[Hammarlund] SP600 - No Xtal Notch - SOLVED

[David Wise]

Besides my low gain on the 13kHz IF Bandwidth
setting, I also failed to get any notch action on
the .2, .5, and 1.3kHz crystal positions.

I did an in-circuit test first.  I applied 300V,
through a 10M-ohm DMM, to pin 1 on V9's socket.
The meter read about 0.1V, or 10nA.  Then I probed
L37, which in the 1.3kHz position is 18k off ground,
with a microvoltmeter.  Nothing, so it was C114 not C113.

C114 is the bottom half of the capacitive voltage
divider feeding V9's grid.  Together with C113, it
tunes L37 to offer an adjustable-Q load to Y8, and
more importantly, it completes the L36-Y8 series
circuit.  I removed this 270pf mica cap and put it
on my Heathkit IB-2 impedance bridge.

"D" (Dissipation factor) read approximately
.0005 at 1kHz, which implies an ESR of about
250 ohms.  This is apparently enough to ruin
the Q of Y8's parallel resonance, because when
I put in another mica with a D of 0, I saw a
notch on the scope, and on the speaker I could
hear hets being cancelled.  It's still not as
good as I remember on my RAO-3, but it's
definitely working now, at least somewhat.

This circuit appears to be extremely sensitive
to cap quality; the original C114 has better D
than mylar.  It still puzzles me, because
outside T3, C114 is effectively paralleled by
R43, which is only 100k.  Not to mention the
intentionally low-Q L37-R47.

The notch action is most useful on the 1.3kHz
position; it's almost absent on .2kHz .  Maybe T3
could benefit from a full recap.  Or I could
spring for a new crystal...  Nah.

Dave Wise