Every morning, when I come in to work, I turn on my radio. One morning out of, maybe, ten, I get a low hum drowning out the news. The pitch of the hum gets higher and higher until it’s inaudible. About a second later, the tone is audible again and becomes steadily lower. Once the tone becomes sub-audible, it doesn’t come back. If I turn off the radio, the tone picks up at the same pitch it was when I turned it off, even if I leave it off the whole day. I usually just turn the volume down for five minutes and it’s fine. Why does this happen?
In case it matters, it’s a digital clock radio, about ten years old, with a wire antenna stuck to the side of my cube with a T-pin.
Hum is either feedback or interference. The most common reason for this, especially if it’s an older radio, is dirty controls. Cleaning the controls is not hard to do, but probably not something you can do unless you want to try cracking the case open. Does the audio sound scratchy when you rotate the adjustment knob? Does anything else (like tapping the case) affect the symptoms?
If it is digitally tuned there is a circuit called a phase locked loop that controls the frequency of the receiver. The circuit contains an oscillator and a frequency reference as well as a means of measuring the difference, when you select a frequency the PLL circuit generates that value by adjusting its oscillator in respect to the frequency standard until the correct phase relationship is reached. Probably the problem is caused by drift in the oscillator or frequency standard. I have seen this happen with one of my receivers, don’t know why, but as you stated, it seems to happen once up and down and then go away. In my case it is not overpowering, just audible. This is with a Grundig YB-400. Not something that is likely to be readily repairable, if this is the case.
You might be the subject of Secret Navy Mind Control Experiments. Or maybe you’ve just got a bad capacitor in your Heterodyne circuit. A local oscillator that’s unstable as it warms up can produce the slow chirp you describe.
Let’s hope it’s that heterodyne circuit. I’ll let you know if I turn up wearing an “Irish Pub” Marmaris t-shirt with no idea how I got it, though. I didn’t think I’d be fixing it, since I bought it for a dollar at a yard sale. I’ll probably just put up with it until I change jobs. Thanks.