Fender Passport PD-150, Service Manual

Page: 6 / 48

PASSPORT
®
PD-150
(This is the model name for warranty claims)
6
The DSP Reverb PCB Module
The DSP Reverb PCB module receives the input
REV/AUX signals at pin 4 of connector CN401. The
DSP Reverb’s IC’s consist of U404 (AL1101 ADC),
U405 (AL1201 DAC), and U406 (AL3201B). The
outputs from U405 are fed to U402B and then buf-
fered by U402A before feeding back to pin 6 of CN
401.
Signal Processor & System EQ Circuit
The signal processor is partially located on the mix-
er board but is an integral part of the power
amplifier section. The operation of this circuit is per-
formed by filtering the signal from U11A/B which
drives the bases of Q5 and Q7 to control the vol-
tage level at FETs Q4 and Q6 respectively. This in
turn changes the filter characteristics of circuits
which at low levels boosts the low and high fre-
quencies of the signal. At high signal levels with the
gate voltages approaching or at 0v, the circuit low-
ers the boost of the low frequencies in response.
This circuit is post aux return and therefore does
not modify the response of the signal at the aux
send outputs.
Power Amplifier PCB module
Note: In cases where the amplifier circuits are iden-
tical between the left and right channel, only
components related to the left channel will be dis-
cussed for simplicity.
The first stage of the power amplifier is made up of
op amp U201A that acts as the input buffer and
muting circuit for minimization of turn on/off pops.
Q202 in balance with the input resistors of this cir-
cuit are designed to provide a deep muting effect.
This circuit will be called into action if the tempera-
ture of the heat sink also rises above 80 degrees
Centigrade. (Typical HS temperature is around 50
for normal operation). The output of the first stage
feed trimmer resistor TP201 which are part of the
balanced input section of low noise op-amp U202A
which sets the amplifier gain (factory set at approx-
imately 30.5dB). The output of the second stage
directly drives the biased gates of lateral power
FETs Q204 and Q205.
Speaker short circuit protection is provided by
U203B. This is accomplished when the output is
shorted to ground, the normal differential signal ap-
plied to U203B is absent at the negative terminal
while at the same time, the second stage is now
forced into open loop gain. The drive signal is inte-
grated into C207 forcing a positive voltage at the
output of U203 that pulls the gate of FET Q203 into
conduction from its previous negative bias off point,
nulling the output of U202A. With the output of
U202A minimized the output current of the amplifier
is minimized until the short is removed. At this time,
C207 will discharge allowing normal operation of
the amplifier.
Note: Some small DC offset will be
present when the amplifier is in the short circuit pro-
tect mode.
The power amplifier board also provides the ±15v
for the Mixer and DSP reverb boards as well as the
supply for the SMPSU FAN and the LED PCB
which indicated power and thermal protection mod-
es. The power amplifier board is also connected to
the GM type fuse holder and aux DC power input
connector.
The SMPS PCB Module
This unit is not recommended to be serviced in
the field except by replacing the complete mod-
ule as a total assembly
.
The following information
is provided to assist in the trouble shooting of the
unit. Please note that the mains fuse must be re-
placed with exactly the same grade of fuse for
safety regulations and to avoid risk of damage and
potential fire hazard.
The mains input to the SMPSU allows for operation
from 90-130V in 115V setting and 190-250V in the
230V setting. The output of the SMPSU is ±42V
DC for the main power supply rails of the power
amplifier. The unit also has connectors for mains
power input, mains power switch and chassis, earth
and signal grounds.
Service Tip: You can connect to the aux DC input
(with GM fuses installed) to test the output voltage