[R-390] R-390 Related Activities

[Tom-WB3AKD]

Tom

I have not swept my (A) yet, as the mechanical filters pretty much define
the passband.  I ressurrected an R-390 last fall and have been tinkering
with it on and off over the past few months and that is the one I sweep
aligned as the audio was not all that hot and I had used the CW alignement
technique from the Tech Order.   Once I set up the sweeper to check the bad
8KHz mech filter I had opened up and fixed, I just went ahead and checked
the () while the sweeper was set up.

I must have messed up the CW alignment or else the procedure is just
inadequate because even the shap of the xtal filter was pretty bad.  I went
ahead and adjusted the .1 pass band so it was symetrical (the capacitor on
the top of Z501.  Now, mind you, I was monitoring the IF output jack with
the scope and not the Det output tha the ASA procedure calls for.  That
helps to get rid of the delay effects that cause multiple humps.

Anyway, once the xtal filter looked good, I went to the 2KHz position and
started squeaking the transformers over the the same center frequency as the
xtal filter.  This meanse that I won't have to retune the receiver too much
as I decrease the BW on a weak signal during operation.

After the 2 KHz was looking good, I widened the various BW and retweaked to
keep the pass band centered and to keep the humps even.  I basically just
messed around 'til it looked good.   Sounds much better now as there is less
receiver induced distortion.

As for the (A), the mech filter by itself, definitely has a bumpy passband
and some experimentation yesterday suggests that the external capacitors
largely just resonate the end transducer coils.  Adding the capacitors
increased the amplitude, but did not significantly change the passband.
Bumps look to be 2 dB or so, as you said.  I have not muscled the (A) over
to the Lab bench for sweeping but will do that soon as I am curious how the
NIB 8KHz filter I put in there looks in situ with (presumably) all the right
parts.  One thing I am trying to do is evaluate the quality of the repair on
the old filter befor I bake it out and seal it up without actually putting
it back in the receiver.  My guess is that with all the high Q resonators in
a mechanical filter would make the phase response squirrel-ly and that would
be good for increased audio distortion, although from listening to it either
it is not all that bad, or my ears are not all that good.  Given that the DF
system version of the R-390(A) (whatever the designation is) changed out the
Mechanical filter IF strip for an LC filter IF strip, my guess is that the
phase distortion is certainly measureable, if you care that much.

Since the HP 3336B generator is GPIB controlable, I'll probably make some
serious attempt to make a bunch of automated measurements on the pass bands
in a few months or so after I clear all the backlog of other things and have
time to do the software and calibrate the setup.  An interesting science
project, if nothing else.  I'll post the response curves or links thereto,
when it happens.

Probably more discussion than you were looking for.

regards,

Tom