[time-nuts] Vintage Frequency Measurement

Scott Stobbe scott.j.stobbe at gmail.com
Mon Feb 13 10:35:51 EST 2017


Hi Mike,

First of all, Wow what an interesting read, thanks for sharing some of the
history and your experiences with the 105. A second thanks for uploading
the manual, which I found to be a great read, as with most old test &
measurement product manuals, they are far from just marketing fluff.

Bare with me, I'm not well versed in early radio history, but, I also found
it neat they choose to crystal calibrate on the 3rd harmonic of the VFO to
help prevent injection lock and for increased sensitivity (but that may be
true of all frequency meters of the era, don't know).

Based on the manual, the thermometer is thermally mounted to crystal
holder, allowing one to temperature compensate the crystal calibration
point. I didn't see a mention as to what crystal cut they used. I would
guess it is one with a flat tempCo with no turning points for the linear
thermometer scale to be used effectively.

Attached is a plot taken from the manual, the VFO stability strip-chart

On Mon, Feb 13, 2017 at 12:24 AM, Mike Naruta AA8K <aa8k at comcast.net> wrote:

>
> On 02/12/2017 01:08 AM, Scott Stobbe wrote:
>
>> I was inspired recently coming across a Lampkin 105 frequency meter, as to
>> how  frequency measurement was done before counters.
>>
>> Certainly zero-beating a dial calibrated oscillator, would be one
>> approach.
>>
>> Is there a standout methodology or instrument predating counters?
>>
>>
> Hi Scott.  That Lampkin 105 is a sophisticated design.  I did some
> research that you might be interested:
>
> For the 2015 November ARRL Frequency Measuring Test, I fired up my old
> Lampkin frequency meter.  For their 100th Anniversary, QST was encouraging
> the use of “vintage” equipment for the FMT, and the Lampkin was designed in
> the 1930s.
>
> I (AA8K) did surprisingly well, coming within 322 Hertz on 40 meters, 202
> Hertz on 80 meters, and 18 Hertz on 160 meters.
>
> The Lampkin 105-B was designed by Guy Forest Lampkin BSEE, who got his
> first ham license in 1924.  In 1933 he was selling the model 102, that was
> checked with the Federal Radio Commission and commercial laboratories to be
> within 3 to 15 cycles at 1,712 kc.  He was also selling a “foundation unit”
> of the Precision Micrometer, Band Spread condenser, Special Isolantite coil
> form, Temperature compensator, Adjustable pad condenser, and complete
> circuit details for $14.50.  Lampkin Laboratories moved from 146 West
> McMillian Street, Cincinnati, Ohio to 8400 Ninth Avenue N.W., Bradenton,
> Florida 33506 in 1935.  It was incorporated in 1942.  Precise Power
> Corporation had acquired Lampkin Laboratories in 1971/Oct.  At that time
> Lampkin Labs had 17 employees and wasn't advertising their 107B Digital
> Frequency Meter because they were selling as fast as they could make them.
> The last known address was 12297 US Highway 41 North, Palmetto, Florida
> 34221.  Voluntary Dissolution 2007/April/27.
>
> The 105-B is a fascinating design, able to measure frequency to 0.0025%.
> Signals can be measured from 100 KHz to 175 MHz.   It works similarly to
> the later World War II BC-221 frequency meter.  It can receive, or transmit
> the internal oscillator 2330-2670 KHz.  A diode generates harmonics that
> can beat against the signal to be measured.
>
> The variable condenser has a precision-machined tubular stator and a
> tapered, conical rotor.  They are made from steel and brass and copper
> plated.  The parts are proportioned such that, due to the differential
> thermal expansion, the temperature coefficient of capacity is a few parts
> per million per degree Celsius at all positions of the rotor.  The rotor is
> moved in and out of the stator on a micrometer screw.  The large dial and
> turns counter give a dial band spread of 8,000 divisions over 42 feet!  The
> inductor is wound on a six-ribbed form of polystyrene.  Since the thermal
> expansion of polystyrene is greater than copper, as coil temperature
> increases, the turns are pulled from circular to hexagonal, and the average
> diameter of the coil decreases.
>
> Thermal design is utmost in the Lampkin MFM.  In addition to the L/C
> circuit, the vacuum tubes and circuitry are mounted on the rear, with the
> chassis cut-away to keep it from heating the front where the L/C and
> calibration crystal are mounted.  Wires connecting the L/C and crystal are
> very small diameter to reduce the thermal path.  Even the power transformer
> is bolted to the outside of the cabinet.  The 7.5 MHz calibration crystal
> (no oven) is held against the front panel.  There is a glass thermometer
> mounted to the front panel.  It is custom-marked with a correction factor
> for that specific unit.  I left the Lampkin turned on from October, but the
> temperature soon stabilized.  The metal 6J7 tube has the Bakelite cap
> removed, to eliminate changes due to moisture absorption in the Bakelite.
>
> Striking features of the Lampkin are:  The very smooth tuning with almost
> zero backlash.  Turning the dial clockwise lowers the frequency, but that
> is because the micrometer screw is a right-hand thread and moves the rotor
> into the stator, increasing capacitance and lowering frequency.  Increasing
> frequency moves the micrometer post outwards through the center of the
> dial, like Pinocchio's nose growing.  The outboard transformer looks clunky
> at first, until you realize why he did it.
>
> Modifications.  I replaced the 1 ampere line fuse with a 0.5 ampere for
> additional protection.  (I have been wary of unattended equipment ever
> since an un-fused Radio Manufacturing Engineers receiver monitoring RTTY
> autostart almost burned our house down in the 1960s.)  Measuring frequency
> requires table look-ups and correction math.  Since I did not have the
> original manual with calibration data, and the end-stop was nowhere near
> 000 on the counter dial, I decided to calibrate it myself.  At the same
> time, I added ferrite inside the coil to drop the oscillator frequency down
> into the 160 meter band.  Using my OpenHPSDR, I calibrated by noting the
> Lampkin dial reading for every 100 Hertz on the HPSDR.  During the FMT, I
> used a Collins 75A-4 receiver in AM mode and adjusted the Lampkin 105-B to
> zero-beat the signal.  Using the Lampkin dial reading and looking it up in
> the table, I could interpolate for the frequency between the two
> calibration values.
>
> During the test, I noticed that the Lampkin was varying because the line
> voltage was changing.  The next time I will build the recommended line
> voltage regulator using two 0C3 (VR-105) tubes and a 60 Watt ballast lamp.
>
> I uploaded my mods, photos of the inside, and a scan of the manual to
> mods.dk.  I tried to ftp and email the manual to BAMA/eDebris without
> success.
>
> See attachments
>
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