Speaker Matching
Use the output tap most nearly equal to total speaker
impedance. If the load impedance falls between two out
-
put terminal values, use the terminal of lower impedance.
Speaker Power Distribution
Total power distributed to all speakers should be not greater
than the power rating of the amplifier system. The
70.7V
distribution outlet permits connection to a large number of
speakers, each speaker operating at its required
powt!r level.
Computing impedance
is
not necessary for this
application.
Each speaker
is
equipped with a line transformer having
various power taps. Select the tap which yields the power
desired for that speaker.
OPERATION
ADJUSTMENT OF POWER DRIVER BALANCE
CONTROL
The Power Driver Balance Control
(R116 on Power Driver
PCB), balances the outputs of transistors 0104 and 0105
on the PCB. If this control is not adjusted properly, high-
frequency distortion results. If adjustment is indicated, use
the following procedure:
Step 1. Connect a 16
-
ohm dummy load across terminals
2 (16 ohms) and 5 (common) of
TB2 (see
Figure
121.
Step 2. Apply a 20 kHz sine wave to terminals 1 and
2
of the INPUT terminal board (TBlI.
Step 3. Adjust VOLUME control for 100
-
watt output
(40 volts).
CONTROLS AND INDICATORS
Step 4. Connect a frequency distortion analyzer
(pre
Two operating controls are on the front pane1:a VOLUME
ferred) or an oscilloscope
to terminals 2 and 5
control and a primary power ON
-
OFF switch. A pilot indi-
of TB2 (16
-
ohm speaker output) and observe
cator is lit when the power is ON.
output.
HIGHPASS Fl LTER
A
two.section highpass filter is provided with the 15948 to
protect driver loudspeakers from excessive low
-
frequency
power demands. Filter use is indicated when high power
output from the amplifier is applied to driver loudspeakers
not equipped with protective crossover networks. The
3 dB frequency of the
highpass filter is 400
Hz
(see
Figure
2).
A screwdriver
-
operated FILTER IN
-
OUT switch is located
on the rear of the chassis (see Figure 6). The switch is
placed in the OUT position when the
15948 is ;hipped
from the factory.
SERVICE
If a malfunction occurs, service should be performed by an
ALTEC Qualified Service Representative. For factory ser
-
vice, ship the 15940 prepaid to Customer Service. ALTEC,
1515South Manchester Avenue. Anaheim, California 92803.
For additional information or technical assistance. call
(714) 774
-
2900, or
TWX
910
-
591
-
1142.
Main chassis component locations are shown in Figures 10
and
11, The 15948 schematic is shown in Figure 12. Com
-
ponent locations on the power driver PCB are shown in
Figure 13.
ACCESS
Remove the four screws securing the front panel, then
open and lower the hinged front panel to gain access to the
chassis interior.
If the ALTEC 42526 Shelf Mount Cover accessory is in
-
stalled, remove
by
reversing the steps of the cover installa
-
tion procedure given in the associated instructions.
Step 5.
Remove four screws securing front panel, then
open and lower panel.
Step 6.
Adjust R116 on Power Driver PCB (see Figures
5
and 12) until minimum distortion
is
observed
on distortion measuring instrument.
Step 7. Close front panel and secure with four screws
previously removed.
ADJUSTMENT OF OUTPUT
"Q"
BALANCE CONTROLS
Output
"a"
Balance Controls R136 and R137 on Power
Driver PCB (see Figure 5) balance the bias current of power
transistors
01, 02. Q3 and 04. Inadequate adjustment of
these controls may result in distortion and excessive current
drain from
one or more power transistors. If adjustment is
indicated (such as replacement of one or more power tran
-
sistors), use the following recommended procedure:
Step 1.
Turn VOLUME control fully counterclockwise
(01.
Step 2.
Remove four screws securing front panel, then
lower panel for access to interior.
CAUTION
the chassis
is
opened for service. This
procedure should be referred to a quali
-
Step 3.
Turn Output
"a"
Balance Controls R136 and
R137 fully clockwise (see Figure
5).
Step 4.
Turn on power and allow a 5
-
minute warmup
period.
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