Fender Stage 1000 Service Manual - Stage® 1000/1600
STAGE® 1000/1600
(This is the model name for warranty claims)
5
CIRCUIT DESCRIPTION
This section provides an overview of any new or
unusual circuitry incorporated into this amplifier
model. The purpose is to aid the service technician
by providing insight into the design areas most
likely to present a challenge in troubleshooting.
This overview is focused for the efficient use and
security of Fender® proprietary information.
PRE-AMPLIFIER
The guitar input signal is fed into the high pass filter
at U1–A before splitting into two signal paths. The
normal channel signal path (U1 to U4A) is active
until FET Q2 is turned on and either Q4/Q1
(DRIVE1) or Q3/Q5 (DRIVE2) are turned on activat-
ing the DRIVE channels signal paths. Differential
amp U6–A and follower U6–B adjust the signal level
and provides +1.65VDC bias voltage to the uDSP
effects card. The preamp is in series with the digital
effects card, therefore, disconnecting the card
breaks the overall signal path. The low pass filter at
U4–B recovers the signal from the uDSP card and
sends it to the external effects loop and power am-
plifier.
POWER AMPLIFIER
“Flying Rail” power amp topology. The output stage
is referenced to ground, and the output signal is de-
rived from the center tap of the high voltage power
supply (C86 & C87) which is floating. Diff–amp U7–
B amplifies voltage developed at the load sense re-
sistors (R120–123) to provide negative current
feedback. Inserting an 8Ω load at Ext. Spkr (J7)
turns on FET Q20 creating an attenuation network
(R117–119) to compensate for the increased cur-
rent at 4Ω total load impedance. This allows the
amplifier to have a similar frequency response run-
ning either an 8 or 4Ω speaker load.
Thermistor RT2 monitors the heatsink temperature.
Its resistance increases with temperature and at
approx. 100 ºC turns on xstr Q23. This shorts out
C77, turning on FET Q22 providing a muting func-
tion at the power amp input. The amplifier remains
muted until the heatsink cools down, at which point,
normal operation is restored.
POWER SUPPLY
Standard bridge rectifiers for both high and low
voltage sections, with zener regulation (D52–53) for
the +/–16V supplies. Half–wave rectifier (D46–47,
C48) supplies U11, 7805 regulator for +5V supply
for the uDSP card and digital user–interface. Q31
drops the 5V supply to 3.3V which powers U12 and
U13.
DIGITAL USER INTERFACE
The uDSP effects card generates clock signals HD0
(~45Hz) and HD1 (~90Hz) which control the 4053
CMOS switch to read REVERB level, FX level and
TIME/RATE pots on the display PCB. The DC level
(0–5V) of the pots controls the reverb level, fx level,
and fx time/rate. HD2/3 send serial data to the Dis-
play PCB which is then converted to the parallel bits
that power the TUNER/TIMBRE LEDs. HD5/6 con-
tain clock and serialized data from the REVERB
and FX encoders as well as the TUNER/TIMBRE
buttons. HD7 is a bi-directional serial bus sending
TIMBRE settings to the U12 EEPROM which stores
them during power down.
DIGITAL NOISE
The uDSP effects card generates some high fre-
quency noise (>50kHz), which is a normal product
of DSP processing. This noise is not audible and
does not effect amplifier performance, but may be
visible when observing the output of the amplifier on
an oscilloscope.
.