Electro-Voice 7600 - Adjusting R23 and R24, the Negative and Positive Current Limit Trimpots

Operating and Servicing Instructions for the Electro-Voice 7600 Power Amplifier
the unit is in the Dual
mode with 8 Cl 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
sinewave generator for a 1
kHz, 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 R26 on the compon¬
ent layout in Figure 12.
Rotate the shaft of R26
slowly clockwise until the
spikes are minimized in
the distortion waveform
as shown in Figure 9.
6. Repeat steps 3 through 5
for Channel 2.
7. Check the ac idle current
draw. With both channels
at idle, the ac line current
should be approximately 1
amp or less. If the idle
current draw exceeds 1
amp, rotate R26 counter¬
clockwise slightly on both
channels until the idle
current is no more than 1
amp.
8. 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
previously removed.
9.5 Adjusting R23 and R24,
the Negative and Posi¬
tive Current Limit
Trimpots
Shown in Figure 11 is an
asymmetrically clipped waveform
caused by an improperly adjusted
positive current limit as deter¬
mined by R24. Had R23 been imp¬
roperly adjusted, the negative half
of the waveform would be clipped
as well, but its degree of clipping
is a function of R23 only and is
independent of R24.
In the following procedures, you
will be adjusting the current limit
thresholds by varying R23 and
R24 in such a way so as to insure
symmetrical clipping.
1. Turn power off and dis¬
connect unit from power
source. Make sure the
unit is in the Dual mode
with a 2 fl load connected
to the channel under test.
If you do not have a 2 fl
load, you may use a 4 fl
load or even an 8 fl load.
The higher the load impe¬
dance, however, the less
likely it is you will have
symmetrical clipping on
peaks into lower impe¬
dance loads.
2. Remove the eleven screws
securing the top cover.
Refer to Figure 1 for the
screw locations.
3. Connect the sinewave gen¬
erator output of the anal¬
yzer to the input of Chan¬
nel 1. Rotate the input
level control of Channel 1
to its full clockwise posi¬
tion. Rotate the input le¬
vel control of Channel 2
to its full counter-clock¬
wise position.
4. Find R23 and R24 on the
component layout in Fig¬
ure 12. Reach into the
amplifier and rotate the
shafts of R23 and R24 to
their full clockwise posi¬
tions.
5. Apply power to the ampli¬
fier and adjust the sine¬
wave generator for a 1
kHz, —10 dBu (0.245 V
rms) output level. For this
adjustment, it is not nec¬
essary to maintain a con¬
stant 120 V ac line input
under load.
6. Increase the level of the
generator until the output
of the amplifier reaches
42.43 V rms (which corre¬
sponds to 900 watts out¬
put into the 2 Cl load).
The ac line current draw
will be approximately 19
amps. Although the fuse
installed is a 15 amp fuse,
it should not blow during
this test unless the amp¬
lifier is operated contin¬
uously for an extended
period of time.
If a 2 fl load is unavail¬
able, use either the 4 or 8
fl load and increase the
generator output level ac¬
cordingly to the corres¬
ponding rated output po¬
wer.
7. While monitoring the dis¬
tortion waveform on the
oscilloscope, rotate R23
counter-clockwise until
the negative half of the
waveform just begins to
visibly clip (more pro¬
nounced spikes will ap-
14
Electro-Voice • a Mark TV Company