[Hallicrafters] SX-100 stability

[johnf]

A few thoughts:

The N150 designation indicates the amount of negative chg in capacity.  It
is expressed in negative parts per million per positive degree C change.
Inductor drift is positive with temp....Formulas?  Never did find any.

Check the back of the ARRL handbook for a listing of N-drift color codes.

The dogbone temp caps used by Hallicrafters are notorious for being the
source of trouble & drifting after 40+yrs.

Chging to all NPO zero drift wud not stop the drift as it wud not offset the
inductor drift.

The N-drift caps need to follow the temp chg of the inductors.  If the coil
form is ceramic I have sometimes found it necessary to put the N-cap in
physical contact with the ceramic form by using clear heat shrink tubing
over the cap & form.  Just hanging the N-cap out in the  breeze does not
always have a good result.

Invest in a can of freeze mist to assist you in locating which
parts have the greatest effect on freq vs temp.  A few squirts can save a
lot of time.

Dipped silver micas have a small neg drift characteristic which can be used
to advantage.  When you need a bit more N drift, chg a NPO disc to silver
mica.

The original values are a good place to start in the effort.  However, you
likely will not find the same cap values & N-drift readily available.  Free
running oscillators are the distant past.  Direct synthesis is the here and
now.

Hence, you must use what you can find and compute the effective Neg drift of
the NPO + Neg caps in parallel and look at it as a "block" of Neg drift.

I have re-comp'd Collins 70K-2 PTOs, Hallicrafter VFOs, Heath LMOs, etc.  It
takes time to sort out things and a good counter to measure the result of
any change.  Slug tuned Inductors tend to drift higher in freq w/age.  The
slugs tend to lose permeability w/age.  So, sometimes you must increase
total C to put the osc back on freq.


Cheers!

John
K0LFA


----- Original Message -----
From: "Dave Moorman" <dmoorman4 at attbi.com>
To: "Hallicrafters List (qth.net)" <hallicrafters at mailman.qth.net>
Sent: Sunday, February 17, 2002 1:54 PM
Subject: [Hallicrafters] SX-100 stability


> I was listening to the net this afternoon when the subject of frequency
> stability and the SX-100 arose.
>
> The suggestions to clean the tuning cap was a good one.  Also checking the
> OA2 is a good idea.  A drifting B+ on the local oscillator is not going to
> help.  A worn out LO/VFO tube (12AU7) is not going to help, either.
>
> It would be interesting to know if the amount of drift is the same on all
> bands, or different for each band.  If it's the same it could be drift in
> the BFO.  This is not very likely, though.
>
> In practice, it was difficult to make a stable oscillator, and the higher
> the frequency, the less stable oscillators were.  Heat was the major cause
> of drift.  The SX-100 has several temperature compensating caps in the LO
> circuit.  C46 and C113 are a couple of examples.  C46 is shown as 100
> PFd/N150.  The N150 is a negative temperature coefficient, which probably
> means that as the temp goes up, the capacitance goes down.  Anyone know
for
> sure?  At any rate, the designer tried to compensate for the effects of
> temperature change by installing caps whose values change with a change in
> temperature, so, in a perfect world, the oscillator would be perfectly
> stable.  That usually ain't the case!
>
> Someone suggested replacing the temp coefficient caps with NPOs, which do
> not change with temperature.   I'd be reluctant to do that because it
would
> defeat the circuit designer's efforts completely and probably make the
> situation worse.
>
> If cleaning and tube replacement don't help, you could try replacing the
> temp compensating caps with new ones with the same specs.  Maybe they have
> changed over time.    If it doesn't, then trying some caps with slightly
> different temperature coefficients would make it better or worse - trial
and
> error.  There are formulas which tell you what of temperature coefficient
to
> use.  I'll see if I can find them.
>
> Dave   K9SW
>
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