a. We need to mention at this point that hum is usually not a problem with a transistorized
control console. However, the tube type consoles with AC filaments are subject to hum distortion. To
eliminate this problem, you need to adjust the hum balance control, which is found in the power supply
section.
(1) To make the hum adjustment, set all input selector switches to the center or off position.
If there is an input that cannot be turned off, it should be terminated in a resistance, since it may pick up
hum in a manner similar to an antenna.
(2) After all input circuits are either off or terminated, turn the master gain all the way
clockwise. If the preamplifier is in the circuit following the selector switch, turn the preamplifier gain or
mixer to minimum, fully counterclockwise. Now you are ready to adjust the hum control for minimum
hum in the output of the program line.
b. The switch contacts can be a source of much trouble due to their position in the line relative
to the amplifiers.
(1) When a switch spring becomes bent, it does not close properly; therefore, you must adjust
it for good contact.
(2) If a contact becomes burned or otherwise pitted, erratic action may result. Switch
contacts should be checked carefully and adjusted or aligned for smooth action. This is true for both
rotary and lever action switches.
c. When setting up the output level, the important thing to remember is that your accuracy can
be no better than the VU meter reading. In other words, you cannot properly set the output unless your
VU meter is accurate. Depending upon the console, you may find various methods of calibrating the
VU meter.
(1) If you find the meter is improperly calibrated, it must be recalibrated against a known
standard. In some cases, it is necessary to change the resistors in the pad circuit to get the correct VU
meter calibration.
(2) You should realize the importance of output level control since this signal goes to a
recorder, a live broadcast, or a live CCTV network. Therefore, any distortion which results from the
overdriven output causes a loss of intelligence.
d. The troubles diagnosed here are only a few that may cause the symptoms described. Some of
the troubles may have many other symptoms which enable you to determine the most likely cause of
malfunction.
(1) As we have already mentioned, some of the hum problems are found in both transistor
and tube-type sets. However, we know that troubles with hum are less frequent with transistor units,
because they do not use heater voltages. Most of the hum picked up in a transistor unit are from the
5
Previous Page
