[Milsurplus] ARC-2: Smart People Needed

[David Stinson]

I'm so glad I know people smarter than me.

So I'm trying to find a non-destructive way to tighten
the IF bandwidth of the ARC-2.
The IF tunes 1.0-1.5 MC as you tune the RF,
making things a bit more "tricky."
Simplified circuit of the RT-91/ARC-2 receiver IF:

http://home.netcom.com/~arc5/ARC-2/ARC2IFCir.jpg

All the DC circuits (screen, fils) have been removed 
for clarity. Mixer plate circuit outputs to 1ST and 2ND
if, which are tuned IF amplifiers, then to 3RD IF amp
which is actually an untuned DC amplifier, then to 
the 4th IF which has only a bandpass filter on the output.
So I've omitted the 3RD and 4TH in the diagram because
the selectivity is in the Mixer plate, 1ST and 2ND IF.

My first thought was to reduce the coupling to the 
tuned circuits, unloading them and thus increasing "Q."
I changed the 800 pFd coupling caps first to 240 pFd,
then down to 100 pFd. The cap to the 3RD IF I
changed to 50 pFd. 

After each change, I realigned the IF according to 
the procedure. I noted no significant reduction in 
stage gain, then plotted the bandwidth.
To my surprise, reducing these coupling caps
actually made the IF circuit *broader." 
A lot broader.

OK Smart People- I gotta be missing something
here that's probably obvious to you.
Why would reducing the coupling here make
the IF broader??
73 DE Dave AB5S