for difficulty can be isolated. Since voltage appears at
the cathode only when the 6AU6 Tube is operating, a
shorted blocking condenser connecting the cathode to
the transformer will not be detected except under oper-
ating conditions.
If the 150A Microphone Base is suspected after these
tests, (failure of cathode voltage to appear or failure of
the heater to draw current) the 6AU6 Vacuum Tube in
the 150A Base can be changed as follows:
Remove the 21B Microphone. Place the 150A Base on
its cable connector which makes a convenient withdrawal
tool. Remove the two oval head screws near the name-
plate. Grasp the cable connector firmly and gently
slide the exterior shell of the Base off of its interior
parts. The 6AU6 Vacuum Tube is now accessible for
replacement. Normally this tube should have a life
expectancy of two years or more operation. When
replacing the outer shell note that the nameplate should
be opposite the latch on the Cannon connecter.
Do not attempt to make any adjustments or repairs
to the 2 l B Microphone. If the rear insulated portion has
accumulated dirt, it may be removed with a cotton swab
dampened, but not wet, with alcohol.
Complete repair facilities are available in either New
York or Hollywood.
GENERAL
We believe every facility has been provided to
permit you to obtain dependable, trouble-free opera-
tion while enjoying the high quality performance possi-
bilities of your Altec 21B Microphone. Enclosed with
these Instructions are some suggestions for use of the
microphone. We will appreciate hearing from users
with further suggestions for best results in order that
your experiences may be included in future issues of
our suggestions for use of these systems.
ALTEC LANSING CORPORATION
161 Sixth Ave., New York 13, N.Y. 1161
North Vine St., Hollywood 38, Calif.
P 5 1 9 A
* VOLTAGES MEASURED WITH 150A MICROPHONE BASE DETACHED. ALL
VOLTAGES TO GROUND EXCEPT AS SHOWN.
FIGURE H
7
