[Hallicrafters] SX-100 stability

[Dave Moorman]

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