[R-390] Filters

[Tisha Hayes]

If your gain is drifting there is something else going on that is probably
not directly related to the filters. That is the problem you really want to
dig into.

I had been following your posts over on eHam (I also suggested you come
here). I would start out by looking very closely at your voltage supply to
make sure it is not wandering all over the place, that your tubes are in
good shape (swap individual tubes from your known good IF deck into the
deck in question, keep track of what ends up where or you will end up with
two wonky decks). Make sure your AGC voltage is not doing strange things
and whatever oscillators are involved in the IF deck are also stable.

It sounded like you had a radio that had a whole skew of problems (from the
eHam posts over the past few weeks). There is some interdependence across
the different decks. Like you found out, part of your B+ supply actually
resides on the audio deck.. things like that.

You have one advantage in that at least one of your radios is a known-good
quantity so it will make troubleshooting a bit easier.

Generally the IF filters are either going to work or they will not (due to
the "killer cap" blowing out the input sides by letting B+ get into the
filters). Sometimes the foamy insulation in a filter will deteriorate into
a black sludge and the filter will be mechanically loose inside and rattle
and may break off a lead. Sometimes it will get filled with goo and the
filter curve will not look right if you swept it with a signal generator.

I think that deciding to pull out the IF filters and replace them with a
different technology is a bit premature at this time. You may end up
masking or minimizing the original problem but you will not really have
fixed the issue. There "are" filter-like swaps (like going with Clevite
ceramics, if you could even find them, heck, we would all love a set of
original Clevite ceramic filters but they are rarer then hen's teeth). You
could take some sort of DSP-like approach to an IF filter, an LC type
approach (like the original R-390) or something else like crystal filters
(with appropriate impedance matching).

Generally the mechanical filters should not be seeing any DC potential at
all; they are isolated by capacitors. The IF signal levels involved are low
millivolt range (maybe even high microvolt levels). In a properly aligned
IF deck the filters should exhibit very sharp curves and good linearity.

If you really get stuck you could always send your IF deck out to someone
who works on the radio all the time (there are probably a dozen people who
are real magicians). It would be allot cheaper than sending in an entire
radio for restoration. A properly set-up R-390A is going to be an
eye-opening experience for you.

-- 
Ms. Tisha Hayes/ AA4HA


*"neque est alter hujus universi locus quam anima"*
*('For there is for this universe no other place than the soul or mind')*
*-Arthur Schopenhauer*