The power supply also houses the components asso-
ciated with the vacuum tube in the 150A Base together
with the transformer through which the signal passes
for transmission to following amplifiers or controls.
While a cathode follower has a low output impedance it
cannot be loaded with low values of resistance or im-
pedance. Therefore, if the M11 Microphone System is
to be operated into a mixer or into a preamplifier hav-
ing an input transformer, the output transformer in the
power supply is necessary.
If the M11 Microphone System is to be followed by a
pre-amplifier in a permanent installation intervening
transformers can be eliminated. In this case transmission
should be directly from the cathode of the cathode fol-
lower through coaxial cable, then through a blocking
condenser to the grid of the following amplifier.
HUM BALANCE
A screw driver adjustment for hum balance is accessi-
ble through a small hole in the front panel of the power
supply. Experience to date indicates that this control is
not necessary but it is provided as an added precaution
in the event that it is needed under some unusual
circumstances.
LOW FREQUENCY CUT-OFF SWITCH
The response of the condenser microphone inherently
extends to very low frequencies. In some cases this is a
disadvantage where rumblings at low frequencies are
caused by ventilating systems or traffic. The low fre-
quency cut-off switch located on the power supply
reduces low frequency response at a rate of 6 DB per
octave, being down approximately 6 DB at the frequen-
cies indicated—20 cycles, 40 cycles, or 120 cycles. The 40-
cycle position gives low frequency response at least
equivalent to the best commercial microphones.
SERVICE INSTRUCTIONS
Difficulties with the M11 Microphone System are
most readily diagnosed by means of voltage measure-
ments using a high resistance voltmeter, (at least 10,000
ohms per volt).
P518A Power Supply.
Remove the chassis of the
power supply from its case. The chassis is attached
to the front panel by four oval head screws and
slides out when they are removed. This is most
readily accomplished by laying the case on its back.
P519A Power Supply.
Remove the front panel
exposing the resistor board and components.
The power supplies will appear as shown in Figure H
which shows the position of parts on the chassis with
component values labeled. Typical operating voltages
are shown.
With the Microphone System in operating condition,
check voltages in the power supplies as shown. If volt-
ages are abnormal, remove the microphone from the
base. Unless the microphone needs repair a l l voltages
should remain unchanged except for momentary surges.
The input cable can be removed to check the power
supply alone.
With the base and cable detached the heater supply
voltage will rise to about 13 or 14 volts and the plate
and screen voltages will rise to about 310 to 325 volts.
No voltage will appear at the cathode since the 6AU6
Vacuum Tube is not connected and no current is flow-
ing through its load resistor.
Assuming that the power supply alone is operating
properly, add first the cable, then the base, then the
microphone. Thus the component which is responsible
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