preamplifiers, amplifiers, long microphone leads, and other types of inputs. The tube-type control
consoles are different because of the AC filaments. The hum balance adjustment procedure has already
been discussed; however, this adjustment suffices only if all components are in good condition.
(a) For example, if you attempt to make the hum adjustment and find that the control
does not have any effect, there is another reason for the hum. In some models of the control console, the
hum adjustment is nothing more than a DC voltage that can be varied from one side of the filaments to
the other.
(b) In essence, this applies a positive potential on the filaments with respect to the
cathode, causing a current from the cathode to the filaments rather than from filaments to cathode.
Thus, any hum which might ordinarily be coupled from the filaments is blocked.
(c) Now if you cannot make the hum adjustment with the balance control, there is a good
possibility that the voltage is missing from the hum balance adjust. Depending upon the console model,
you may have as much as 40 volts at the center tap of the balance control. This voltage is usually
obtained from a bleeder circuit in the B+ supply. Therefore, when there is a hum control failure, you
should check the power supply voltage and the balance voltage source.
(2) Recall that the muting relay is used to mute the speaker. If this fails, sound may be fed
back from the speaker to the microphone. There are two possible places to look for the cause of this
trouble one is the relay which does the muting, and the other is the switch which controls the relay. Of
course, you can easily determine whether the switch is faulty by using another selector switch to see
whether or not the relay is activated.
(a) If the relay is activated and the speaker is muted, you know that the trouble is in the
switch.
(b) If the relay is not activated, then it should be checked.
(c) Another thing to remember about sound feedback is that the higher audio frequencies
usually cause more difficulties than the lower frequencies. Also, do not overlook the possibility of
sound feedback from extension monitors in adjacent rooms. These sources of sound can also be
troublesome.
e. Since the microphone is the signal source in an audio system, it can also be the source of
troubles. The microphone in most all instances requires a preamplifier as indicated on the block
diagram (fig 1-1). There are many troubles that are attributable to the microphone itself, such as broken
cords, loose connections, and corroded terminals--all of which could cause weak or erratic output. This
is in addition to any other internal damage to the microphone; and, of course, any of these symptoms
could be localized to the microphone by substitution of a known good microphone.
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