Fender PRINCETON 112 PLUS Service Manual - page 2
SPECIFICATIONS
Part Number:
022-6704-010 (120V)
022-6734-010 (240V Australia)
022-6744-010 (230V UK)
022-6764-010 (230V)
022-6774-010 (100V)
Input impedance:
Greater than 1M Ohm
Nominal input level:
100mV
Power output:
63 Watts R.M.S. @ 5% T.H.D
Rated load impedance:
8 Ohms
Effects loop:
Nominal level
-10 dBv
Output impedance
1k Ohm
Input impedance
16k Ohms
Dimensions:
Width:
18-5/8 inches
(47.3 cm)
Height:
16-1/2 inches
(41.9 cm)
Depth:
9 inches
(22.9 cm)
Weight:
28 lbs.
(12.7 kg)
THEORY OF OPERATION
PREAMP
Input jack J1 contains the switching circuit for the inhibit function of the 4053 CMOS switch
(U7). Without a plug inserted into J1 -8Vdc is available to pin 6 of U7A. This inhibits the CMOS
switch from functioning, effectively muting the preamp. When the input plug is inserted into J1,
+8Vdc becomes available at the inhibit line allowing the CMOS switch to function.
TROUBLESHOOTING TIP: When the channel switching circuit does not operate, first check
that the inhibit voltage toggles properly. In some cases a faulty CMOS switch can load down the
inhibit line. Desolder pin 6 (of U7) and check that the voltage toggles between -8Vdc to +8Vdc.
If so, the CMOS switch may need to be replaced.
The first stage (U1A) sets up the initial response of the amplifier. It provides a second order
high pass filter with a 3dB down point at 60 Hz, and a gain of about 3.5. The output of U1A
splits to feed the clean and overdrive channels. For the clean channel, the signal feeds the
treble and bass controls, which provide boost and cut respectively. The signal then flows to the
Auto-pad circuit (U1B). It provides excellent headroom and low noise. At upper volume control
settings, it also rolls off low frequencies.
The signal split from U1A feeds the overdrive channel through a special tone shaping circuit
located just prior to U2A. This circuit is a third order bandpass filter with a peak boost at 2.2kHz.
It provides the optimum frequency response for the distortion circuit that follows. U2A provides
the variable gain adjustment that drives the distortion circuit. It also acts as a bandpass filter
with 3dB down points at 90Hz and 4kHz. U2B is the distortion circuit. LD1 and LD2 provide
diode distortion.
The Contour control adjusts the overall tone quality of the overdrive channel by emphasizing or
notching the mid range frequencies. In the fully CCW position the response is fairly flat up to
about 2kHz, where a gentle roll-off occurs. In the fully CW position, the circuit provides a -20db
notch at about 1kHz. The signal then feeds the overdrive volume control and then to the treble
and bass controls.
U3B sums the clean and overdrive channels through CMOS switches U7B and U7C. The output
from U3B feeds the reverb drive circuit (U4B) and summing amp U3A. U4A is the reverb
sensing circuit. The output of U4A drives the reverb volume control and then feeds through
U7A, which sums the reverb with the dry signal at U3A. The output of U3A drives the preamp
out jack, is normalled through the power amp in jack, and feeds the power amp input.
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