开云体育

Paul

On my BITX20, and the BITX40, I used an AF design that is different
from the LM386 that Farhan specified. There are some schematics of this
in the BITX20 Group files section under K7HKL. My AF section uses two
NPN's (2N2222)as AF amps, driving an NPN/PNP pair of power transistors
for about 4 watts peak AF output. With this the AF level is more than
adequate.

There have been cases of low receive volume in BITX20 units being traced
to RF transistors that showed less than optimum gain at 10 & 14 MHz.
That was the case on my first BITX20 unit. I found that supposedly good
2N2222's from a surplus source were much inferior to some 2N3904's that
I used to replace them. In another BITX20, VFO instability was traced
to use of a transistor having too low an hfe factor.

Work on my BITX-LF is going slowly, due to other more pressing work on
(1) a 74HC4046 based H&P circuit, (2) installing and configuring DSL
for Internet access, and (3) it is now spring in Idaho and outdoors
activities are competing with ham radio activities!

So far, my BITX-LF has the receiving AF, BFO (3.579 MHz), BFO-mixer,
transmitter AF, and Solid-state T/R switching sections completed.
I have a basic 3.579 MHz IF filter soldered together but not yet tested for
shape factor. The above-mentioned 74HC4046-based H&P VCO will become
the BITX-LF VFO if all works as anticipated.

It will be interesting to see how your LF version of the BITX evolves.
One major problem I have encountered thus far is finding suitable miniature
inductors for 160 KHz work.

Arv K7HKL
_._

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