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

One the biggest problems in the transmitter is the selection of low noise tubes for the audio and FM oscillator sections. In the file of the transmitter there are some recommendations made by Westinghouse for FM noise hunting. By the use of an oscilloscope, the frequency of the noise components may be identified. By now, most of the spare tubes have been picked over for lowest noise and in the future, more tubes may have to be obtained to find the best combination to meet the FM noise level requirement of 60 dB below 100% modulation. There is a vibration problem from the blower motor so tubes have to be selected for both low microphonics and low heater to cathode hum.

The AM noise problem in the transmitter was solved by the addition of 4 mfd of filter to an existing 12 mfd in the 1600 volt supply. Since the AM noise is mostly 120 cps, there must be a large unbalance in the high voltage rectifier system, which by June 1952, wasn't found. In a three phase, full wave system, the ripple frequency would be 360 cps if the system is balanced.

In the course of working with the transmitter, several bad joints were found. One was at each end of the coupling system between the driver and the final. Before the second bad joint was found it was noticed that the output power would suddenly drop and the driver tubes would heat up because of the detuning. This was the cause of the periods of low power output.

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