Circuit Description - Fender G-DEC Junior Service Manual

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 in-
formation wherever possible.
MAIN PCB - ANALOG INPUTS/OUTPUTS
Op-amp U8-B provides the high impedance instru-
ment input with 15db of gain. Op-amp U8-A is a
configured as a three pole active filter. The filtered
output signal (Lin) drives the encoder portion of the
CODEC U13 of the uDSP processing section of the
amplifier. Signal VQ2 is the D.C. reference voltage
from the CODEC thus centering the output signal
about this level. Op-amp U2-B is configured as a
differential input amp recovering the signal after
uDSP processing and conversion back to an analog
signal in the decoder portion of the CODEC U13.
The LOUT+ and LOUT- signals are the input to the
differential amp which acts as a three pole active low
pass filter. The output of U2-B is applied to a unity
gain buffer amp U2-A which drives op-amp U1-A.
U1-A is a unity gain buffer when no headphone is
connected and in turn drives the power Amp I.C. U3
via R36. FET Q1 provides system muting during
power up/down in conjunction with the power sense
circuits comprised of D10, Q2, Q4.
MAIN PCB – POWER/HEADPHONE AMPLIFIER
U3 is a 15 Watt monolithic power amp device which
drives the 8 ohm speaker. Fender conventional cur-
rent feedback is applied to the power amplifier via
current sense resistor pair R24/R26 and the voltage
feedback to the amp via R28/R30. Current feedback

CIRCUIT DESCRIPTION

(This is the model name for warranty claims)
affects the power amplifier output resistance so it re-
acts with the speaker load similar to a tube amplifier.
The main internal speaker (8ohms minimum load) is
connected at P3/4. The internal speaker is discon-
nected if a connection is made to the Headphone
Jack J2.
Op-Amp U1-A and U1-B act as the headphone am-
plifier when a headphone is plugged into J2. The
gain of U1-A goes from unity to 21 dB when a head-
phone is plugged in. U1-A drives the left side of the
headphones and U1-B drives the right side of the
headphones.
uDSP System
The uDSP system is integrated on the main PCB of
the G-DEC Jr. The UDSP system is comprised of the
microprocessor U10 (TMS320VC5501) and associ-
ated memory (static RAM U14 and flash ROM U11).
Signals to and from the uDSP chip are in serial digi-
tal format. Signal input is from the encoder portion of
the CODEC U13 and output to the decoder portion of
the CODEC U13. The CODEC converts the analog
input signal to digital format for processing.
processed output digital signal and it is then con-
verted back to analog format and sent to the power
amplifier.
All analog controls; LOOP Start/Stop switch; and
headphone present status are read via the analog
multiplexer U6-A. The output of the multiplexer is
read serially by the right channel of the CODEC and
sent to the uDSP system
Note: Troubleshooting the uDSP system to com-
ponent level beyond verification of power
supplies, clock, and data signals is NOT recom-
mended.
6
®
G-DEC Jr
The