[Milsurplus] ARC-2: Smart People Needed

[Ian Wilson]

Hi Dave,

Are the IFT primaries and secondaries coupled by both magnetic (proximity)
coupling and capacitive coupling (the 5pF caps) or only by the caps?

73, ian K3IMW



On Tue, Apr 1, 2014 at 9:52 PM, David Stinson <arc5 at ix.netcom.com> wrote:

> 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
>
>
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