[EdHolland]

Interesting, and similar to an experience I had recently. A while back, I bought a nice Yaesu shortwave receiver. It suffered from terrible hum on some AM stations, but going up in frequency to the shortwave section, this diminished and went away.

I was convinced there must be an issue in the PSU, so replaced the caps, and bridge rectifier. No change. One day I had run the car down into the garage with the radio running, and noticed a hum that had not been present while driving. The garage is right beside my radio room. Eventually, I traced the hum to a USB charger I was using as a power supply for another radio project. Grounding the negative lead of the charger cured the hum - presumably a modulation hum of some kind on both radios.

So, I think you are on the right track with your detective work. With my case, certain stations in the AM broadcast range were affected more than others. I would suggest testing your transmitter at several different frequencies if possible. Also, check general reception of broadcast stations to see if there is any 60 Hz or 120 Hz present - and are any stations affected when the transmitter is running.

Good hunting!

Ed

[TV MAN]

I was messing with things last night and found that if I plugged my transmitter into a different outlet (different circuit) that the hum was a lot less. This is with all breakers on. If I plugged it back into the bench outlet, the hum was louder. I also found when I touched the plug from my CD player to the input jack, the hum increases. It used a 5v USB charger. I thought it may be the problem, so I plugged back into the bench, unplugged the charger and the hum is still louder regardless of the charger. So, whatever is on that line will come in on either the transmitter line or the CD charger line (same circuit). So, I have some hunting to do. In thinking back, this is the same circuit that my brother would have used back years ago for his ham equipment. Huh … I also experimented with intentionally laying the antenna right next to the extension cord to the transmitter with no change. Ed, now that you mention it, I do have hum issues in certain areas of the AM dial, which come and go. I have found our TV interferes, and our refrigerator (of all things) also puts out some kind of weird interference.

[Ron Ramirez]

So Tim visited me today and brought his transmitter along. Very neat, very compact. Looked good, Tim, good job.

He fired it up at my workbench and I fired up a Crosley "Jewelers' Radio" in another part of the basement.

While there is a little RFI in my basement, his transmitter came through loud and clear with minimal hum.

So, Tim...could the problem be with that particular electrical circuit at your place?

[Phlogiston]

I was messing with things last night and found that if I plugged my transmitter into a different outlet (different circuit) that the hum was a lot less. This is with all breakers on. If I plugged it back into the bench outlet, the hum was louder. I also found when I touched the plug from my CD player to the input jack, the hum increases. It used a 5v USB charger. I thought it may be the problem, so I plugged back into the bench, unplugged the charger and the hum is still louder regardless of the charger. So, whatever is on that line will come in on either the transmitter line or the CD charger line (same circuit). So, I have some hunting to do. In thinking back, this is the same circuit that my brother would have used back years ago for his ham equipment. Huh … I also experimented with intentionally laying the antenna right next to the extension cord to the transmitter with no change. Ed, now that you mention it, I do have hum issues in certain areas of the AM dial, which come and go. I have found our TV interferes, and our refrigerator (of all things) also puts out some kind of weird interference.

The TV has a switching power supply and the fridg might have an inverter. They want to run the compressors on HVAC at 400hz rather than 60 for efficiency OR, like on my well pump, they run the motor at a variable rate. Any of these switching devices might cause some RF interference - though on the 3 devices that I have, I have not noticed any. The switching supply on the laptop on my bench is another story altogether. It generates interference at a high level on the broadcast band and what looks like "dead" carriers on VHF.

[TV MAN]

I still haven't found the source of the hum in the house. I took it again to another property and there was very little hum. It seems really dependent on what is plugged in. I tried a EMI filter, but that didn't help at all. It may be the nature of this to have some hum. Other than that, it is loud and clear. I will try to post some completer pics a little later.

[TV MAN]

Here is the finished product. I still will do some refining, but it is working. 

   
   

[sam]

looks great tim 

so when does production start 

sam