AN ALTERNATE "SOUTHWEST TECHNICAL PRODUCTS" THEREMIN DESIGN

by Fred Nachbaur, Nelson BC, November 1997

I'd been hoping to try out my "Dis*Player" pitch and volume display
design on the original "Southwest Technical Products" theremin design,
to assure compatibility. The unit I had built in the 70's had some time
ago been given to my son, who lives over 1500 miles away. Imagine my
surprise when one of my radio tech colleagues mentioned one day that he
had one tucked away somewhere in his basement!

He agreed to let me play around with it, and my surprise deepened when
the unit he has turned out to be somewhat different from the one I had
built. I am documenting the differences in this article, for reference
by future (and present) theremin historians.

The biggest difference is that instead of semi-adjustable coils, this
(presumably earlier) SWTP design uses fully variable coils of the type
once used in the antenna circuit of AM radios. These are quite long
(about 2"), and have a slug that is tuned by long threaded screws. Two
of these (the pitch reference oscillator coil and the volume slope
filter coil) are mounted on the pc board, at right angles to the board,
such that the slug screws protrude through the front panel. Small knobs
are affixed to the slug screws. These form the user adjustments for
pitch and volume null. The other two (pitch oscillator and volume
oscillator) are mounted on L-brackets at the outer edges of the board,
and therefore lie parallel to the pc board.

A check on an LCR meter revealed that the inductance of these coils is
variable between about 40 and 280 microhenries.

Related to this difference is the fact that there are no fine-tuning
controls. Instead, the coil slugs are used for fine tuning. Being a
multi-turn affair, the need for a separate fine tuning control is
obviated using these coils.

The network consisting of R3, R21, and potentiometer R22 in the pitch
oscillator are instead replaced by a single 1K resistor (R3).
Similarly, in the volume oscillator, R19, R23 and potentiometer R24 are
replaced by a single 1K resistor (R19). The fact that these "added"
parts have numbers higher than the other parts in those areas of the
circuit is evidence enough for me that this unit is the "original"
version of this design.

Interestingly, the copper side of the pc board is labelled "142", just
like the later version. On the silkscreen side there is a legend, "Sw
Tec".

The only other circuit change is that resistor R13 is 100K, instead of
1 megohm as on the later version.

The board appears to be somewhat longer than I remember the version
that I had. In fact, the holes in the corners of the board line up with
the holes in the front panel. 1/2" standoffs separate the pc board from
the front panel, and longish wood screws hold the assembly in place
against the front of the case.

The plywood case is varnished, rather than having the fake leather
"Mac-Tac" type of finish as was on the later version. Like the later
version, the antennas are made from triangles of pc board material.
However, they are held to the top of the case using wood screws passing
through a hole in the center of the antennas, instead of being glued
into place like the later version.

The transistors have different part numbers, though functionally they
are similar. These are listed below:

Q1, Q2, Q4 - MPS3638
Q3, Q5 - MPS3708
Q6, Q7 - TI744D

Interestingly, the owner of this particular unit had somewhere along
the line replaced Q4 (the volume oscillator) with an old-style
germanium unit (part number not legible). Despite the considerable
difference in specification between this and the silicon transistor
intended for this position, the unit works fine.

It would be interesting to determine how many of these "original" units
were sold, before changing the design to the more commonly seen version
using semi-adjustable coils and fine-tuning potentiometers.

(C)1997