[Milsurplus] [ARC5] Receiver Filter Adaptor Simple ARC 5 xtal receiver?

Tue Feb 16 16:01:31 EST 2016

Did you gents have access to the previous emails on this topic and view the video also??

The video pretty well covers the ways to add selectivity.
The selectivity of the LW and MW Command Sets receivers seem pretty adequate for the services they will be receiving.

The cathode bypass i described some emails back. I tried this around 1985 but it had no effect whatsoever. As i commented here, if i redid this experiment ( it’s #603-B on my list now ),

i’d series an RFC with the cathode resistor to see if that would help. The crystal i used, i’m sure it was good; it was ‘removed from service’. 

If by negative resistance you mean regenerative IF – i found this truly mickey-mouse and impractical to use in real life band conditions.  And i’m aware that Roy Paffenberg in his booklet

on the ARC-2 recommended this means to improve the selectivity. After trying it in a Command Set, and i think back now, in my old Lafayette HE-30 eons ago, i completely rule it out as

nothing but a textbook demonstration of the principle.

-Hue 

From: Milsurplus [mailto:milsurplus-bounces at mailman.qth.net] On Behalf Of Dennis Monticelli
Sent: Tuesday, February 16, 2016 11:55 AM
To: Bruce Long <coolbrucelong at yahoo.com>
Cc: David Stinson <arc5 at ix.netcom.com>; ARC5 at mailman.qth.net; Milsurplus at mailman.qth.net
Subject: Re: [Milsurplus] [ARC5] Receiver Filter Adaptor Simple ARC 5 xtal receiver?

Bruce,

I looked into a XTAL filter for the ARC-5.  It will work for the high IF versions but not the low freq IFs due to high crystal resistance.  For example an 85KHz crystal Rs would be 50 to 100K.  By contrast a 2MHz rock would exhibit around 200 ohms.

Various forms of negative resistance generators would work well and could be added with less invasiveness than inserting solid state active IF filters in a low IF receiver.  They also work with the higher IFs, albeit with less shape factor than a crystal filter.

Dennis AE6C

On Tue, Feb 16, 2016 at 9:47 AM, Bruce Long via ARC5 <arc5 at mailman.qth.net <mailto:arc5 at mailman.qth.net> > wrote:

There may be a minimum mod way to add a xtal filter to a ARC-5 receiver.  I thought of this several years ago but have not had the time to give it a try.  The ARC-5 receiver has two tubes serving as IF amplifiers.  Both tubes have a cathode resistor to ground bypassed by a by pass capacitor.  I thought about disconnecting the cathode bypass capacitor and replacing it with a crystal at ( or near) the IF frequency.   The xtal will serve as a cathode bypass but only at the xtal amplifier series resonant frequency.  A small variable capacitor could be added in series with the xtal to adjust its in -circuit resonant frequency.  The first and second IF stages could be adjusted to slightly different frequencies to give a basic bandpass characteristic at the cost of a little gain.

Receiver gain will be affected by the xtal series resistance so attention to this parameter is warranted.  If xtal series resistance is a problem it ought ot be possible to add a transistor or op-amp negative resistance generator to cancel most of the xtal Rs thereby wiggling past that possible limitation.

As we are talking about disconnecting only lead and adding a two port network to the cathode node and ground, restoration to original condition should be quite easy.

  _____  

From: David Stinson <arc5 at ix.netcom.com <mailto:arc5 at ix.netcom.com> >
To: kgordon2006 at frontier.com <mailto:kgordon2006 at frontier.com> ; ARC5 at mailman.qth.net <mailto:ARC5 at mailman.qth.net> ; Milsurplus at mailman.qth.net <mailto:Milsurplus at mailman.qth.net>  
Sent: Tuesday, February 16, 2016 11:53 AM
Subject: Re: [ARC5] Receiver Filter Adaptor

Thank you for this, Ken.  Good info.
This level of circuit changes are forbidden in "my world," but
the ideas and concepts are still fertile fields to plow.
73 Dave S.

----- Original Message ----- 
From: <kgordon2006 at frontier.com <mailto:kgordon2006 at frontier.com> >
Subject: Re: [ARC5] Receiver Filter Adaptor

> Well, David, et al. Your musings are well taken here.
>
> Now, considering a simple method of increasing selectivity in 
> our BAs, we
> might take a look at Mike Murphy's WU2D youtube video "ARC-5
> Command Receivers Part-4" on selectivity.
>
> Near the end of this video, Mike shows how he improved the 
> selectivity of a
> BC-455 by modifying the first IF transformer into a crystal 
> filter.

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