sb620 IF parts

[Ed Mosher]

Mike and all,

The SB-620 used a Heath # 40-776 coil in the oscillator circuitry, which
has a value of 23.6 uH (nominal as I recall).  This is a tapped coil, and
in a Hartley Oscillator.  So, from ARRL Handbook references, it is tapped
from 10 to 25% of total turns from the ground (low-z) point.  That should
be enough information to wind one of your own if you have an LCR meter.
Since I don't have the original Heath print for the part, I can only
guess that the variable range was +/- 20% at most.

Note that you must also replace the IF coil with a Heath #52-101, which
is tuned for 3395kHz with a shared cap (1800pf) and one 130pf on each
leg.  Use the old resonant freq formula to calculate the inductance
needed for this 1:1 transformer.  Alternately you can use a tuned circuit
similar to the one for frequencies above 3395, but you won't have the
narrow bandwidth of the double-tuned IF transformer.

Hope this helps get some of you on the right track.

                                     Ed Mosher, WA8ZVO at Juno.com
                                Formerly  "One Of The HAMS At HEATH"

                        The most important things in our lives are NOT
things!

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