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Prob best to ask me.....?

Got a mail from you I think???

The mod tran will act like a speaker as it essentially is without the paper cone.

Fill it full of glue and bolt it down!

Do that ONLY when you have a working system tho!

Str.
'It's better to give than receive' so why RX when you can TX!

         15W, 60W and 100W models available.

                   Buy one from me, NOW!


                                              ;)

Offline RadRoadrunner

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Just to verify, your v2.5 transmitter will work with a 6900 KHz crystal, correct?
Please QSL At:  sansswl@gmail.com

Radios: Tecsun PL-600, Tecsun PL-380
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Offline radiogaga

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Heh, competition for the pescadores? Hola, hola

Offline RadRoadrunner

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Nah, just the closest to 6925 I could get at the time.
Please QSL At:  sansswl@gmail.com

Radios: Tecsun PL-600, Tecsun PL-380
Antenna: 1.25m Coil Antenna + Whip Antenna on PL-600

Offline Josh

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Some folks have used superglue applied liberally to the laminations to silence same.
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Offline SV1XV

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Re: What exactly could cause a modulation transformer to ocscillate?
« Reply #10 on: July 22, 2018, 1742 UTC »
The cause of the "singing transformer" is magnetostriction, i.e. small change of dimensions due to the magnetic field. All power distribution transformers generate hum noise because of magnetostriction. Also most warship sonar transducers operate on this principle.

Offline Josh

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Re: What exactly could cause a modulation transformer to ocscillate?
« Reply #11 on: July 23, 2018, 1824 UTC »
Here we see magnetostriction put to good use;
http://www.wa3key.com/filters.html
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Offline moof

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Re: What exactly could cause a modulation transformer to ocscillate?
« Reply #12 on: July 23, 2018, 2319 UTC »
Drench in krylon triple thick spray glaze x2.  It will most likely still sound like a tiny flea band but hey you dont need to have a receiver on all the time.

Offline redhat

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Re: What exactly could cause a modulation transformer to ocscillate?
« Reply #13 on: July 24, 2018, 0026 UTC »
If you can't fix it...feature it!  Call it a modulation confirmation indicator.

+-RH
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Airspy HF+, MLA-30/Mini-whip/Chi-Town Loop
Please send QSL's and reception reports to xfmshortwave [at] gmail [d0t] com

Offline Pigmeat

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Re: What exactly could cause a modulation transformer to ocscillate?
« Reply #14 on: July 24, 2018, 0136 UTC »
If you can't fix it...feature it!  Call it a modulation confirmation indicator.

+-RH

A brilliant solution! LOL.

I thought I'd finally gone around the bend the night it happened to me. I emailed Fearless Fred when I got in to get his opinion on it, he thought it was hilarious, then he explained what it was to me and told me not to sweat it it unless it started happening without something there to act as a resonator under the heat sink. I still laugh about that night.

 

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