WEVC at Evansville College, the First Two Years
By Bill Visher

The transmitter is a Westinghouse Model FM-3. This has an output rating of 3000 watts and is operated at 1000 watts at WEVC. It was first installed in a station in Flint, Michigan. but according to all the information that was gathered during the initial tests, it is thought that they never had it operating. The basic line-up of the transmitter is simple because it used direct FM modulation of the oscillator. The oscillator is an electron coupled circuit operating at about 10 Mcs. It is followed by two 829B's which triple twice resulting in the 91.5 Mc carrier frequency. This is amplified straight through by two 4-250's in push-pull, followed by the output triodes connected as grounded grid. The audio is fed to the transmitter at a 10 dBm level (decibels compared to 1 milliwatt level as zero) and amplified, after the pre-emphasis correction is applied, by a triode connected 6SJ7 followed by a 1614 modulator tube. The varying current is passed through a 6H6 diode connected across the oscillator tank circuit to produce the FM modulation. For a complete explanation, see the instruction manual.

It will be found that the frequency control of the transmitter is excellent. In some thousand hours of operation, the center frequency of the transmitter hasn't changed more than 300 cycles per second from the original adjustment. Our standard of frequency is the frequency measuring service supplied by Harvey Kees. Under modulation that isn't symmetrical, the frequency indication on the FM monitor may shift up to 300 cycles from the frequency under no modulation. It has never been determined whether this indicated shift is due to the transmitter or the monitor or both. It is considered normal. Therefore to read the frequency to a greater accuracy than 300 cps, a period of no modulation is required. Under the adjustment procedure of the transmitter given in the instruction book, several tests are given for checking the tubes in the frequency control system for gas or other leakage. It is recommended that these test be made after every two or three hundred hours of operation. These tests can't be made with the full power for reasons that are apparent from a study of the High Voltage interlock system.

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