Circuit Description - Fender KXR 60 Service Manual

KXR 60

CIRCUIT DESCRIPTION

Preamp
The Channel One input will accept a Balanced or Unbalanced signal via a ¼" Phone plug. U1B provides a
gain of 1, and rolls off high frequencies at about 128kHz. The signal then couples to the second half of the
input stage.
Located within the negative feedback loop of U1A is the Channel One Volume control. It provides a
minimum gain of 1, and a maximum gain of 15. At maximum gain, Capacitor C6 rolls off the high
frequencies at about 47kHz.
The input stage also features a function that shorts across the Volume control via a switch contact in the
¼" input jack. This causes U1A to remain locked into a Unity gain mode, disabling the Volume control, until
a plug is inserted into the jack. This reduces noise in the input circuitry. Channel Two is identical to
Channel One.
The two channels sum together to feed the Reverb Drive stage U3A, and the Summing Amp U4B. U3B
senses the return signal from the Reverb Pan . R24 and C19 provide a slight boost in the high frequencies
. R23 and C18 roll off low frequencies. This produces a fairly bright reverb sound. To eliminate noise,
C17 limits the high frequency response to about 5kHz. All ground connections in the Reverb circuitry are
routed through a dedicated Reverb Ground trace to the Star Ground point on the circuit board. This
eliminates any bleed through of noise into other audio ground paths.
U4B sums the dry signal with the Reverb return signal and then drives the Equalizer circuit. Gyrators U5B,
U6B, and U6A are used for the Low (100Hz), Low Mid (338Hz), and High Mid (1588Hz) bands. The High
band uses a single pole shelving filter (R40,C31) at 4kHz. U5A performs the Boost/Cut function, and also
drives the Effects Loop.
The Tape in jacks are buffered by U7A and sum with the main signal just prior to the Effects Loop. The
Effects Loop Send is capable of providing a pseudo balanced signal if desired. This helps to reduce noise
and hum. The Effects Loop Return will accept an Unbalanced or Balanced input.
U7B buffers the signal from the Effects Loop, and drives the Master Volume control. R55 and C39 boost
the low frequencies at 220Hz. Between the output of U7B and the Master Volume control, C40 and R56
boost high frequencies. The boost in Low and High frequencies provide a slight "Smile Curve" response.
DELTACOMP™
U8B is a unity gain amplifier that drives the power amp stage. U9 (CA3080A) is an Operational
Transconductance Amplifier (OTA). In conjunction with U8B, it acts as the gain reduction circuit for the
Deltacomp™. The attack/release circuit for the DeltaComp™ contains the Diode, Resistor, and Capacitor
network that drives the base of Q8 (MPSA63). Comparator U4A senses the output of the power amplifier.
As the amplifier approaches clipping, the output of U4A toggles negative. The negative voltage through
Diodes CR14-17 charges Capacitors C54-C57 all at once in parallel as a one-pole filter through a single
time constant made up with R84. R84X (C54+C55+C56+C57) = 2.2K x 8.8uF = 19.4mSec.