Marantz 250 - Performance Verification Procedures; General Test Procedure Setup; DC Balance Adjustment and Test; Total Hum and Noise Measurement

c ~ PERFORMANCE VERIFICATION
TEST PROCEDURE
A. Test Equipment
Refer to Table 1 for required test equipment.
B. Preliminary Procedures
1. Set up the test equipment as shown in
Figure 5, with the instrument controls
set in the following positions:
Line Switch он
Variable-Line Switch Variable
Watt Meter Switch On
Variac 0 (Fully CCW)
Load — Resistive он
Load — Capacitive он
Scope Output 5V Range
Scope Gain Minimum
AC Voltmeter 30V Range
Scope Vertical 20 mv/cm
Scope Horizontal,
Line Sync 1 ms/cm
Scope Input DC
Scope Trace Centered
(Input Shorted)
2. Connections, between the output termi-
nals of the Model 250 and the resistive
load, must have negligible resistance with
respect to the resistance of the load itself.
Applicable resistance adds to the total
load, resultingin inaccurate measurement
of output power.
3. Insert a shorting plug in each input jack
of the Model 250.
4, Remove the top cover of the Model 250.
C. DC Balance Test
1. Connect oscilloscope to Channel A out-
put terminals of the Model 250.
2. Turn Line Switch to ON and adjust variac
to 120 vac.
4.
After relay energizes, adjust Channel A
(Left Hand Heatsink) amplifier board po-
tentiometer R504 for an indication of a
“line” zero (i.e.,0V +50 mv) as indicated
on the oscilloscope.
Repeat test for Channel B (Right Hand
Heatsink).
Total Hum and Noise Test
1.
2.
Connect the voltmeter across the Chan-
nel A output terminals of the Model 250.
With shorting plugs in the input jacks,
the meter should read 0.25 mv or less.
Repeat test for Channel B.
Turn Line Switch to OFF and adjust
variac to 0 (fully CCW).
Bias Adjustment Test
1.
Preset the bias adjust potentiometer R526
on each amplifier board fully CCW.
After discharging the filter capacitors
using a bleeder resistor, disconnect all
red and blue wires from the capacitor
terminals.
Turn Line Switch to ON and slowly ad-
vance variac while observing the voltmeter
and wattmeter. The relay K301 should
energize at 105 volts or less.
Turn Line Switch to OFF. Reconnect all
red and blue wires to filter capacitors.
Turn Line Switch to ON and advance
variac to 120 volts.
Observe wattmeter reading. Adjust the
Channel A amplifier board potentiometer
R526 to increase wattmeter reading to
10 watts above the initial reading.
Adjust the Channel B amplifier board
potentiometer R536 to increase watt-
meter reading to 10 watts above reading
taken with left channel biased properly.
Turn the Line Switch to OFF. Remove
shorting plugs from the input jacks.
NOTE: AC line voltage
must be maintained at
120 volts at all times
during remaining tests.