February 2017
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 wonder if Mr. Casas (or a neighbor sharing the same utility transformer) has recently installed a new central air conditioning system. Many of these systems employ a condenser (the outside unit) having a variable-speed fan. The fan is a three-phase motor powered by rectifying the AC mains power, then using an inverter circuit to convert the DC bus voltage into a three-phase AC voltage. The fan provides tachometer feedback so that the inverter provides three-phase power at the frequency needed to maintain fan speed to the degree desired by the control circuitry.
The inverter operates using pulse-width modulation techniques. A chopper frequency in the range cited — six to eight kilohertz — is not unreasonable, being sufficiently high to be significantly greater than mains frequency yet low enough to avoid the magnetics and copper losses that would otherwise occur if the frequency were higher.
The problem is that some of this chopper operation may be leaking back into the AC mains supply. The DC power filter capacitor(s) in Mr. Casas's radio do an acceptable job of smoothing the AC mains-frequency ripple; for cost reasons, however, there is no reason why the manufacturer would have chosen capacitors having equivalent series resistance (ESR) sufficiently low to filter kilohertz-range noise.
My suspicion is that Mr. Casas's radio is receiving power-line-conducted chopper noise. Since it comes via the power system, it will be presented to the radio's audio amplifier regardless of the volume control setting.
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.
