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What items cause resonating in cb or amp during modulation?
- Thread starter Limeybastard
- Start date
LB - As was said earlier, check the main power transformer. Tighten all the screws/nuts that hold the laminations tightly together and then as quickly as is safely possible, turn the station on and see if the noise is gone or at least reduced. Transformer plates delaminating is a pretty common problem. Worst case: you might need a new transformer.
It depends on the transformer too. In our tube type 1 Kw AM broadcast transmitters you could hear the transformers singing even when things were 100% fine. You could turn the monitors off and actually hear the program content ringing from the transformers. Then again it was a 2500 volt amp and a half choke input power supply and it fed a modulation transformer as well so lots of iron to irritate there. I would do like Beetle said and tighten everything you can and try it again.
If the noisemaker is a multi-toroid transformer coat it with silicone rubber or RTV if you prefer. Be sure it is silicone based especially if is one that gets hot.
Had a modulation "buzz", sounded like talkback through a speaker with a cracked cone in a Pride DX300 a few years back. It was pretty loud. Figured out it was coming from the overlap between the faceplate/front chassis piece and the rear chassis-box piece of sheet metal. Poked about when I unkeyed, and got a nasty burn on my finger tip. Sure enough, it was a "thermoaccoustic" transducer.
The shield on the output coax that runs from the Load control to the relay underneath came loose at one end. The chassis metal became the ground-return path for the RF current between the Load control and the antenna socket on the back panel. Caused that current to pass through the sheet-metal screw holding the two halves of the sheet-metal chassis. High resistance caused a tiny arc under the head of the screw, buzzing along with the voice modulation. Sure did get it hot.
Replacing the output coax and bolting the sheet-metal lap with a machine screw, nut and tooth washers made it quiet again.
Don't see that every day.
73
The shield on the output coax that runs from the Load control to the relay underneath came loose at one end. The chassis metal became the ground-return path for the RF current between the Load control and the antenna socket on the back panel. Caused that current to pass through the sheet-metal screw holding the two halves of the sheet-metal chassis. High resistance caused a tiny arc under the head of the screw, buzzing along with the voice modulation. Sure did get it hot.
Replacing the output coax and bolting the sheet-metal lap with a machine screw, nut and tooth washers made it quiet again.
Don't see that every day.
73
i have experienced this before with radios that have had all AMC limiters removed.
I always referred to it as "RF ringing in the chassis", because it is magnetostriction IMO.
don't really know how to explain it other than to say that if you have a path for RF that was designed for certain frequencies, then adding all the harmonics of that frequency to that path causes all the ferromagnetic materials in that path to, well, ring.
you can hear it coming from the PC board in a chassis with no speaker connected.
LC
I always referred to it as "RF ringing in the chassis", because it is magnetostriction IMO.
don't really know how to explain it other than to say that if you have a path for RF that was designed for certain frequencies, then adding all the harmonics of that frequency to that path causes all the ferromagnetic materials in that path to, well, ring.
you can hear it coming from the PC board in a chassis with no speaker connected.
LC
Typical case of RF rectification caused by a bad connection. We had to be careful of that in our broadcast installations and usually placed a stainless steel star washer between mating surfaces to ensure a good connection over the years especially if aluminum panels were involved. This is one reason some radio installations seem to check out fine yet cause a lot of RFI. Things look great visually or even with an ohmmeter or analyzer but when RF is applied a poor connection somewhere can begin to emit a tiny little spark and fry away causing all sorts of grief on the air. Sometimes you can actually make out the audio and other times it is just a raspy buzzzzzzzz.
The first radio I ever bought (used DX66V 
) had some "modifications" done internally, this included removing the AMC function.
I decided to learn more about the radio, so after some research I discovered I had bought an "export use only" radio. Spooky Stuff
Well, long story short.. I first heard "talkback" coming through my bedroom TV.
I decided I'd take a stab at correcting the issue, I came here and asked a lot questions.
Eventually I got the radio sorted, producing a nice sine-wave, and the "TV talkback" (interference) disappeared.
Over the past year I have observed some quirky things when I get a radio on the bench that has had AMC opened too far (or eliminated entirely) Odd things can happen, including:
"Talking through my external speaker" (without talk-back)
Horrible squealing with talk-back
Self oscillation, through some feedback or RFI issue, I keyed the Galaxy and it was "modulating the carrier" seemingly without any audio input, the needle jumped and people couldn't hear me if i tried to talk... After I de-hacked the radio those problems went away.
But, I know your setup is "fresh" (within a year old) and nobody has literally hacked on the bloody thing; just some minor adjustments have been made, if I recall the other threads correctly.
If this noise doesn't happen with just a dead carrier, then I'd try lowering the mic gain quite a bit and see if you still notice the noise under modulation. (AMC may be opened up a tad too much)
&
Can you still detect this noise with the amplifier turned off?
) had some "modifications" done internally, this included removing the AMC function.I decided to learn more about the radio, so after some research I discovered I had bought an "export use only" radio. Spooky Stuff
Well, long story short.. I first heard "talkback" coming through my bedroom TV
.I decided I'd take a stab at correcting the issue, I came here and asked a lot questions.
Eventually I got the radio sorted, producing a nice sine-wave, and the "TV talkback" (interference) disappeared.
Over the past year I have observed some quirky things when I get a radio on the bench that has had AMC opened too far (or eliminated entirely) Odd things can happen, including:
"Talking through my external speaker" (without talk-back)
Horrible squealing with talk-back
Self oscillation, through some feedback or RFI issue, I keyed the Galaxy and it was "modulating the carrier" seemingly without any audio input, the needle jumped and people couldn't hear me if i tried to talk... After I de-hacked the radio those problems went away.
But, I know your setup is "fresh" (within a year old) and nobody has literally hacked on the bloody thing; just some minor adjustments have been made, if I recall the other threads correctly.
If this noise doesn't happen with just a dead carrier, then I'd try lowering the mic gain quite a bit and see if you still notice the noise under modulation. (AMC may be opened up a tad too much)
&
Can you still detect this noise with the amplifier turned off?
Glad to know I'm not the only one that can hear it! I was worried for a second there.