[R-390] Leigh Sedgwick AGC Cap 2uFd replacement

[J.Byers]

On Jan 7th 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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Leigh, the high voltage rated plastic caps from computer monitors may supply you 
with a cheap, but high quality  replacement for you 2uF paper job.
They are used in the scanning circuits and the higher capacity ones (2.2uF in some models) 
are used as waveform correction in series with scan coils.

Also some models (17 inch and up) have a 2uF 400v or even 630V cap
 (at least in Australia, where we use REAL line power!) across the line input socket 
Usually largish rectangular jobs often white in colour, but should fit.

The others in the scanning circuits are in the 'J' series intended for high current pulse work 
and would provide very reliable service in the R390 where they would be merely idling along.
Usually you can get old monitors off junk heaps, but if you have trouble finding one then
I have a bundle of brand new ones somewhere in my shed and will post  you one 
if you can give me a physical mail address 

Cheers
John Byers