Note
QST, Feb 1941, p. 71.
QST, Mar 1946, p. 188.
Millen Catalog, 1946.
Lafayette catalog number 88 (1947), p. 106.
Walter Ashe catalog, 1954, p. 9.
Originally called the "Rice Variarm," that name was dropped at least by the end of WWII, after which is was called an ECO (electron coupled oscillator).
The 90700 Variarm ECO was the commercial realization of a design published in the January, 1941 issue of QST by Henry Rice, W9YZH. It operated on the 160, 80, and 40 meter bands. The arm on the left side of the oscillator box provided vernier frequency adjustment once the basic frequency range had been set with the large knob. On top of the box is a calibration chart. The 90700 had a transformerless power supply.
Large band spread with 100 divisions over the frequency span, with main tuning (knob) and vernier tuning (arm) controls.
The high impedance output may be connected directly the crystal socket of the transmitter. No separate matching network is required. A high capacity grid circuit and temperature compensation result in very low drift (not actually specified). Provides chirpless keying, is immune to vibration, and has no hand capacity. Accurate calibration with an easy-to-read chart. Built-in power supply operates under a constant load under keying.
The only differnece between the two models is the output frequency:
90700 – 3500 to 3650 kHz for use on 80 and 40 meters,
90701 – 1750 to 2000 kHZ for use on 160 and 80 meters.
Caution: The unit does not use a power transformer, so that a shock hazard exists. Observe polarity when connecting to other equipment. The use of an isolation transformer is highly recommended.
Tubes: (1) 6K7, (1) 25L6, (1) 25Z6