[Hallicrafters] Help with SX-101 Mark III

[Dave Moorman]

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.