I just got done redoing the input network on my old FL2100b (adjust 15m/10m
coils and caps) and was
happy with the result..so I went back to the AL-1200 I've mentioned here before
to fix it's issues.
1) Opening up the front panel and exposing the AL-1200 input coils, yes indeed,
the ferrite had
fallen out of the 10m coil. The 15m ferrite had been turned in too far for my
hex tool to reach it.
2) 10M couldn't be adjusted. I noticed it had just 1 turn for the "coil" ...so
I disconnected the
coil and replaced it with a wire (basically just across the bandswitch). 10M
looks good now. (it had
been centered lowerbefore I lowered the L by removing the one turn coil)
I wonder if they had tuned 10M when they manufactured it..since it shipped with
the green wire
disabling 10m.
3) I was surprised at the poor way the wires had been arranged. The leads from
the input coils to the
bandswitch seemed excessively long and the wires weren't arranged to minimize
coupling. Also the wire
insulation seemed cheap pvc? (not teflon). I rearranged things, just because I
had seen how dressing
had affected 10M/15M on the fl2100b.
4) Surprisingly, the input network is a Pi network (C's shunting both sides of
the L) on 160/80/40,
BUT just an LC network on 20/15/10. I know an LC is perfectly fine but was
expecting Pi on all bands.
I looked hard at this, wondering if maybe someone forgot to install some micas,
but it appears to be
the design.
I looked at the al-1200 user manual, which lists the input network values, and
it does seem to show
Pi network for all bands. It shows 3-1/2 turns for 10M
from the user manual
160M 1300pf, 28t, 2000pf
80M 1300pf, 18-1/2t, 1500pf
40M 500pf, 11-1/2t, 470pf
20M 220pf, 6-1/2t, 180pf
15M 150pf, 3-1/2t, 120pf
10M 150pf, 3-1/2t, 180pf and 27pf? (not sure why they list two here would
think it should be
100pf or so)
So this is odd. My input swr looks good now, except maybe on 40M it could be a
little better (1.6:1).
So I'm kind of thinking what I have is what Ameritron ships, but it's different
than the above in the
manual/schematic.
But maybe the above is what it's supposed to be?
Has anyone looked at an AL-1200 input network in their amp recently? Have you
seen 10/15/20 as an LC
network, or different values for L and C compared to what the schematic lists
above?
I just got done fixing my fl2100b 10m and 15m input network and had to
remove/add 1 turn and adjust
one side of the Cs. So I'm curious about all this input pi network stuff.
-kevin
ad6z
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