Marantz 250 - Page 10

METER DESCRIPTION
Audio from the amplifier output terminals is applied
to METER RANGE switch S2, which selects as a
reference two different power output levels: 0 dB =
0.75W or 0 dB = 75W.
Output from the range switch is applied to the
input stage (Q403) of the meter drive amplifier.
Two diodes (CR411, CR412) limit excessive voltage
spikes from reaching the input stage.
The output stage (Q404) drives a bridge rectifier
circuit (CR415, CR416, CR417, CR418) which
generates DC power for the VU meter (M2). Two
zener diodes (CR413, CR414) connected back-to-
back protect tne meter and rectifier circuits from
possible overload.
Overall amplifier gain is controlled by potentiom-
eter R430. This adjustment calibrates the meter for
0 dB = 0.75W* when the METER RANGE switch
is in the 0 dB position.
Meter calibration for the +20 dB position of the
METER RANGE switch is accomplished by adjust-
ing potentiometer R424 for a meter indication of
0 dB = 75W*.
DC feedback for the drive amplifier is accomplished
by R426.
Zener diodes CR409, CR410 regulate supply volt-
age for the meter drive amplifier.
*NOTE:
82 load.
82 load.
0.75W corresponds to 2.45V across an
75W corresponds to 24.5V across an
100,220 — VOLT AC CONVERSION
220-VOLT AC CONVERSION
To convert the Model 250 to 220-volt operation,
perform the following steps:
1. Remove the top cover.
2. Orient the Model 250 so that the rear panel is
facing toward the viewer.
3. Locate TB1, the strip located on the power
transformer half shell facing the rear panel,
which terminates the power transformer pri-
mary wires.
Unsolder the black and white power lead-in
wires and all jumpers from TB1.