Acoustat Monitor 3 Service Manual

Page: 1 / 28
left right
Acoustat Monitor 3 Service Manual

Extracted text from Acoustat Monitor 3 Service Manual (Ocr-read)


Page 12

INPUT AND EQUALIZATION AMPLIFIER

This is the first stage of the Servo-Charge amplifier and is in operation in both
the operating and standby modes of the amplifier. This stage amplifies and
processes the signal from the preamplifier other audio source and drives the
phase inverter and inephase (roar plates of the speaker) high voltage amplifier.

The Input and Equalization Amplifier provides approximately 20 dB of gain
at typical settings of the Gain control and includes the equalization networks
and the HF Balance control.

R5 acts as the input termination and sets the input impedance of the amplifier.

R9, R10, C12, and Gill comprise the "wall effect" equalization network to
smooth the base response of the dipole output of the speakers in the presence
of an acoustically reflective rear wall. If the speakers are to be operated

a considerable distance from a wall, or in "free air", this network can be
effectively bypassed by the addition of V2 megahm resistor in parallel with
C12.

ClB, C19, and Rl8 comprise the midrange equalization network.
MS and CV comprise the high frequency equalization.

Rlé, RléA, and R17 comprise the High Frequency Balance control network
and determine the relative contributions of the midrange and high frequency
equalization networks.

Rl2, R13, and RM comprise the overall gain control network and C15 acts to
limit the DC offset of the stage by reducing the DC gain to unity.

In addition, on amplifiers prior to serial number l700, a higher level input

was provided which was designed to be driven by the speaker outputs of any
tuner/amplifier on which a preamplifier was not provided. This provision

was seldom utilized and was eliminated in the process of changing the main
input provision From a DIN (RCA phone) jack to a V4 inch diameter telephone
lack . This is a two conductor (mono) iock with only tip and sleeve connections.

If required in individual cases, the "receiver input" provision may be sub-
stituted tor the preamplifier input on the later amplifiers by moving the tip
connector lead from the telephone iack to the printed circuit board so that the
input to the amplifier is the iuncticn of R6, R7, and RB rather than the junction
of R5, R9, and Cl2--after installing R6, R7, R8, and CH on the printed
circuit board (R6 is substituted for the factory installed wire iumper in this
procedure).

Page 27

SYMPTOMS

Unit shuts down
to standby, but
comes back on
immediately .

of

Unit will not

shut down to

standby mode.
or

Unit will not
turn on from

CAUSE

Defective LC.
or Arcing tube

Detective LC. ,
Q5 Transistor or

CURE

A detective integrated

circuit can cause this problem.
The OP-ll device was found

to be particularly prone to this
rare problem. An arcing tube
can also affect proper operation
of the turn on circuitry.

The attending tube will

usually arc at the shutdown

to standby point and bring
back the amplifier to the

full 0n condition. This tube
can sometimes be spotted in a
darkened room. The simplest

standby. bad Relay solution is to replace all
four tubes.

White noise in Leaky 2N3440 Be sure the noise isn‘t

background below (Ql,Q3) originating Upline in the

level of music

preamp or elsewhere by removing
the input to the servo charge
amplifier. It noise persists

replace QI or Q3.

Amp distorts ,
weakened output

sometimes accompanied
by a low level hum.

Detective 02,04,
Ql or Q3 transistor

Check/Repiace detective
transistor.

Intermittent drop

in volume, about
50%, accompanied
by low level hum.

Intermittently open
iunction mm4003
transistor(2N5416 on
later units)

Replace Q2 and Q4

Unit turns on from standby

due to AC line surges-

sometimes induced by an
air conditioner,ilourescent

lights etc.

Earlier production amplifiers
are sometimes found to do this.
Eliminate the problem by
installing a 680 ohm resistor

in series with the cathode
(banded end) of CRI4. This
resistor is factory installed
_-eginning with s/n I700.