Electro-Voice 7300A - Adjusting R26, the BIAS Trimpot; Adjusting R23 and R24, Current Limit Trimpots

Operating and Service Instructions for the Electro-Voice 7300A Power Amplifier
9 SERVICE INFORM¬
ATION
WARNING! No user servicahle
parts inside. Extremely hazardous
voltages and currents may be en¬
countered within the chassis. The
servicing information contained
within this document is only for
use by Electro-Voice authorized
warranty repair stations and qual¬
ified service personnel. To avoid
electric shock DO NOT perform
any servicing other than that
contained in the Operating In¬
structions unless you are qualified
to do so. Otherwise, refer all ser¬
vicing to qualified service per¬
sonnel.
NOTICES Modifications to
Electro-Voice products are not
recommended. Such modifications
shall be at the sole expense of the
person(s) or company responsible,
and any damage resulting there¬
from shall not be covered under
warranty or otherwise.
9.1 Trimpot Adjustments
Figure 12 is a component
layout of the main circuit board
for one channel (both channels
use the same board). The sche¬
matic of the amplifier is shown in
Figure 13. Several trimpots are
provided for adjustment. Resistor
R26 adjusts the bias. Resistor R23
sets the negative current limit
and resistor R24 sets the positive
current limit. These two resistors
also affect the symmetry of clip¬
ping. The LF Cancel trimpot, R39,
minimizes distortion caused by
ripple on the power supply lines.
9.2 Equipment Needed
To precisely adjust the
trimpots, you must have the fol¬
lowing equipment:
® 1 — Oscilloscope (Tek¬
tronix 2445 or equivalent)
® 1 — Distortion analyzer
(Sound Technology 1700B
or equivalent)
® 1 — 16 amp ac ammeter
® 1 — 4 0 load rated at 600
watts
© 1 — 8 0 load rated at 300
watts
@ 1 — Small non-conducting
flat-blade screwdriver or
set of plastic TV
alignment tools
© 1 — 12 in jumper cable
with alligator clips on
each end
® Miscellaneous handtools
(to remove the top cover)
NOTE: If you need to verify the
amplifier’s performance against
the rated specifications, you must
be able to maintain the ac line
voltage constant at 120 V ac (or
240 V ac if wired according to
Figure 2b). Therefore, we recom¬
mend a suitably rated variac (50
ampere rating at 120 V ac).
9.3 A djusting R39, the LF
Cancel Trimpot
Shown in Figure 6 is a
distortion waveform resulting
from an improperly adjusted R39.
Notice the near sawtooth appear¬
ance of the waveform. The trace
in Figure 7 shows the resulting
waveform after R39 is properly
adjusted. Notice the reduction in
ripple.
To adjust R39 for minimum rip¬
ple, follow the procedures below:
1. Turn power off and dis¬
connect the unit from its
power source. Make sure
the unit is in the Dual
mode with 8 D loads con¬
nected to each channel.
2. Remove the eleven screws
securing the top cover.
Refer to Figure 1 for the
screw locations.
3. Connect the sinewave
generator output of the
analyzer to the input of
Channel 1. Rotate the in¬
put level control of Chan¬
nel 1 to its full clockwise
position. Rotate the input
level control of Channel 2
to its full counter-clock¬
wise position.
4. Apply power to the amp¬
lifier and adjust the sine-
wave generator for a 60
Hz, 0 dBu (0.775 V rms)
output level. For this ad¬
justment, it is not nec¬
essary to maintain a con¬
stant 120 V ac line input
voltage under load.
5. Find R39 on the compon¬
ent layout in Figure 12.
With a non-conducting or
plastic-shaft screwdriver,
adjust R39 for least
amount of ripple as shown
in Figure 7.
6 . Repeat steps 3 through 5
for Channel 2.
7. Turn off the generator’s
output signal. If you have
concluded with the test
and alignment procedures,
disconnect the amplifier
from its power source and
re-install the top cover
with the eleven screws pre¬
viously removed.
9.4 Adjusting R26, the
BIAS Trimpot
Shown in Figure 8 is a
distortion waveform resulting
from an improperly adjusted R26.
Notice the pronounced spikes at
the crossover point in the wave¬
form. The trace in Figure 9 shows
the waveform with less pronoun¬
ced spikes after R39 is properly
adjusted.
To adjust R26 for the proper bias,
follow the procedures below:
1 . Turn power off and dis¬
connect the unit from its
power source. Make sure
Electro-Voice ® a Mark W Company
13