[Hallicrafters] Help with SX-101 Mark III

[Kenneth D. Grimm, K4XL]

I just basically said what you did, Dave, in a message to Markus.  You
are right, C92 is listed as a "molded" paper cap...maybe even an evil
"Black Beauty."  It is obviously suspect, but C94 is a 560 pf 500V mica,
and despite conventional wisdom about mica caps never going bad, I wish
I had a dime for every bad mica cap that I've found causing problems in
an oscillator circuit.  Since Murphy is always around, it will probably
be C95, a 10pf N4700 that is probably rarer than hen's teeth!  8*)
73,
Ken K4XL
k4xl at arrl.net
*** BoatAnchor Manual Archive ***
On the web at http://bama.sbc.edu or
FTP site info: bama.sbc.edu login: anonymous p/w: youremailadr


----- Original Message -----
From: "Dave Moorman" <dmoorman4 at attbi.com>
To: "Hallicrafters/qth.net" <hallicrafters at mailman.qth.net>
Sent: Saturday, May 11, 2002 11:04 PM
Subject: Re: [Hallicrafters] Help with SX-101 Mark III


> It sounds like a little too much instability to be coming from dirty
switch
> contacts, Markus.  A more likely culprit would be a capacitor
associated
> with the BFO.  That would include C94 560 pF, C95 10 pF N4700 (a
temperature
> compensated capacitor), C96 470 pF, and C92 .022 mF.  C92 is almost
> certainly a paper cap and almostly certainly subject to extensive
> degradation of its performance over time.
>
> The other caps are probably of mica or some other construction which
is much
> more stable over time.  However, they are in the part of the BFO
circuit
> which determines its frequency and are among "the usual suspects" for
that
> reason.
>
> Just to be sure, you might want to squirt the switchs with a contact
cleaner
> and run them through their positions before replacing caps.  I suppose
that
> if V8, a 6SC7, is defective is defective in a serious way, that could
also
> cause some instability.
>
> Maybe someone with more experience with the SX-101 BFO circuit will
have
> some more suggestions.
>
> Dave K9SW
>
>
> > From: "Markus Thomerson" <k5wtx at arrl.net>
> > Date: Sat, 11 May 2002 21:34:44 -0500
> > To: <hallicrafters at mailman.qth.net>
> > Subject: [Hallicrafters] Help with SX-101 Mark III
> >
> > Hello to the list.
> > I just obtained a very nice SX-101 Mark III and I need some advice.
The BFO
> > has an intermittent instability.  I can be listening to a CW (or SSB
for
> > that matter) signal and the BFO will start to warble and then take
off,
> > sometimes up in frequency and sometimes down and then it usually
returns to
> > normal.  It doesn't matter how long the radio has been on.  Any
ideas and/or
> > pitfalls would be appreciated.  I am just beginning to learn how to
work on
> > tube receivers and I apologize in advance for the bandwidth.
Thanks,
> > Markus.
>
> _______________________________________________
> List Administrator: Duane Fischer, W8DBF **for assistance**
> dfischer at usol.com
> ----
> Hallicrafters Collectors International: http://www.w9wze.org
> ----
> http://mailman.qth.net/mailman/listinfo/hallicrafters