[Milsurplus] ARC-2: Smart People Needed

[Ray Fantini]

With all due respect for what you are doing may I propose a different idea? Not that what you are attempting is wrong but the original design of almost all AM equipment prior to sideband was wide by design and that effective IF filtering has to take place before the input to the first IF amplifier with a filter being between the output of the  mixer and the first IF.
The AN/URC-7 was a huge three part HF AM transceiver intended for use on the old two to three MHz marine band and although the receiver was a newer design then the ARC-2 it suffered the same problem of being too wide. There was a modification kit that involved removing the first IF amplifier tube, think it was a 6BA6 and replacing it with an assembly that fit into that socket that had a Collins mechanical filter and a tube on a little chassis that fit into the same socket as the original first IF amplifier tube.
Think the modification may have accounted for the URC-7 being used up until the demise of HF AM operation on the marine band, that and the fact that the radio was all crystal control so it required no skill to operate.
I would speculate that something can be built  that replaces the first amplifier tube that incorporates a filter and  a newer tube to fit in the place of the original first IF amplifier tube and if you wanted to return the radio to original just remove the assembly and put the original tube back in place. To me that would be way more simple then trying to change the design of the IF response.
Have no idea where or if it can be found but the schematic of the adapter used on the AN/URC-7 would be real useful as a guide.

Ray F

-----Original Message-----
From: milsurplus-bounces at mailman.qth.net [mailto:milsurplus-bounces at mailman.qth.net] On Behalf Of David Stinson
Sent: Wednesday, April 02, 2014 12:52 AM
To: milsurplus at mailman.qth.net
Subject: [Milsurplus] ARC-2: Smart People Needed

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