Marantz Service Bulletin

Page 1

modelinumhea: WW-l bulletin numberm"

date "

_ GENERAL . .. MG-l 7 4/27/71
for serial numbers to
z _, ALLW-_...__._._H."in, . , , ,, W V W.
x subiect

PROCEDURES FOR REPLACEMENT' OF POWER
TRANSISTOR AND ADJUSTMENT OF POWER
_BIAS_AND_DC_BALANCE

In most cases, the failure of power transistors is accompanied by
the failure of associated components as well. This can include
driver transistors, bias transistors and resistors. To avoid
damage to newly installed parts and loss of additional time, always
check the components preceding the power transistors.

After verifying that all circuit components have not been damaged,
it will then be necessary to readjust bias and DC balance (offset),
in the amplifier. To make these important adjustments, it is
necessary to have the following test instruments:

A. DC millivoltmeter capable of reading 5 millivolts
' DC or a calibrated oscilloscope with DC amplifier
capable of such reading.

NOTE: 1. Terminals of meter or scope must be floating
above ground, since the points to be measured
are at +B or -B potential.

. 2. If a millivoltmeter or the DC scope are not
available, a milliammeter of 200 milliamperes
full scale can be used. However, in this case
one end of the resistor indicated on Table I,
Column 5 must be disconnected and the milliam-
meter connected in series with the resistor.

B. An AC line wattmeter with a range of 0-150 watts.
C. An AC line voltmeter, for appropriate line voltage.

D. Variable Autotransformer (Variac).

General Bias Adjustment Procedure

Outlined below for your reference, is a simplified common bias
adjustment procedure. For specific data about adjustment pots,
voltage drops, etc., please refer to the attached table.

NOTE: Do not connect any signal source to
the equipment under test. If possible,
short the input jack with a shorting
plug or similar device.

0

-2-
Marantz Company,lnc., P.0. Box 99. Sun Valley, California 91352 ~(213) 767~9750 ' 875-0810 - A Subsidiary of Superscope, Inc.

Page 2

model number bulletin number dale

GEHERAL M611 4L22/le
ior serial numbers I to

AT T. ,_ ...,. ,_ W, 7.
subiecl

PROCEDURES FOR REPLACEMENT OF POWER
TRANSISTOR AND ADJUSTMENT OF POWER
:u' : : : I p -ALANCE_-_

A. Connect the unit under test to the variable auto-
transformer (variac), and the AC line wattmeter.
Before applying power set the variac to 0 volts.

8. Slowly advance the control on the variac while ob-
serving the AC wattmeter. Many Marantz components
incorporate special time-delay circuitry to prevent
transient surges to the loudspeakers when power is
applied. You will note this when slowly advancing
the variac as the unit under test will show a jump"
in line consumption (or a relay will actuate), when a
certain minimum input voltage is reached. All adjust-
ments are performed at 120 volts or 220 volts, or 240
volts AC input, whichever is appropriate.

The wattmeter should never indicate a reading beyond the

rated idle power consumption of the unit under test. If l
it does, and cannot be adjusted by following steps D and l
E, shut the unit off and recheck circuitry for malfunctions.

C. Connect the millivoltmeter or oscilloscope across the
appropriate emitter resistor indicated on Column 5 of

Table I, on one channel or connect the milliammeter in 1

series with the resistor.

l

D. Adjust the bias pot* until the proper voltage drop* or
bias current* is obtained.

E. Repeat steps D and E for the remaining channel. 1

Alternate Procedure For

Setting Up The DC Bias

For Models 15, 16, and 168 where idle power consumption can be
measured independently for each channel, sometimes it is more
convenient to adjust the proper bias by measuring the idle power
consumption of the unit and adjusting the bias so that the con-
sumption reaches the value indicated in Column I of Table I.

For the units which combine the two power amplifiers from a single
power supply, this procedure cannot be adhered to, but nevertheless,
after the bias has been adjusted by the procedure given previously,
the total power consumption of the unit must be checked against the
figure given in Column I, Table I.

-3-

*SEE DATA TABLE . _
Marantz Company, Inc., P.O. Box 99, Sun Valley, California 91352 '(213) 767.9750 - 8750810 - A Subsndlary of Superscope. Inc.

or ,,