Rusco IF Transformers/Camfield Superheterodyne

     Selectivity is a term used to describe how well a radio receives one station clearly and rejects interference from all other stations. In the early 1920s broadcasting stations were few and far between and selectivity was not recognized as too much of a problem. It only took a few short years to change though. By 1927 the broadcasting band was crowded with stations, and the ability to select one station without interference from other stations was a problem of fundamental importance.
     There was a limit to how close each station could be to each other, frequency wise, because of the sidebands of the transmitted carrier wave. An audio signal with frequencies up to 5 KC will create carrier wave sidebands of 10 KC wide (5 KC below the carrier wave and 5 KC above the carrier wave). Laws were thus enacted to limit the transmitting frequency of each station to multiples of 10 KC.
     The ideal RF frequency response, or band pass, of a broadcast radio set would then be 10 KC wide; no more and no less. The band pass would have a flat top, to allow equal amplification of the different audio frequencies, and the edges would drop off as quickly as possible (simulating a rectangular shape, rather than the more familiar bell shape). It was with these objectives in mind that the Rusco Type 10 K.C. Band Pass Filter was designed.
     The Rusco Type 10 K.C Band Pass Filter passed frequencies between 90 KC and 100 KC; the center frequency being 95 KC. This created a need for the Rusco Type 95 K.C. IF Transformer. The Rusco band pass filter and IF transformers were introduced in March, 1927, in the Camfield “Super-Selective Nine” superheterodyne. The radio was probably the most advanced set of the time. You couldn’t buy one fully assembled (legally), but you could build one.

     Interior view of the Rusco Type 10 K.C. Band Pass Filter. The identical coils are positioned 90 degrees offset from each other to reduce the magnetic coupling between them. The tuning and coupling capacitors are mounted under the phenolic base panel. The plate and B+ battery connections are on the left and the grid and filament connections are on the right. There is a number “100” written in pencil on the phenolic base board near the plate lead. Likewise, there is a number “99” written in pencil on the outside of the aluminum case near the plate lead. These numbers could indicate that the center frequency of this particular band pass filter was about 100 KC.

Schematic diagram of the Rusco Type 10 K.C. Band Pass Filter.

     Interior view of the Rusco Type 95 K.C. IF Transformer. I have four of these transformers and the primary/secondary coil assembly was loose around the wood dowel on all four transformers (you can feel the coils move when the transformer is tilted back and forth). I suspect the wood dowel has shrunk slightly during the past 85 years. The primary coil is closest to the dowel. The primary inside wire near the dowel is the plate connection and the primary outside wire near the secondary coil is the B+ battery connection. The secondary coil is wound around the primary coil. The secondary inside wire near the primary coil is the filament connection and the secondary outside wire is the grid connection. The B+ battery connection and the filament connection are at a low AC signal voltage point and that is why the transformer is wound like this (there is very little capacitive coupling between the coils). The plate lead and the grid lead both have a relatively high AC signal voltage and that’s why these terminals are kept away from other conductive surfaces. The primary coil is shunted with a .0025 mfd capacitor, visible in the photo on the right. The primary inductance is 1.20 mH and the secondary inductance is 12.4 mH. The primary coil in the IF transformer looks to be identical as the four coils in the band pass filter but there could be a slight difference between them.

     A band pass measurement of the band pass filter indicated that it was still in excellent electrical condition. The band pass seemed relatively flat at the top and the response fell off very quickly on each side. The lower -3 dB point was at 97.05 KC. A maximum peak occurred at 103.80 KC and the upper -3 dB point was at 106.61 KC. The bandwidth was 9.56 KC and the center frequency was 101.83 KC. A plot of the band pass might resemble a crude rectangle. I am impressed with it. The Rusco Type 10 K.C. Band Pass Filter was a major improvement over the previous types of “filter” transformers.
     Of the four Rusco Type 95 K.C. transformers that I have two of them are probably NOS (never used) and two of them are definitely used. The used transformers peaked at near 95 KC, just as expected, but the NOS transformers peaked way up at about 108 KC. Why? Perhaps the NOS transformers were factory rejects and they remained unused in some storage box for 85 years? Who knows? The table below records the measured parameters of the transformers.

* center frequency

     Schematic diagram of the Camfield Super-Selective Nine, as copied from the March, 1927, Citizens Radio Call Book. This receiver is a combination TRF/superheterodyne; a switch on the front panel converts the set from a 5-tube TRF radio into a 9-tube superheterodyne radio. This is a nice feature to have since the radio can be set to the TRF mode for local stations (locals would probably overload the superhet mode) and save battery consumption. The radio can be set to the superheterodyne mode for distance getting and super selectivity response. The superheterodyne part of the receiver is shown with dashed lines.
     The radio is a 2-dial TRF receiver or a 3-dial superheterodyne receiver. The first dial turns one variable capacitor and tunes the RF antenna stage. The second dial turns two capacitors ganged together and this tunes a TRF amplifier and the first detector. The third dial turns one variable capacitor and this tunes the oscillator.

     Diagram showing how the parts are positioned on the sub-panel, as taken from the March, 1927, Citizens Radio Call Book. The Rusco IF transformers and band pass filter are in the upper right quadrant. The receiver uses Camfield TRF transformers in the front end. The audio section uses the massive Silver-Marshal No. 220 audio transformers.

     Rusco advertisement from the March, 1927, Citizens Radio Call Book.

July 19, 2012 update:

     With the help of a Wavetek sweep generator and an operational amplifier “precision rectifier” I was able to record what the pass band actually looks like on a Fluke oscilloscope. The above photo shows the pass band of the Rusco 95 K.C. IF transformer (my transformer number 3). The sweep generator was set for a minimum frequency of 50 KC and a maximum frequency of 140 KC, with a sweep speed of 900.0 milliseconds. The Fluke oscilloscope was set at 100 milliseconds per division, which equates to a screen display of 10 KC per division. The pass band of the Rusco 95 K.C. transformer has a typical bell shape

     The above photo shows the pass band of the Rusco Type 10 K.C. Band Pass Filter. Notice that the gain is very low (essentially zero) 10-20 KC on either side of the center frequency.
     The top of the pass band has a slope to it; on one side of the center frequency the gain decreases and on the other side of the center frequency the gain increases. The net effect might be about the same thing as if the pass band had a flat top.

November 7, 2012 Update:

eBay photos of my “Camfield Super 9”
(won on eBay in October 2012)

     Front panel view. The engraved front panel is 30 inches long and in perfect condition (other than a few smudges that will easily clean off). There is a small piece of wood trim missing above the panel and one of the small knobs is missing. The massive cabinet is in very good to excellent original condition.

     Left side front angle view.

     Right side front angle view.

     Left inside chassis view. One of the large Silver-Marshall Type 220 audio transformers has been replaced by a smaller “Apollo” type transformer. After close examination I suspect that the transformer was replaced sometime in the late 1920s. This radio came from an estate in Fredonia, New York, and I am probably the first radio collector to get my hands on it. It is like an “attic fresh” find.

     Right inside chassis view. The Rusco 10 K.C. Band Pass filter in this set is contained in a steel enclosure that is painted black (my other example of this filter is contained in an aluminum enclosure). One IF transformer is in a brass enclosure and one IF transformer is in an aluminum enclosure (all other examples of this transformer that I have are in brass enclosures). I suspect the transformer in the aluminum enclosure is an old replacement. The oscillator tuning capacitor is missing.

     One final view of the chassis. The set is dusty and it is missing a couple of parts but it should be an awesome radio when it is fixed up!

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