[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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