Marantz 250 - Introduction to the Service Manual; Circuit Description Overview; Amplifier Circuit Details

INTRODUCTION
This service manual is intended for use by author-
ized warranty stations. The manual contains service
information for the Marantz Model 250 Stereo
Power Amplifier, manufactured by the Marantz
Company, a subsidiary of the Superscope Incorpo-
rated, Sun Valley, California 91352.
Adjustment, maintenance, and troubleshooting in-
formation listed herein should be attempted only
by the experienced technician, one knowledgeable
in solid state amplifier operation and the use of
test equipment. All instructions should be read
carefully and understood fully before proceeding
with any service.
Symptoms (and their remedies) listed in the trouble-
shooting section, are those which might occur in
some units. As the Marantz Company becomes
aware of other field problems, supplementary ser-
vice bulletins will be issued to all stations. To
improve this service, all problems (and their solu-
tions) not covered in this service manual should be
brought to the attention of the National Service
Manager at our Sun Valley location.
CIRCUIT DESCRIPTION
The following circuit description will be based on
Channel A only. Channel B operates in an identical
manner,
AMPLIFIER
The input stage of the power amplifier, fig. 1, is
comprised of an emitter-follower, Q519, and its
current source, Q518. The output of this stage is
coupled through C502 and R506 to the differential
amplifier (Q501, Q502), which drives an inverter
(Q503) whose collector current is tt
through current source Q504. The inverter is с
pled to complementary pre-drivers (Q507, 0508).
The output of the pre-drivers is applied to their
respective drivers (Q510, Q511) which are coupled
to their respective power transistors (Q802, Q804-
0803, 0805).
Output current regulation is accomplished through
a current-sensing network. Excessive current levels
are detected by resistors R531 and R532. Volt-
ages developed across these resistors are applied to
current sensors Q516 and Q517.
When excessive current levels are detected, 0816
апа 0517 develop peak-limiting signals, which are
applied їо 0505 апа Q506. These transistors dis-
able the pre-drivers on excessive output current
peaks, thus limiting peak output current to a safe
level.
Feedback for the amplifier is developed at the junc-
tion of R531 and R532. The feedback is applied
across two loops. Feedback applied across R520
and C509 completes the driver-power output loop.
Feedback applied across R519 and C508 completes
the loop for the entire power amplifier.