The AM Forum

My pristine, obscenely expensive, refurbished Valiant is a joy to behold. With the W2IHY equalizer it even garners unsolicited reports of 'broadcast' audio, and that with the stock modulation components.

Yesterday I was in QSO with a west coast ham on ten meters. He was +10 and I was reportedly the same to him. He mentioned that he hears a buzz or humming and it sounded like it was coming through the mic as audio. The wallet-emptying Valiant has always had that nice, reassuring sound of raw power when it is in transmit mode. Not having used another I assumed this was normal. Now I'm not so sure. My uneducated guess is the laminations in the transformer are loose and vibrating. Not sure, though, as the box remains unopened!

I've heard about using slim wooden wedges or even epoxy or superglue to alleviate buzzing transformers. But, is it worth it? Will the buzzing cause an eventual failure of the transformer or indicate it is already on the way? What is the penalty for leaving it alone?

No doubt "One-L" can provide a diagnosis and prognosis from long distance. Heaven knows, I don't want to join his club of Valiant explorations!

Wooden shims are ofter used between the core and the windings.  Try tightening the screws that hold the xfmr together.  You would have to remove the xfmr from the chassis and remove the end bells to add shims.  And it may not eliminate the buzz.

Fred

Before you tear into it I would look at the audio with an oscilloscope to verify there is hum or noise on your audio.

Are you able to determine which transformer or choke is mechanically humming, or that this is actually is a mechanical hum issue or not? You can start there.

My friend Ken VE3MAW was working on his Valiant and found that the choke in the HV PS was buzzing and the laminations were loose. He tightened up the bolts as Fred DZT mentioned, and the mechsnical buzz/hum went away.

Al VE3AJM

It would have to be a really loud buzz for your microphone to pick up the noise unless you are whispering and running extremely high mic gain.  First monitor with the audio gain at normal and then set to zero to compare.

IF the noise is really there and is originating from transformer or choke lamination noise it may be making its way via a microphonic tube instead of via the microphone.

Again, as suggested by previous posters make sure there really is a problem with your audio.  There is a multitude of possibilities to cause hum in the Valiant including heater to cathode leakage in a tube, failing power supply cap, (bias caps are often overlooked and new stuff can go bad), and a loose mic shield.  Try in on both crystal and VFO control to rule out a VFO issue.

I very much appreciate all of the constructive help. Some really great ideas. I think I will try the easiest first: get in a QSO where the other end can hear the noise and then disconnect the microphone. As W9QE suggested, it may not be coming through the air to the mic at all. That should quickly eliminate that path. If that fails, I'll look at the audio with a scope (I should have thought of that earlier) and see what it shows. The next step will require getting some help to get the beastie out of its cabinet and start to isolate the source.

Thanks to all and to all a good night! Oh, and Happy Festivus Eve, too.....

73
Jack
WA2OLZ

Hello Jack.
Was the guy on the other end using headphones on his receiver?
Many older receivers will cause hum in the headset especially if the audio setting on that receiver is reduced. I suggest before you do anything that you attempt to determine if the other guy was using headphones on his receiver during that QSO.

73

   I have a couple of comments, I know from that darn guy in Texas again. ::)

1st off, the Valiant is likely spraying magnetic fields beyond the cabinet. If the microphone is dynamic, the magnetic fields from the mighty V might induce a hum into the microphone dynamic element. Moving the microphone position, or distance from the transmitter will have an effect if magnetic coupling were the issue.

Here at my station, there is always a scope monitoring what goes to my tuner input. On AM I have 4 cm carrier, offset 1 cm down from center. This way I can easily see -100% and + 150% modulation. A low level hum is visible, and going to line sync will sync the trace if the hum is from the AC main frequency or 2X that. On my Flex 3000 station, I have another scope looking at the Flex output....so two scopes, one before the amplifier, and one after the amplifier. Sometimes on the Flex I need to boost the power 13.8V to 14.5V in order to get good positive peaks.

Jim
Wd5JKO

Jim
Wd5JKO