HFU HF Underground

Technical Topics => The RF Workbench => Topic started by: RadRoadrunner on July 21, 2018, 1937 UTC

Title: What exactly could cause a modulation transformer to ocscillate?
Post by: RadRoadrunner on July 21, 2018, 1937 UTC
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.
Title: Re: What exactly could cause a modulation transformer to ocscillate?
Post by: redhat on July 21, 2018, 2258 UTC
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.

+-RH
Title: Re: What exactly could cause a modulation transformer to ocscillate?
Post by: Pigmeat on July 22, 2018, 0323 UTC
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.
Title: Re: What exactly could cause a modulation transformer to ocscillate?
Post by: RadRoadrunner on July 22, 2018, 0344 UTC
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)
Title: Re: What exactly could cause a modulation transformer to ocscillate?
Post by: Pigmeat on July 22, 2018, 0500 UTC
Try cutting your audio level back & see if that helps. Overdriving the audio too much is a sure way to make mod transformers "sing".
Title: Re: What exactly could cause a modulation transformer to ocscillate?
Post by: Stretchyman on July 22, 2018, 0927 UTC
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.
Title: Re: What exactly could cause a modulation transformer to ocscillate?
Post by: RadRoadrunner on July 22, 2018, 1430 UTC
Just to verify, your v2.5 transmitter will work with a 6900 KHz crystal, correct?
Title: Re: What exactly could cause a modulation transformer to ocscillate?
Post by: radiogaga on July 22, 2018, 1533 UTC
Heh, competition for the pescadores? Hola, hola
Title: Re: What exactly could cause a modulation transformer to ocscillate?
Post by: RadRoadrunner on July 22, 2018, 1611 UTC
Nah, just the closest to 6925 I could get at the time.
Title: Re: What exactly could cause a modulation transformer to ocscillate?
Post by: Josh on July 22, 2018, 1716 UTC
Some folks have used superglue applied liberally to the laminations to silence same.
Title: Re: What exactly could cause a modulation transformer to ocscillate?
Post by: SV1XV on July 22, 2018, 1742 UTC
The cause of the "singing transformer" is magnetostriction (https://en.wikipedia.org/wiki/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.
Title: Re: What exactly could cause a modulation transformer to ocscillate?
Post by: Josh on July 23, 2018, 1824 UTC
Here we see magnetostriction put to good use;
http://www.wa3key.com/filters.html
Title: Re: What exactly could cause a modulation transformer to ocscillate?
Post by: moof 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.
Title: Re: What exactly could cause a modulation transformer to ocscillate?
Post by: redhat on July 24, 2018, 0026 UTC
If you can't fix it...feature it!  Call it a modulation confirmation indicator.

+-RH
Title: Re: What exactly could cause a modulation transformer to ocscillate?
Post by: Pigmeat 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.