I have been using a Sony ICF-9740 AM/FM table radio on my nightstand since 1974. Recently, there is increasing interference on the AM band. It’s not AC “buzz” or “hum” that one would expect from old power supply filter caps; it's more of a high frequency whine — my guess is around 6-8kcs, and it's consistent even when the volume is turned all the way down. I could replace this unit but I would really rather fix it. Any pointers on finding the source of the noise or theories on what might be causing it?
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I have found that a light dimmer can cause buzzing on the AM radio band. If you tune to a strong station that will usually drown the buzzing.
This sounds like interference that is coming from some other device that you probably placed nearby the radio, like a cell phone charger or dock. Sometimes, LED lamps can create nasty interference. So can flat screen TV sets and computer monitors. (Even when they are OFF!) Try unplugging various devices and eventually, you should be able to find the culprit. The other option is to use a transistor radio, and walk around to see where the noise gets worse. This is how I found interference in the past.