[R-390] AGC problems - need help interpreting IF resistance readings

[John Kolb]

Well, of course, more flexible filter options would depend on
your interests.  A good match for the factory SSB opbion
would be to install USB and LSB filters. For more general
SWL listening, a 3 and 6 kHz filters would be nice. For
TTY or CW???

The trick is to either find filters with a 100 kohms in and out
Z or match to the newer filters which are typically 2 kohms.

It appears that all the F455Y-xx filters Y-31, Y-60, Y-120, etc
are 100 kohms centered on 455 kHz. Since the "Y" size case
filters are round, but smaller than the "N" case filters, an adapter
can be made to hold them and provide good shielding. Mounting
the rectangular style filters would be more of a problem.
"Y" series filters show up on ebay from time to time.

The rectangular style filters would be harder to mount. The "V"
case filters, metal, would provide better shielding than
plastic filters such as the "FA" series, and most of the
"V" filters have extremely good shape factors.

One could, of course, buy new filters in an "N" sized case
from Dave Curry, but a couple of those would set me back
more than I paid for my 390  :)

There are Motorola mechanical filters for sale on my site if you
decide on something a little closer to original than the
Clevite ceramic filters.

John
http://www.jlkolb.cts.com

At 08:21 PM 9/5/2005, Dennis Wade wrote:
>Thank you everyone for your responses.
>
>The 8 Kc filter had a high resistance (~120K) short to ground.
>
>When I opened up the filter box, I discovered why the 16 kc filter was
>out of the circuit..it has about 5K from ground to either terminal.
>And, one side was still wired into the common side of the filters!
>That helped me  discover the hard way that I needed to isolate BOTH
>terminals (duh) since one side is all wired in common in order to find
>the second suspect filter.  I also discovered that the tabs on the
>trimmers will only flex so many times before breaking.  One got flexed
>one too many times...luckily it was the one for the 8kc filter.  So, I
>put shrink tubing on the exposed wires and hooked up the remaining
>working ones and fired her up.  Sure enough, no more distortion on
>strong signals, and overall an improvement in audio quality.
>
>Well, if I had to loose 2 filters, I guess those would be the two to
>loose (for my listening anyway.)
>
>I don't feel a need to replace those filters right away.  I happen to
>have an 8 and 16 kc Clevite ceramic filters, and a 4 kc as well.  I'm
>also wondering as I type this if there may be other filter options
>which would give me more flexibility.  (note that this particular
>'390A has the "factory" SSB mods done on it).  Comments are welcome.
>
>           Thank you all again for your help.
>
>                       Dennis
>
>
>
>On 9/5/05, John Kolb <jlkolb at jlkolb.cts.com> wrote:
> > At 02:48 PM 9/5/2005, Dennis Wade wrote:
> > >Well, I think I've isolated the problem to the output side of the
> > >mechanical filter group, so its  either a filter, a mica cap
> > >(unlikely, since the filter would have to be leaking to ground too),
> > >or the switch itself.
> >
> > snip
> >
> > >There is no DC path from input to output of a mechanical filter, correct?
> >
> > Correct, the input coil will measure somewhere around 50-60 ohms, as
> > will the output coil, but infinite between the pair. Also, should 
> be infinite
> > between either coil and filter case (out of circuit)
> >
> > John
> > http://www.jlkolb.cts.com
> >
> >
>
>
>--
>Dennis L. Wade
>KG6ZI
>Carmichael, CA