[ARC5] Selectivity improvement in BC-639

Tue Jun 3 12:34:13 EDT 2014

Even a "cascode" tube preamp made a huge difference in the sensitivity of the BC-639.  I used one on 2 meter AM, with such a device, in the mid 60s.

More than likely the reason the receiver had a BFO was merely to make it easier to spot weak signals.  

The BC-640 transmitter was a real POS.  Very high ratio of size and weight, per (milli?) watt. Very difficult to tune up correctly, and the output was full of spurs.  If I remember right it used 18th harmonic crystals.  Once you finally got it tuned with RF on the proper frequency, you were well advised not to try another frequency!

As for the SCR-522, it is possible to render the receivers quite sensitive by cutting down the tuning condensers to reduce the range to something like 143-149 mc; but be careful -- the shafts are brittle ceramic.  The condenser rotors must be removed from the frame (not hard to do) and the rotor plate assemblies removed, to pull plates safely.  This isn't hard to do either, because the rotor sections are held in place by socket set screws.  The coils are rewound from 2 turns to 3, and from 1 turn to 2, as the case may be.  Subbing 6AK5s for the 9003s in the front end also helps a lot.  I got one of these receivers to an AM sensitivity of close to 1 microvolt.  Later, I made it into a tunable dual conversion 2 meter FM receiver using the Permakay filter and low IF/audio/squelch chassis from an old Motorola solid state packset.  It was a "good idea" and seemed at first to want to work very well, with a sensitivity of something like 0.8 uV for 20 dB
 quieting; but the front end was just too unstable to hold a signal in the narrow filter passband.  If the air conditioner came on, air movement in the room would cause the receiver to drift.

The front end in the BC-639 is very much like that in the 522, so don't try to modify the condensers without taking them apart.

The 522 front end conversion was published somewhere in a late-40s or early-50s magazine, but I can't remember the source.  I still have a copy of the article someplace.

73

Mike
WA4DLF

--------------------------------------------
On Mon, 6/2/14, D C _Mac_ Macdonald <k2gkk at hotmail.com> wrote:

 Subject: Re: [ARC5] Selectivity improvement in BC-639
 To: "W7EKB Ken Gordon" <kgordon2006 at frontier.com>, "ARC-5 Mail List" <arc5 at mailman.qth.net>
 Date: Monday, June 2, 2014, 12:27 PM

 That was supposed to have
 been AMECO nuvistor pre-amp.

 Mac, K2GKK/5

 > From: k2gkk at hotmail.com
 > To: kgordon2006 at frontier.com;
 arc5 at mailman.qth.net
 > Date: Mon, 2 Jun 2014 11:22:46 -0500
 > Subject: Re: [ARC5] Selectivity
 improvement in BC-639
 > 
 > I used a BC-639 and modified SCR-522
 transmitter on 2m AM from 1962 through 1966.  An AM pre-amp
 made the 639 quite usable.  I still have the 522 in rack in
 the garage but sold the 639 about 3 or 4 years ago. They
 were reasonably stable considering the bandwidth.  FAA used
 the 639s in fixed applications into the 60s IIRC. They were
 often paired with the BC-640 xmtrs with split-site
 installations.
 >  
 >
 * * * * * * * * * * * 
 > * 73 - Mac,
 K2GKK/5 * 
 > * (Since 30 Nov 53) * 
 > * k2gkk hotmail com * 
 > * Oklahoma City, OK * 
 > * USAF & FAA (Ret.) * 
 > * * * * * * * * * * * 

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