[Eric T]

My 39-45 hums on stations from about 800- 1710. Below about 800, the audio is nice and clear. Above 800, there is an annoying hum behind the station's audio. It does not occur in the SW bands, just the mid-to-upper BC band.
Any ideas?

[BrendaAnnD]

Tunable hum. Try repositioning your antenna, or move the radio to another locale.

[Electrothaumaturgist]

On the 42-400, I get a hum only on the strongest station on the dial. It completely goes away when I turn the antenna just slightly. This is mildly annoying, as four of the presets come in perfectly without touching the antenna, then for the fifth (and strongest station), I have to pull the radio away from the wall, and just move the rotator a little bit. Tried to adjust the preset oscillator and antenna, which works, but then the volume isn't as loud as the others. It seems that it is picking up the hz cycle from the wiring in the wall at that particular frequency. (Perhaps there are wires not exactly grounded properly at the outlets, or a neutral is reversed in this old house?). I came to the conclusion that a different location in the house would solve this problem (as it never hummed at all out in the garage-even with the fluorescent lights running!). Keep us posted, Eric.

[audiomagnate]

Consider yourself lucky. My 39-45 hums on all stations.

[marty]

Hi All;
Maybe, Eric's Radio just doesn't know the tune.. And so it only partially Hums..
THANK YOU Marty

[Electrothaumaturgist]

^^^ GOOD ONE!!!

[codefox1]

Devices undreamed of or uncommon in the day now routinely interfere with over the air broadcasts. Battery chargers/eliminators, dimmers, computers, wall warts of any kind are prime suspects, many more things as well. If you can solve your problem by moving the set a bit, agree, consider yourself lucky! Not all that much music on AM, just curious, does the set stutter or lisp on talk shows?

[Eric T]

I'm kind of reluctant to move this radio because then I'll have to re-arrange ALL of the darn furniture in the living room (wife is a bit OCD).

I think I'll try setting up a long wire antenna in the attic for it this weekend. Before we moved, I had a long wire antenna running around my back porch, and the radio worked great.

[codefox1]

Make it so!

[Eric T]

Well I have a long wire up in the attic.

My shortwave reception is much better, but the hum is still there on the upper AM broadcast band.

My next experiment may be to shut off all the breakers and bring them on one at a time to see if I can narrow down the noise source.

[Eric T]

I got home from work today and all the ladies were out and about, so I took the opportunity to do some troubleshooting. I shut off all the 120V circuit breakers for the house and ran an extension cord to my radio from the garage. Then I turned on the circuits one at a time until the noise came back. That narrowed down the noisemaker to one area of the house. Then I started pulling plugs in that room until the noise stopped. That unmasked the culprit: an audio amp/speaker set we use for the TV. It's probably got some bad electrolytics. I'll see if it can be fixed this coming weekend.

That is a load off of my mind. I was afraid I was going to have to pull the chassis out of the radio again.

[Mondial]

If that audio amp/speaker is fairly modern, it probably has a switching power supply or a Class D switching output stage. Both will generate tons of RFI unless properly designed and filtered. There should be RF chokes and bypass filters at the power input and speaker outputs. If not you may have to add them or simply unplug the amp when you want to listen to your radios.

This is quite a common problem with modern electronics and unfortunately there may be no simple solution.

[Eric T]

You are correct, it is a modern amplifier and subwoofer/speaker set intended for use with a computer system. A few months ago, it developed a motorboating sound which I remedied by replacing a leaking electrolytic capacitor in it; it is full of cheap caps. It did not cause interference at our last place, so I'm fairly sure that lost another cap or two. I'll probably just replace them all this time around.

[audiomagnate]

Bad caps in the power supply of another piece of equipment can cause hum in a radio in another part of the house? Hmmm. I mean hummm.

[Eric T]

Well, today I replaced a 4700uf and two 470uf filter caps in the amplifier that were out of spec and the interference on my radio has completely stopped. And on top of that, the low-level motorboating from the amplifier's subwoofer has stopped, too.

Edit: I had tested this out while plugged into a different circuit from the radio, and had good results. However, after putting it back in the same room (and plugged into the same electrical power circuit) there is still a bit of RFI. Fortunately, there is an outlet from a different circuit a couple feet away, and the noise is gone again.