[Hallicrafters] SX 101A Challenge

[Bill Smith]

I just cured a similar problem in the SX-117.  The first conversion
oscillator was very unstable over a short frequency range, but within that
range, it was miserable!  The problem would vary from a slight instability
to such an outrageous outflow of frequency modulation that the receiver
could almost be blamed for an attempt at singing a song.  It even bothered
the cat.  I tapped every part in the receiver, and used a soldering iron and
freeze mist in an attempt to find a sensitive part, but no luck.

I was quite prepared to remove the oscillator parts and replace them all
(which I have never done), but the poor receiver was spared when I
associated the problem with a 3-30pf trimmer soldered to the top of the
tuning capacitor.

The trimmer is the calibration capacitor used to align the oscillator to the
dial frequency while the front-panel calibrate air-variable cap is set at
half-stator.

I temporarily removed one leg of the variable cap, and the problem
disappeared.  It is a round device, marked 3-30 NPO and as mentioned, was
soldered directly to the top of the tuning capacitor.

The cap was replaced.  If you have a similar trimmer in your receiver, I
would strongly suspect it is the culprit.

Hope this helps.

73 de Bill, AB6MT
billsmith at ispwest.com


----- Original Message -----
From: "Rocco Lardiere" <lardiere at ix.netcom.com>
To: <hallicrafters at mailman.qth.net>
Sent: Thursday, September 12, 2002 10:46 PM
Subject: [Hallicrafters] SX 101A Challenge


> To the Hallicrafters Experts:
>
> OK - I am stumped.  My SX 101A local osciallator is warbling slightly,
> and I can't stabilize it.  This is not the usual temperture drift.  This
> is a slight delta frequency, continuous, perhaps only back and forth a
> few cycles, but it is constant from turn on to whenever.  It just sounds
> awful on CW.  It is more pronounced at higher frequency bands.  The
> problem is in the 1st conversion oscillator - I can hear it clearly when
> I couple my test receiver to the tube and listen to it directly.
>
> I have rebuilt and resoldered the components around V3, the 12By7A, and
> it is less prone to vibration now.  This problem is not related to
> vibration or turn on temperature changes.  The B+ and bias to the
> cathode look clean and stable.  The warble is not a steady beat note -
> it is just back and forth at a few hertz, but not at a stable low
> frequency.
>
> I have tried tugging on every freq component, including the main
> variable cap.  I resoldered everything in sight.  Everything looks
> solid.
>
> This is a really beautiful 101A - very, very clean, so I want to save
> it.  I suspect the receiver is like new because this problem has been
> there from very early in its life.
>
> Any hints?
>
> Rocco Lardiere N6KN
> _______________________________________________
> List Administrator: Duane Fischer, W8DBF **for assistance**
> dfischer at usol.com
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