Fender Ultimate Chorus Service Manual - Theory of Operation (Cont)
ULTIMATE CHORUS
THEORY OF OPERATION (CONT)
U3A, which contains a tone shaping circuit that enhances the distortion characteristics. The signal is then
fed to the PRESENCE control, DRIVE VOLUME control, and to U3B. U3B drives the tone controls, and
feeds the signal through JFET Q3 to summing amplifier U3A.
REVERB AND EFFECTS LOOP
The output from U4A splits to feed U4B and the reverb drive circuit U5B. U5A senses the return signal
from the reverb pan and feeds the NORMAL REVERB (R60) and DRIVE REVERB (R63) controls. When
switching between channels, JFETs Q4 and Q5 select the reverb signal from the respective reverb control
pot. This allows different levels of reverb for each channel. The reverb signal and dry signal are summed
at U4B. The output from U4B drives the MONO EFFECTS SEND jack (J3), is normalled to the MONO
EFFECTS RETURN jack (J4) then splits to feed the chorus and stereo circuitry. U7A represents the left
channel, and U7B the right channel. The output signals from U7A & U7B drive the STEREO EFFECTS
SEND jack (J5), and are normalled to the STEREO EFFECTS RETURN jack (J6), and feed the left and
right power amplifiers.
CHORUS
The input signal to the Chorus circuit is capacitivly coupled to U6B, which drives the Bucket Brigade Device
(BBD) U13. Trimpot R75 set the DC bias for U6B and U6A. When properly adjusted, the ouput from U6A
will display symmetrical clipping. (Refer to the adjustment procedure on the service diagram). The BBD is
clocked by U12 at 180kHz.
The 180kHz carrier clock is Frequency Modulated by the Low Frequency Oscillator (U11), which deviates
the carrier at maximum DEPTH from 60kHz to 300kHz. The RATE of deviation runs from about 0.3Hz to
8.7Hz. The FM deviation on top of the BBD delay results in a pleasing choral effect.
C39 and R72 (@U6B) provide pre-emphasis above 7kHz. At the output of U6A, R84 and C47 provide the
de-emphasis. This helps to reduce noise. C43, C44, and C46 filter out clock noise. The Dry or Chorus
signal is selected by JFETs Q6 and Q7, buffered by U7b, and fed through the STEREO EFFECTS LOOP
to the right channel power amplifier.
CHANNEL, CHORUS, AND REVERB SWITCHING
The A.C. voltage from the power transformer secondary is tapped off through resistor R134, and presented
to the footswitch jack (Approx. 20 V RMS). By rectifying the positive or negative side of the waveform, a
D.C. control voltage is created and sensed by comparators U10A and U10B. Diode’s CR57 and CR59 set
up the reference voltages at the non-inverting inputs of the comparators. Therefore the control voltage (@
inverting inputs) must toggle above and below the reference voltage to cause the output of the comparator
to toggle between +15Vdc and –15Vdc. The output of comparator U10A controls the indicator LEDs and
JFETs Q1 through Q5. JFETs Q1 and Q3 select the output of either the NORMAL or DRIVE channel.
When the CLEAN channel is active, JFET Q2 mutes the signal that feeds the input of the DRIVE channel.
This prevents crosstalk. JFETs Q4 and Q5 select the reverb return signal through the NORMAL REVERB
or DRIVE REVERB controls. This allows different levels of reverb for the NORMAL and DRIVE channels.
The output of U10B provides a control voltage to the following three sections of the Chorus circuit:
The base of transistor Q13, which enables the Chorus indicator LED.
The Low Frequency Oscillator U11.
JFETs Q6 and Q7, which determine if the dry or chorus signal is fet to U7B.
INHIBIT/MUTING
The inhibit circuit provides two functions. First, without a guitar cable plugged into the input jacks, all of the
JFETs will mute their respective signal paths. Channel switching will not operate. The muting will be
disabled when a cable is inserted at the input jacks. Secondly, if a guitar cable is connected to one of the
input jacks, and then the power is switched on, the muting circuit will be disabled after a short delay (about