[R-390] Mechanical Filter Issue

[Leigh Sedgwick]

Well, for the record, the 2.2uF electrolytic worked fine in
replacing C551 but did nothing to cure the problem describe
below.  So, more investigative work is required to correct
this issue.  In the process of repeaking the IF and RF
alignment, however, I found a significant problem with with
the IF can, T207.  As it peaks up, there is a pop and the
sensitivity immediately drops way down.  It can be brought
back up by detuning the coil slug and then adjusting it back
slowly.  When peaked, the voltage slowly decays back down so
I'm guessing I've got a bad cap in here to replace.  I'm
having fun now.  I won't bother the list anymore until I get
this puppy running correctly. 

Thanks to all for the help.

73 de Mike
K7PI

Leigh Sedgwick wrote:
> 
> Well, I'm making progress.  The 2 khz filter has been
> replaced and is working fine now.  I have completed an
> alignment of the radio and it is working reasonably well but
> needs some additional tweaking.  It was interesting that
> although all of the RF slugs appear to be identical, I had
> to switch a couple of them to get them to peak at a
> reasonable spot in the coil form so I am not confident that
> they are same.  I recommend marking them in a clear and
> concise manner prior to removal so you don't run into the
> same problems that I have had.
> 
> On to a new subject.  I read that one can check the AGC
> capacitor C551, by switching between fast, medium, and slow
> AGC attack and that the signal strength should not change.
> Well, when I switch to the slow position, the audio
> increases dramatically and the signal strength, as measured
> by the carrier meter, drops approximately 20db.  Should I
> replace C551?  This cap is a fixed paper dielectic, 2.0uF
> +-10% rated at 500VDC.  There are no direct substitute caps
> available that I am aware of.  I have a 2.2uF electrolytic
> rated at 450VDC.  Has anyone replaced this cap with a an
> electrolytic with success?  I guess I'll leave it installed
> in the chassis and install the replacement cap below.
> 
> Any thoughts?
> 
> Thanks again to all that have provided responses to help me
> with my issues.
> 
> 73 de Mike
> K7PI
> 
> Leigh Sedgwick wrote:
> >
> > Hi Gang,
> >
> > On the a different subject.  My restoration R-390A project
> > is just about finished.  I rebuilt the RF deck and recapped
> > the IF deck and AF decks.  The smoke test was positive with
> > signals on all bands, although it is way out of alignment.
> > But I have a couple of issues.
> >
> > 1.  Three of the four mechanical filters appear to work
> > fine.  But, when I switch to the 0.1, 1, and 2 bandwidth
> > positions, there are no signals.  Is there any quick check I
> > can make to ensure it is the 2 khz mechanical filter and not
> > something else in the circuit?  It looks like a bit of work
> > to replace and I would hate to go through that effort only
> > to find out it is actually something else.
> >
> > 2.  I tore the RF deck apart with the idea I would get it
> > rebuilt within a week or 2.  No such luck...work issues
> > caused a significant delay of almost a year before I was
> > able to get back and finish it.  During that time, I lost
> > track of the ordering of the slug racks and was forced to go
> > with a "best fit" approach to reassembling them.  Other than
> > the fact that the alignment is totally hosed up, will I be
> > faced with any other issues, i.e., are they all identical or
> > are there differences I cannot see visually.  I saw
> > something on the FAQ page that some are different.  They all
> > looked the same to me.
> >
> > Thanks for any help.
> >
> > Happy New Year
> >
> > 73 de Mike
> > K7PI
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