WorkingPro™ 400 & 700
(This is the model name for warranty claims)
5
CIRCUIT DESCRIPTION
This section provides concise information about
new or unusual circuitry designs incorporated into
this amplifier model. The purpose is to aid the
service technician by providing insight into the
design areas most likely to become obstacles in
troubleshooting. Information is focused for its
effective use while maintaining the security of
Fender® proprietary information wherever possible.
Overview
The WorkingPro series consists of the WorkingPro
400 and the WorkingPro 700 amplifiers. Both
amplifiers share the same preamp, and differ only in
output power. All PCB assemblies are the same in
both amps. The WorkingPro 400 has a smaller
power transformer than the 700, and has two less
power transistors in the power module assembly.
The circuitry is derived from the classic SWR
amplifiers & is designed to deliver the same tone
and performance as the rest of the SWR amplifier
line. New electronic features to SWR include the
Bass Intensifier effect, a tuner mute function, and a
new footswitching scheme.
Preamp
The following discussion follows the signal path
through the functional blocks of the amplifier: The
signal is introduced at J1 & sees a 3.9M
Ω
input
impedance. U3A provides input gain, which is
reduced by the input pad switch S2.
U3B provides additional gain, and serves as a
buffer for the Aural Enhancer circuit, which provides
part of the signature SWR tone. U5A provides gain
to make up for the attenuation of the passive Aural
Enhancer circuit.
The gain control is a passive shunt-style with
external taper caused by R36. U5B provides
additional make-up gain for the passive gain
control.
The next block consisting of U7, U9, & U11 is the
Bass Intensifier circuit. The path through U7B is the
clean signal. U9B is a lowpass filter with a variable
cutoff frequency set by R71. This feeds a
compressor circuit at U11. The compressed low-
frequency signal is then mixed with the clean signal
at U7A, resulting in an extreme bass boost. Q3
disables the Bass Intensifier when switched from
the front panel or the footswitch.
The next stage comprising of U6 & U8A is the tone
section. This is the same architecture as any other
SWR amplifier, and boosts each frequency range
by +/- 15dB. The frequency response of this stage
is flat when all of the level knobs are at their center-
detent position.
The Effects Blend control is the standard SWR
circuit. When there is no plug in the Effects Return
jack (J203), the "EFF_RETURN_R" signal, going to
J203 ring, is open circuit, and the
"EFF_RETURN_T" signal, going to J203 tip, is
grounded. Therefore, the effects return control
does nothing if there is no plug at J203.
When a jack is inserted into J203, the
"EFF_RETURN_R" signal is grounded, so the
Effects Blend control wiper is grounded. As the
control is turned, the grounded wiper will attenuate
either the dry or the wet signal.
The Master Volume control is also a passive shunt-
style attenuator, with make-up gain at U14B. The
limiter circuit is also included at U14B, and
compresses the signal at high levels. The amount
of compression is set by R98 (Limiter Adj), and is
adjusted at the factory to a pre-determined level.
The stage at U15B serves as a gain buffer for the
power amp, and also provides the mute point for
the tuning mute.
The switching circuitry allows control of the mute
and Bass Intensifier functions. The architecture of
the two switching circuits are basically the same.
The circuit looks for a momentary pulse that can be
generated by either the footswitch, or the front
panel switch. Since the two switches are in parallel,
both the front panel switch and the footswitch will
remain operational when the footswitch is plugged
in. The pulses from the switch are debounced, and
then toggle the state of a flip flop, which drives both
the control circuitry and the LED indicators. Mute &
Bass Intensifier functions are controlled by driving
J111 switching FETs in the respective circuits.
