[Hammarlund] SP-600 Setting the BFO
Richard Knoppow
1oldlens1 at ix.netcom.com
Mon Jan 30 15:44:43 EST 2012
I am also including a brief note on setting the BFO.
There are two interacting adjustments for the BFO. One is
the trimmer on the top of the can and the other is the
inductor that is connected to the dial in front. The _range_
of the BFO depends on where the main inductor is set.
Getting the right range and also centering the BFO requires
juggling of these two. The range is approximately +/- 3.0
khz. Its determined by the position of the main inductor. In
order to change is its necessary to loosen the shaft coupler
from the knob in front to the inductor. The stop for the
front panel knob is just behind the front panel. By
loosening the coupler its possible to turn the inductor
shaft independantly of the stop. If the range is incorrect
this inductor should be adjusted in small increments, then
the coupler tighened again and the center frequency set
using the top adjustment. At some point you will find both
will be correct. The deviation is not the same on both sides
of zero because the main inductor is linearly wound and
exact calibration of the front panel dial would require it
to be square-law. However, over its relatively narrow range
its close enough.
The BFO, like the entire IF, should be adjusted to the
actual resonant frequency of the crystal filter. Once set
its pretty stable. If you find the BFO shifts from a cold
start look at C-134, a 51pf cap. I've found a couple of bad
ones. There is another of these caps in T-2 for the 4mhz IF,
C-99, I've also found a couple of bad guys. The symptom is
that loss of sensitivity when in double conversion mode due
to shifting of the tuning as the set warms up. If its tuned
when warm it will not be fully sensitive when cold and
vice-versa. Evidently H got a lot of bad 51pf silver micas
somewhere along the line. Getting into the BFO can is a
PITA, requiring the removal of the front panel and maybe the
logging dial.
BTW, I usually put anything I am working on in a shallow
cardboard box to catch any small parts that may fall off.
Yet another note, if you must remove the main tuning
capacitor tie a length of fine solder to the U shaped spring
and fasten the other end to something on the chassis. If the
spring decides to go flying the solder will hold on to it.
--
Richard Knoppow
Los Angeles
WB6KBL
dickburk at ix.netcom.com
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