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Before attempting any calibrations, the limiter should be oper-
ated for
approximately
15
minutes.
This avoids subsequent drifting.
WARNING:
The full AC line voltage
is
present
at several
points inside the chassis.
Be careful to avoid
personal shock
when you work on
the limiter
with
the covers removed.
5.3.1
POWER SUPPLY
The positive
and
negative voltages are most important
for the
correct function of the entire
unit.
Therefore, these should be
checked first, before attempting any calibration
or
repair.
Con-
venient check points are the metal
tabs (collectors) of Q1
(Q4)
for
+18 V (±0.6 V), and Q2
(Q5)
for -18 V (±0.6 V).
There are
no
adjustments provided
in the
power supply.
5.3.2
Q-BIAS ADJUSTMENT
This control was factory set and should not need periodic ad-
justment.
It is
very important
for
correct operation
of the limiter
that this control be correctly
set, and
therefore we strongly urge
you
to MAKE SURE
that adjustment
is
necessary before undertaking the
procedure.
This may most easily
be done by applying
an input signal
(1
kHz,
0
dBm)
with
the limiting turned OFF.
Turn
the
Output con-
trol full CW and turn the Input control up until the output reads
dBm
on an external voltmeter.
Remove
Q7
from the socket on
the
P.C.
board
and
observe the meter indication.
It
should rise
to
+10
dBm
(±0.5 dB).
If it
does not
so indicate, the
Q-Bias adjustment
should be performed.
Reinsert Q7 into
its
socket, being careful
not to bend the pins
.
Initial control settings
for
Q-Bias adjustments
are:
Input
=
Output
=
Attack
=
Release
=
Ratio
=
Meter
=
Mode Switch
=
Q-Bias*
=
full CCW
full CW
full CCW (switched
full CW
20:1
+4
,
Avg
A/B
full CCW
to
OFF position)
(*Q-Bias
is
set with internal trimpots R13 and R113)
Apply
a
1
kHz
0
dBm
signal to the input
and
turn
the Input
control full CW until the output reads
+1
dBm on
an external
voltmter.
Slowly turn
the
Q-Bias adjust R13
(R113)
CW until the
output drops
3
dB to
-2 dBm.
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21
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Troubleshooting
