[Hallicrafters] More SX101 Drift Problem

[kiyoinc at attglobal.net]

ah6gi/4 here.  I've work mostly on Heath's, haven't used my
SX-101A since about 1970.

A couple questions, how much drift are you talking about?  Can
you describe it in terms of how many kc from a cold start,
every 5 minutes for the first half hour, then on a half hour
basis for another couple hours?

This will let you gauge the magnitude of the problem. 

The SX-101A was a drifter but there were several reasons for
it.

1) All oscillators are freerunning, ie drifty. It's been 30+
years since I've seen the schematic but I don't recall a single
crystal locked oscillator in it.

2) the bandswitch resistance participates in the frequency
determination.  I remember the art of "rocking" the bandswitch
onto frequency.  This is on two of the oscillators.

3) relatively high frequency VFO.

If you have a frequency counter, measure the frequency and
drift of each of the oscillators.   

I think that the "heater" is useless.  As I recall the SX-101
series had an always-on VFO tube.  That makes a bigger
difference than the heater strip.

I suggest:

1) De-Ox-Id the bandswitch, especially the wafers that control
the oscillators.

2) diddle-stick the caps and VFO tube.  You're looking for
something that'll make the frequency jump when you tap it.

I found a bad cap in the inside of a Heathkit factory sealed
LMO.  It had a bad solder joint on the loop of wire attached to
the ceramic tube. It happens.

It could also be a tube-pin to tube socket problem.  De-Ox-Id
and inserting the tubes several times might help.

If you can find it, clean the electrical/mechanical connections
on the tuning capacitor.  This is a problem with Heath's

What you will end up with, if everything is right, is a
radio that will drift less than a kHz from a cold start,
because it's never cold. 

For normal operating, you won't have to adjust the receiver. It
will be drifting as the entire case heats up. The VFO tube is
already hot.  You could measure the drift with a counter but
you can't hear it.

If you were listening to a non-stop SSB transmission, you might
touch up the dial once in a while, perhaps every 5, 10 minutes. 

In real world operating,the SX-101 and other quality
boat-anchors, are as stable as the newest computerized radios.

de ah6gi/4