[R-390] thanks

[Shoppa, Tim]

K1LKY writes:
> The 60 volts will appear on an ungrounded chassis if the line filter  
> is working correctly.  It's not cleat to me how the line filter can  
> cause a mechanical audible hum (that is, not heard in the audio  
> output, but rather coming from the radio itself.)
> Can someone explain this to me?  (I certainly can explain the 60 volts.)

Not from 390A experience, but from other ham and consumer radio experience:

Line filter caps have this tendency to begin failing on line voltage peaks. So what you get is a 60Hz or 120Hz noise coming from the capacitor as the layers begin to arc over. If it's not in a metal can, it is very very common to see wax or fumes coming from a wax capacitor at this point, and sometimes you can see little sparks inside if the arc-over is happening near the surface. It is also actually audible, either hearing a 60Hz/120Hz buzz or a curdling noise as wax melts or vaporizes. In a metal box there's much less to see but I can believe that a 60 Hz or 120Hz buzz could be heard.

Sometimes line filter caps go from "OK" to "completely charred and a pile of hot wax" in a matter of minutes. In other cases it is a very gradual failure taking years, maybe only making noise on high humidity days etc. In old consumer radios it is very very common to see bulging or deformed wax capacitors with a pile of molten wax under it, or a black-charred chassis on top of it, and the radio still (at least mostly) works.

Tim.