Sae mark 31 b service manual

Page 1

v MARK 3IB
" SOLID STATE STEREO AMPLIFIER

SERVICE MANUAL

' Price; $2.06

HIE )IHVIN

Page 2

MK 3IB

Distorflon

Percent

Harmonic Distortion (mm) vs. Power (2 Channels Driven)

0.i
0.07
0.05
0.0)
0.02

otai

p.007
9.905

0,003
0.002

MARK XXXIB CIRCUIT DESCRIPTION

Each channel of the MARK XXXIB is an all
Complementary amplifier powered with symmetrical
+40V and -40V power supplies. Each channel
comprises parallel-fed complementary differential
inputs pairs, complementary driver stages, and
complementary Darlington output stages. A portion
of the output signals, determined by R13 (3.9K
Ohms) and R14 (200 Ohms), is fed back to the
negative inputs of the 2 complementaw differential
input amplifiers which results in an overall gain of
25.5dB (19 X voltage amplification).

Minimum DC offset voltage at the MARK XXXIB
speaker output terminals is provided by balanced
(matched) 'lOOKOhm resistors (R2, R3, R11, R12,
R52, R53, R61, R62) from base-to-ground, at the
inputs of the complementary differential amplifiers

Excessive input signals are limited at the input to the
MARK XXXIB by back-to-back diodes (D1, D2, D51,
D52) connected between the 2 differential inputs of
the complementary input amplifiers.

The common emitter resistors in each of the
complementary differential input amplifiers are tied
to the opposite ends of a 51V zener diode lD7, D57)
which provides a regulated current supply for the
complementary differential input amplifiers. Because
this regulator diode is fed through 2K-Ohm resistors
from the +B and -8 power supplies, it will float or
"ride in conjunction with a portion of the input
audio signal.

2 5 to so too 200

Output in Wat-ts

The DCcoupled complementary driver transistors
(05, 06, 055, 056) have 62eOhm emitter resistors
(R15, R16, R55, R56) which also feed
overload-protection control transistors (07, 08, 057,
058). These overload transistors are, in turn, driven
by a portion of the voltage across (R31, R32, R81,
R82) which is proportional to the output transistor
current. If less thana 4-Ohm load is driven by the
MARK XXXIB, high-signal currents will cause the
overload~protection transistors to conduct, thus
sinking the driver-transistor emitters which will limit
the signal to the output transistors. The speaker-load
value that will initiate protection is determined by
the resistor protection transistors as well as by its VEE
characteristics.

Bias voltage for the complementary Darlington
output stages is provided by an integrated-circuit
transistor array (011, 061). A bias voltage control
(R20, R70) is provided to adjust for minimum
distortion, Due to close thermal contact of the bias
[C to the output-transistor heat sink, the bias voltage
will track accurately with the bias-voltage needs of
the output transistors to maintain minimum crossover
notch at all operating temperatures. This bias-voltage
lC also assures no possibility of output-transistor
thermal runaway.

A thermal-sensitive circuit breaker is wired in series
with the power transformer primary and will open if
the heat sink becomes excessively hot.

inter-modulation Distortion vs. Power

ati

0.07
0.05

0.03
u.oz

Distortion

atai
o.ou7
0.005
0.00:
atom

Percent

I 2 5 i0 so

Output in Watts