[BC] Our Wild TECH Youth - more memories - Dual Crystals

[Charles Lewis]

From: WFIFeng at aol.com
Subject: Re: [BC] Our Wild TECH Youth - more memories

That in turn fed my dual-crystal micro-power AM transmitter.
(Built for this very purpose!) The x-tals came out of dead CB rigs. I 
picked a
pair that gave me 630Khz (a somewhat quiet freq) and that was it.
 

        Mine was a vacuum tube rig.  I had a lot of old military surplus
        crystals in the 5-7 mHz range.  Like you, I employed a pair of
        crystals with the difference frequency I wanted.  It seems I
        recall that I too used a frequency in vicinity of 630 kHz.  I
        used a 12AT7 as a dual oscillator.  The RF output stage was a
        6AQ5.  I built it neatly on a little military electronic module
        chassis.  If I recall correctly, I powered it from the accessory
        jack of my Hallicrafters S-76 receiver.  I used a 12 watt
        Heathkit mono amplifier as modulator.  I used an audio output
        transformer as a plate modulation transformer.  I ran it carrier
        current to cover a college dorm.

        I built this rig to solve a problem.  Believe it or not, I was
        the only person in this dorm at a small school in Western North
        Carolina who had a fairly sensitive FM receiver.  I was bothered
        continually by people wanting to come to my room to  hear
        sportscasts on my Eico FM tuner. Nearly everyone did have an AM
        radio.  I would tune in the desired program on FM and send them
        back their own room to listen to it on their AM radio.

        It wasn't long before it dawned on some of these guys that I
        could also broadcast records.  I soon wound up with a fellow
        acting as program director who would collect LP albums from
        residents and load an evening's worth of music onto my changer. 
        In warm weather when the windows were open, I would sometimes
        hear requests yelled out the window to me. 

        It turned out that by a quirk of the power distribution system
        we had a good signal in a residence nearby known as the "home
        ec" house that was inhabited by young ladies studying to be home
        economics teachers.  They became regular listeners to the
        evening music radio service.

        There was one very interesting aspect to the two crystal setup
        that I had not thought about initially.  These two crystals
        operating around 6 mHz  were identical and were in an identical
        environment.  I had built one of the oscillators with a
        "rubbering" trimmer capacitor.  In the beginning, I tweaked that
        oscillator so that the difference frequency  was zero beat with
        a signal I could hear weakly on the frequency at night with only
        the oscillators running.  This turned out to be a very stable
        way to generate the operating frequency.  Since both oscillators
        drifted more or less the same and the difference frequency was
        only about 1/10 the oscillators' frequencies, this rig was very
        stable without using an oven.  An AM broadcast  transmitter
        normally had to use a crystal oven back then to stay within +/-
        20 Hz with a crystal oscillating at the assigned frequency.  I
        checked it again a couple of months later, and I found that it
        was still almost perfectly zero beat with the commercial
        station's frequency.  I have often wondered if that scheme was
        ever employed commercially to obtain frequency stability

        Charles Lewis
        S9SS

        BTW, there is a somewhat dramatic photograph of my SteppIR ham
        antenna in the DX column of the July issue of CQ Magazine.