E.I.S./Leutz Model C/K Hybrid

     Plans and parts for the Experimenters Information Service (E.I.S., or Leutz) Model C were first offered in the summer of 1923. The IF transformers in the first Model Cs were the RCA UV-1716 type. In early 1924 RCA recalled the UV-1716 transformers because they were about to introduce their own superhet and they wanted to make it difficult for people to build the Leutz superhets. As a result, E.I.S. substituted ACME 30 KC and General Radio 271 IF transformers in the Model C until they could make their own IF transformer (Leutz Model C IF transformers).

     My example of the Model C has the General Radio 271 IF transformers and that dates the set to around 1924. My set complies with most of the specifications of a Model C, but deviates in a few minor ways; the two front panel meters are eliminated and the extra panel space is utilized for an E.I.S. Type K antenna adapter. This set is a three-dial superheterodyne that uses three separate tuning capacitors to tune in any particular station (in association with tap switches on the antenna inductor). The set needs a little TLC but it has got the potential to be a show winner some day. The following are a few particulars of my set:

1. The radio operates on the standard 1920s superhet circuit
2. The front panel measures 40” long by 8” high by 1/4” thick
3. E.I.S. oscillator coupler, filter transformer, and Type K antenna adapter
4. General Radio IF transformers, audio transformers, tuning capacitors, rheostats, and tube sockets
5. Western Electric bypass capacitors

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     E.I.S advertisement for the Model C superheterodyne from the July, 1923, Radio News magazine.

     I was in for a few surprises when I started to measure the frequency response of the IF transformers. First, when I measured the DC resistance of the filter transformer (or Radio Frequency Coupler, as Leutz calls it) I found that the secondary winding was open. The wires for the secondary winding were secured to screws that held the formica endplates in place, and additional investigation revealed that the builder had fastened the external wiring to one of the wrong screws. This mistake must have been made sometime after the set was first built or else the set never would have worked from day one! Second, when I measured the frequency response of the filter transformer I discovered that the peak was at 66.7 KC (more than double what it should be at 30 KC)! The primary and secondary windings appear to be identical. They had the same values of resistance and inductance, 26 ohms and 22.75 mH respectively. Each winding was shunted with a .00025 mfd capacitor. A simple calculation revealed that the correct capacitance to peak at 30 KC should be .00124 mfd. The set had been wired with the wrong capacities across the primary and secondary windings! This radio probably never worked very good from day one!

     Why did the builder use .00025 mfd capacitors across the primary and secondary coils? He may have got that value from the parts list for the Model C in the 1925 edition of Modern Radio Reception. Furthermore, on page 73 of the same book Leutz states this: “Fig. 36 also shows the standard Type C Radio Frequency Coupler. This coupler has two windings each identical in inductance value and each tuned with a fixed condenser of .00025 MF in shunt which gives a frequency of 47,000 cycles.” It’s all very confusing because the Model C had two different IF frequencies; 30 KC in the early sets and 47 KC in the late sets. The two different IF frequencies would require two different couplers, or at least different capacities on the coupler. Perhaps the 30 KC coupler was supposed to use .00124 capacitors in shunt with the primary and secondary windings? I don’t know. How would a home builder of the 1920s ever know there was a problem though? It was a difficult task in the mid 1920s to build a complex superheterodyne without access to expensive test equipment. I installed .00125 mfd capacitors across the primary and secondary windings of the coupler and obtained a peak near 30 KC.

     Coupling had a large effect on the filter transformer. The table below shows the results obtained with the filter transformer with three different distances between the coils. As with the Como superhet, the General Radio Type 271 IF transformers in the Model C did not survive well. Luckily, one of the General Radio transformers in the Model C had good continuity on the primary and secondary windings. However, the frequency response of the apparently good General Radio transformer was way out of specs (it peaked at 19.1 KC instead of 30 KC).

     Blueprints for the E.I.S. Model C superheterodyne. The schematic of the antenna adapter is included at the lower left.

     The above photo is a close up view of the filter transformer. The primary and secondary coils can be moved along the dowel and the distance between the two coils changes their magnetic coupling. The coupling has a great effect on the operation of the transformer. The secondary coil is on the left and the primary coil is on the right. Both coils are shunted with a fixed capacitor (the secondary capacitor is visible on the lower left, the primary capacitor is just out of view on the lower right).

     Another view of the Model C oscillator coupler.

     The above photo shows a view of the audio section. Most of the parts are General Radio. The two tall boxes are Western Electric bypass capacitors. All of these parts were specified in the E.I.S. blueprints.

     The above photo shows the IF section of the set. The three General Radio Type 271 IF transformers are in the foreground. The E.I.S. filter transformer is composed of the two coils on a wooden dowel in the center of the photo. The oscillator coupler is on the right.

     The above photo shows the RF front end of the set. The E.I.S. Model C oscillator coupler is on the left and the E.I.S. Type K antenna adapter is on the right. Other parts include the General Radio tuning capacitors and tube sockets.

     Another view of the Type K antenna adapter.