[R-390] thanks

[2002tii]

Tim wrote:

> >> 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.

I replied:

> > This is a plausible source of mechanical buzzing, but in this case it
> > may be inconsistent with the reported finding of 60 volts on the
> > ungrounded chassis.  Unless all four capacitors were failing in
> > exactly the same manner at exactly the same rate (which is highly
> > unlikely), one would expect that the voltage divider formed by the
> > caps would become unbalanced and there would be a different, and
> > likely varying, voltage on the ungrounded chassis.

Tim then said:

>I don't think we're talking about the same thing. A perfectly good, 
>100% new, line filter will put 60VAC on the chassis if you lift the 
>ground. You don't need any failure of any component or subcomponent at all.

What I was trying to say is that breakdown of line filter capacitors 
is not likely the problem with the original poster's radio, since he 
reported that his ungrounded chassis was at 60 volts.  If the caps 
were failing, I'd expect some voltage other than 60 (and very likely 
varying) on the ungrounded chassis, because the voltage divider that 
determines the chassis voltage -- two caps in parallel, in series 
with another parallel pair -- would become unbalanced.  Since his 
ungrounded chassis is at 60 volts, it appears the divider is in 
balance, which means that the filter caps are likely OK.

Best regards,

Don