Two weeks ago I asked for help on a problem with my NW880/20 purchased from
Dan.I installed the active filter option (at unity gain) and it did a nice
job of filtering. Problem was, when I keyed the rig the amplitude of the
sidetone "knocked the wax outta my ears." This,however, doesn't happen
when the filter is switched out of the circuit.
Several of you were kind enough to respond with suggestions or
sympathetic "my problem too" but it is Bob Hodgins KS4HQ who figured out
the solution. Bob pulled out his copy of the schematic and noticed an
impedence miss-match between the receiver circuit and the filter when the
transmitter is keyed. His solution was to insert a 10K to 20k filter
between the filter's input and its ground.
I tried this last night first with 20k, and found even better results
with 12k. This morning I made a few QSO's but found that if I had the
gain up all the way (like you would with the filter in the circuit) the
sidetone would still be too loud. So...back to the bench.
I just finished inserting a 5.6k resistor between the filters input and
its ground. I simply drilled one new hole next to the input pad and one
next to the ground and used the 5.6k resister as kind of a jumper. This
made it easy to solder the resistor's leads to the already existing pads.
It works great, you cannot tell a difference in the sidetone's amplitude
when you switch in the filter. Thanks Bob for your diagnostic skill. And
thanks to all of you who posted suggestions.
Happy New Year to all,
Len
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Len Barish N2BSC ITVA
Department of Telecommunications ARRL
Kutztown University DLARC
Kutztown, PA 19530 NORCAL
Phone (610) 683-4490 Norcal 40A
FAX (610) 683-4659 NW 8020
If the whole world is a stage, I want better lighting.
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