Power Amplifier - Fender PASSPORT P-80 Service Manual

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 buff-
ered by U402A before feeding back to pin 6 of CN
401.
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 am-
plifier section. The operation of this circuit is
performed by filtering the signal from U11A which
drives the bases of Q5 and Q7 to control the volt-
age 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

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 VR201 which are part of the
balanced input section of low noise op-amp U203A
which sets the amplifier gain (factory set at ap-
proximately 30.5dB). The output of the second
stage directly drives the biased gates of lateral
power FETs Q205 and Q208.
PASSPORT
(This is the model name for warranty claims)
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 C211 forcing a positive voltage at the
output of U203B that pulls the gate of FET Q207
into conduction from its previous negative bias off
point, nulling the output of U203A. With the output
of U203A minimized the output current of the ampli-
fier is minimized until the short is removed. At this
time, C211 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/reverb boards as well as the supply for
the SMPSU FAN and the LED PCB which indicated
power. The power amplifier board is also connected
to the GM type fuse holder and aux DC power input
connector.
Switch Mode Power Supply Unit (SMPSU)
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 not that the
mains fuse must be replaced 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 100V to 240V. The output of the SMPSU is
±36V DC for the main power supply rails of the
power amplifier. The unit also has connectors for
mains power input, mains power switch and chas-
sis, earth and signal grounds.
Service Tip: You can connect to the aux DC input
(with GM fuses installed) to test the output voltage
to the rails before you open the set for service to
get an indication of a possible power supply fault as
it is connected to the rail output. Do not short these
terminals!
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P-80