Setting BFO for CW
This is another feature that is
difficult to understand. Icom designers saved one little coil
with a core and disabled servicemen to set exact value of BFO for
the CW mode. The CW BFO frequency is derived from LSB frequency
by inserting a FIXED coil 0.22 uH (similar approach is used in
IC-275 as well, and maybe others, but not in older models such as
IC-730, IC-740, IC-751A).
This means that your CW BFO is dependent on LSB BFO. If you would like, for example, to lower CW beat to 400 Hz or 500 Hz you simple can't do that. Also older narrow crystal CW filters are sometimes shifted down a bit by aging. A simple remedy is to move CW BFO frequency of 100 Hz or so. You can't do that.
If you need to change CW BFO anyway, locate the BFO compartment on the MAIN board first:
Second, remove the cover of the compartment and locate the magic L126, the fixed inductor 0.22 uH:
If you need to lower the CW beat
decrease the value of the inductor (solder another small inductor
in parallel to L126).
If you need to higher the CW beat increase the value of the inductor (cut its terminal and put a small inductor in series with L126).
Changing the value of L126 doesn't have any impact on the other modes. It changes only CW (CW-N) BFO beat.
In my case I short-circuited L126 completely by a thin wire. This way I got the CW beat around 700 Hz which is just right for my ears.
The above approach assumes that you first check the BFO frequencies with a counter. First check the frequencies and possibly adjust as the Service Manual suggests. Then think about the modification.
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