The Selectoject (as it was originally spelled) was one of the first practical active
audio filters developed to improve receiver reception. It's design, by Dr. Oswald
"Mike" Villard, Jr., W6QYT and Donald K. Weaver, W6VQL, was first published in the
November 1949 issue of QST. By 1950, the design was licensed to the National
Company (of HRO receiver fame) and sold by them throughout the 1950's as the
National's Select-o-Ject was available in three different models, stamped on the
back as SOJ-1, SOJ-2 or SOJ-3. The circuit for each model is identical; the differences
being the wiring harness connections to the receiver. The SOJ-3's harness was
wired for the octal socket on the latest National receivers (HRO-50 and 60 and the B
and D letter suffixed NC-57's, NC-173's and NC-183's). The SOJ-2 was wired for the
octal socket on the earlier versions of these receivers. The SOJ-1's harness was not
terminated and a loose octal plug was supplied to permit the buyer to wire it for any
receiver. So, don't be concerned about finding the "correct" Select-O-Ject for your
particular receiver. A simple wiring change to the octal plug will match any
Select-O-Ject with any National receiver that sports a Select-O-Ject socket. In fact,
you would be wise to verify that the octal plug is still correctly wired for your
receiver regardless of the model number on the back of the Select-O-Ject.
The name Select-O-Ject is derived from "frequency selective amplifier, oscillator
and rejection filter." The circuitry consists of two constant output audio amplifiers.
One can be switched for either zero or 180 degree phase inversion. The other shifts
the phase 180 degrees at a specific frequency set by the "pitch" control. Combine
the outputs of these amplifiers and the resulting signal, at a set frequency, is either
"rejected" if the two outputs are out of phase, or "boosted" if the two are in phase.
The circuit uses regenerative feedback, controlled by the "boost" pot. This control
can be advanced to the point of oscillation. A terminal on the back, marked "key"
allows keying the oscillator, turning the unit into a code practice oscillator.
However, the key terminals must be shorted in order to use the unit as a filter.
Here is an interesting bit of trivia. The Select-O-Ject's Bakelite case was actually
designed for National's TVB-1 "Television Booster." The control labels for the TVB-1
are covered by the two metal faceplates of the Select-O-Ject.
The case was a disaster, with several cracks and a substantial piece of the Bakelite
missing. One knob was missing and the rotary mode switch wouldn't budge. All of
the components and wiring were original, good news, we thought. At a minimum
we would have to replace the paper and electrolytic capacitors, but at least the
electronics had not been messed with over the last 60 years. (The date codes on the
capacitors and potentiometers ranged from the 23rd to the 28th week of 1950,
making this particular example an early model, circa 1950-1951.)
With the condition of the case so bad, our first inclination was to remount the
chassis in a new case. However, we decided to give the case repair it a try. You can
follow up in our Techniques page.
The unit had four 0.05 uF 400 V wax paper capacitors and one 25 uF 450V
electrolytic capacitor, all originals and all leaky. The paper caps were replaced with
0.047 uF 600 V polypropylene film ("Orange Drop") capacitors and the electrolytic
was replaced with a 22 uF 450V electrolytic. While in the chassis, we tested the two
0.001 uF mica capacitors and they tested fine, as expected. All of the resistors were
found to be out of tolerance, so each was replaced as well. The two 12AX7's tested
low and were also replaced. In essence the only electrical components not replaced
were the two mica caps, three front panel pots and the "Reject/Boost" rotary switch.
However, the "Reject/Boost" rotary switch would not turn. So the entire chassis was
turned on its face and the backside of the switch's shaft was sprayed with silicon
lubricant. After letting it sit for several minutes we were able to wiggle the shaft
free. Another shot of silicon and some exercising of the shaft and it was good as
new. We finished up by giving the switch contacts and the three pots a shot of
The connecting cable to the receiver was in good shape, so we simply needed to
remove the existing octal plug and rewire it to match my HRO-60's needs. While the
plug removed, we finished the plug end of the cloth covered cable with a piece of
shrink wrap tubing to keep it from fraying and added a hood to the octal plug
(National did not use hoods on their octal plugs).
There are five leads coming out of the Select-O-Ject. Here is a chart of the lead
assignments and the pinouts for the prewired Select-O-Ject models (remember the
SOJ-1 came with the leads unterminated, so any SOJ-1 wired to a plug, including
mine, could be wired in any sort of manner):
SOJ Lead Assignment SOJ-2 Pin SOJ-3 Pin
Brown 6.3 VAC filament 7 7
Red 200-250 VDC B+ 8 8
Black B- and filament return 5 6
Brown shielded Audio in to SOJ 1 1
Blue shielded Audio out from SOJ 6 5
Click on any picture to enlarge it
The restored Select-o-Ject
Prior to component replacement
After component replacement