I have an all American five AM, AC/DC, radio that I replaced all electrolytic and wax capacitors and installed a polarized two wire line cord. All seems to work well i.e. sensitivity, selectivity and volume! But between stations a horrible hum shows up!Why? Can you help?

By your description I assume you mean it sounds fine when actually on a station? So, is what you hear between stations actually hum or more like 60 Hz buzz (raspy, edgy, noisey as opposed to a single tone 120 Hz Hummmmmmmm)?

Usually tunable noise is external to your radio with causes such as florescent lights, dimmer switches, brush type motors, switching power supplies (like for laptop computers) etc. or even power line problems in the neighborhood.

Does moving or turning the radio cause the amplitude of the noise to change? Your radio probably has a loop antenna which has directivity. If rotating the radio causes the noise to change it is coming from a source external to the radio.

Otherwise if it is internal to the radio it only occurs when the AVC voltage is low and the RF/IF gain is high.

Also, let's make sure we are not talking about motor-boating which also isn't hum. Are you aware of what that sounds like? Lower frequency and more like someone giving you the raspberry or an idling Harley Davidson motorcycle.

Curtis Eickerman

Hi Curtis, thanks for your return. I have ruled out your ideas except the possibility of exceptionally high gain. I will look into the AVC circuit. Thanks again, John

I would say it's a noise source in your house produced by some modern electronics like TV's, light dimmers and controllers, charging devices, almost anything today can produce this noise. In my house I had a 13" TV that swamped the Am dial no matter if it was on or off.

Agree with all. What we call "tunable hum" is a hum that you get when tuning in strong stations as opposed to off station. Odds are pretty good that it is outside interference.

Many AA5 radios have a 0.015 mfd cap (or similar value) across the incoming AC line after the switch to reduce noise from outside sources. Of course since that is ruled out it won't help.

Curtis Eickerman