[R-390] capacitor analysis

[John Saeger]

David Wise wrote:

> Does it drop on all bandwidths, or only one?  On my 1954 Motorola,
> the level was low on 8KC.  I traced it to the mica cap tuning
> that mechanical filter.  In my 30 years (on and off) of restoring
> vintage radios, this was maybe my third bad one.  Micas are pretty
> good, but you should not trust them blindly.

So far the symptoms are the same at 16kc, 8kc, and 4kc bandwidths.  I
haven't spent much time listening at 2kc bandwidth (yet).  Michael Melland
had suggested that it might be one of the filters themselves causing the
dropout.  That a short in a filter could cause a problem when listening at a
different bandwidth.  I was hoping to get some different symptom at some
particular bandwidth if it was something like that.  For example if it's the
4kc filter shorting out, I might expect the signal to drop out completely
when listening at 4kc, but that hasn't happened.  It's just a decrease, not
a complete dropout.  I still need to test the 2kc bandwidth more though.  I
have noticed that the 1kc and .1kc bandwidth positions don't seem to do much
in the way of narrowing the bandwidth, the signal level just decreases when
I select one of these positions.  Maybe this is another reason to worry
about C501.  I don't know.  I don't really understand how this circuit
works.  The book says that C501 is a coupling cap when using 2, 4, 8, or
16kc bandwidths, but it's something different when using .1 or 1kc.  Maybe
its part of some kind of resonant circuit.  Maybe it's gone out of tune.

John