[Hallicrafters] S-38C dial tracking and AGC

[Kevin J Ward]

Once upon a time I was the shop manager for an avionics repair facility. 
At that time we worked on a mix of equipment types; tube type,
transistorized, integrated circuit.  It was an assortment of antique and
state-of-the-art.  A most interesting and challenging beastie was the ADF
(Automatic Direction Finder).  They typically operated from 200 to 1800
KHz, and the older units were continuous tuning with four-section tuning
capacitors.  Talk about tracking problems!

One of the tricks my boss taught me was to deliberately set the signal
generator to the image frequency and check the received signal level.  We
did this before we began the alignment, and again when we were done.

Occasionally the normal alignment procedure (cap at the low end, coil at
the high end) would not work; the results would get progressively worse. 
The trick here was to reverse the adjustments; tweak the cap at the high
end and the coil at the low end.  It sounds like it couldn't possibly
work, but it did.

The IF cans were all double tuned, and the primaries and secondaries were
set to different frequencies in order to get the desired bandwidth in the
IF section.  To do this you load the opposite winding with a 1K resistor
to swamp it while you make the adjustment.  Three iterations were
required to reach the point of diminishing returns.  

Tuning primary and secondary of an IF can for maximum volume on one
frequency will result in the most narrow selectivity, provided the
coupling between windings can be adjusted.  That's a topic all by itself.

Kevin  N2IE

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