Theory Of Operation - Fender Passport PD-250 Service Manual

PASSPORT
POWERED PORTABLE
SOUND SYSTEM

THEORY OF OPERATION

MIXER SECTION
The mixer section is mainly made up of operational amplifiers as gain stages and therefore a technical description
will not be used here. Following is a basic circuit description to familiarize you with the layout of the circuit.
The first section is made up of 4 microphone / line inputs each with a first stage preamplifier for balanced or unbal-
anced inputs (found on Mic input board), one knob contour control, auto-pad level control where the signal splits off
into rev/aux send and main signals with the main signals going to a pan circuit. Here the main signal is split
between left and right channel signal bus.
The second mixer section consists of two stereo pair inputs with RCA and 1/4" stereo phone jack connections. This
circuit consists of an auto-pad level control, low frequency and high frequency tone controls, rev/aux send control
and balance control with signals feeding the left and right audio bus.
The audio bus is connected to three FETs which perform ducking of the signals on the bus, with the exception of
Channel One, which is the drive circuit for the VIP ducking circuit. This function is provided by U2A. Bus signals are
buffered and drive the tape output connections and are directed by SW1A/B which performs the function of switch-
ing this signal as left and right main with reverb mix or left and right summed into left and aux signal on right chan-
nels. Main left and right level controls are auto-pad confirguration and drive the send output connectors. Note:
Channel 1 Mic/Line reverb function will be affected by VIP control making announcements on Channel 1 dry.
The mixer board also contains part of the signal processor circuit and the system EQ circuit for the power amplifier.
These circuits are an integral part of the power amplifier and appear after the return signal path.
These circuits will all be discussed later in this document.
Digital Reverb Section
The digital reverb section utilizes a digital reverb IC M65841SP (IC402) and one dual op amp (IC401A/B) to perform
the reverb function. +5v regulation is on board with input voltage derived from the +15v supply. RC filters make up
the timing functions and gain levels of the processor with regeneration and line driving provided by IC401A/B and
regen mix provided by VR401(this part may be fixed resistor on later models). Foot switch control of the reverb facil-
ity is provided and controlled by TR401 which controls the logic level at pin 2 of IC402 Reverb function is mono but
fed to both right and left busses.
Signal Processor Circuit and System EQ Circuit
The signal processor is partially located on the mixer board but is an integral part of the power amplifier section.
The operation of this circuit is performed by level detecting (Z204/207 on amp board) and storing
(D204/205/209/210, C215, 230 on amp board) the output peaks of the power amplifier to create approximately 0v
bias at the gate of FET Q4 and Q5 on the mixer board. This in turn changes the filter characteristics of circuits
made up around U16A/B which at low levels boosts the low and high frequencies of the signal. At high signal levels
with the gate voltages approaching or at 0v, the circuit becomes flat in response. This circuit is post return and
therefore does not modify the response of the signal at the send outputs.
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