Marantz 7C pre sm

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INTRODUCTION

This service manual was prepared for use by Authorized Warranty Stations and contains service
data for the Marantz Model 7C Stereo Console.

Servicing information and voltage and resistance data included in this manual are intended for
use by the knowledgeable and experienced technician only. All instructions should be read carefully.
No attempt should be made to proceed without a good understanding of the operation of the
preamplifier. The functional description and associated block diagram furnishes signal flow and
functional data about the preamplifier and is provided as an aid in this understanding.

The parts list furnishes information by which replacement parts may be ordered from the
Marantz Company. A description is included for parts which can usually be obtained through local
suppliers.

FUNCTIONAL DESCRIPTION

Figure 1 is a simplified block diagram of the Model 7C Stereo Console showing its functional
elements and the signal flow within the console. The unit consists of two identical channels,
common mode switching circuits, and a common power supply. For clarity, only channel A and
the common circuitry will be described.

SELECTOR switch SR7 contains three individual functional sections (arranged in five wafers).
The three sections provide input, feedback and output routing of the signals. The input section
selects low-level input sources and routes them to the preamp. The feedback section functions
with the preamp. This section chooses an appropriate equalization network to be applied to the
selected input signal. The output section couples the amplified signal to the TAPE switch and
TAPE OUT jack. High-level inputs are coupled directly from the input jacks to the TAPE switch
and TAPE OUT jack via the output section of the SELECTOR switch. When high-level inputs are
selected, the input section shorts the preamp input to ground, thus eliminating interference from
this source.

The preamp consists of V2A, V2B, and V3A. The preamp raises the level of all low-level inputs
while introducing the proper equalization necessary for the different input sources. The gain vs.
frequency characteristics of the preamp is determined by the equalization network selected in the
feedback loop. In the MICROPHONE position, equalization is accomplished with a single resistor.
This sets the preamp gain to 40 db with the frequency response held within 10.1 db from 20 to
20,000 Hz. In the PHONO I and PHONO 2 positions, the desired equalization network is chosen
by the setting of PHONO EQUALIZER switch SL7. The three settings available are OLD 78, R1AA,
and COL LP. In all three positions, the gain of the preamp is set to approximately 42 db. Response
at other frequencies depends on the network selected. Figure 2 shows the gain vs. frequency charac-
teristics for the preamp in the three switch positions. In the figure, the 0 db reference level cor-
responds to a gain of 42 db. In the TAPE HEAD position of the SELECTOR, an adjustable
equalization network is switched into the feedback loop. The variable element is TAPE EQU.

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