Fender Deluxe 112 Plus Service Manual

Introduction

This service manual provides detailed information for the Fender Deluxe 112 Plus guitar amplifier. It includes technical specifications, theory of operation, parts lists, and schematic diagrams to assist qualified technicians in maintenance and service.

Important Notice

The information contained herein is CONFIDENTIAL and PROPRIETARY to Fender Musical Instruments Corp. It is disclosed solely for use by qualified technicians for purposes of equipment maintenance and service. It is not to be disclosed to others without the expressed permission of Fender Musical Instruments Co. All specifications are subject to change without notice.

For warranty repair service, only Fender specified part numbers are to be used. It is recommended they also be used for post-warranty maintenance and repair.

Parts marked with an asterisk (*) indicate the required use of that specific part. This is necessary for RELIABILITY and SAFETY requirements. DO NOT USE A SUBSTITUTE!

A coded naming convention is used in the description of certain parts. The codes and what they mean are as follows:

Capacitor Codes

Hardware Codes

Resistor Codes

Specifications

Product Release No.: PR 291 (This is not a model number)

Part Number:

• 120V Version: 22-6702
• 100V: 22-6772
• 230V: 22-6762

Power Requirements: 180-120VAC, 50-60 Hz, 360W max. / 230-240VAC, 50 Hz, 360w max.

Input impedance: Input 1: 850kΩ, Input 2: 66kΩ

Power output: 94 watts RMS into 4Ω at 5% THD

Effects Loop: Preamp Output Impedance: 1kΩ, Power Amp Input Impedance: 44kΩ, Power Amp Sensitivity: 1.0V for Clipping

Dimensions: Height: 16 1/2" (41.9 cm), Width: 20 5/8" (50.5 cm), Depth: 9" (22.9 cm)

Weight: 33 lbs. (14.9 kg)

Product specifications are subject to change without notice.

Theory of Operation

The amplifier features two inputs: Input 1 (high sensitivity) and Input 2 (low sensitivity). These inputs are connected via series switches to +16Vdc through a third series switch in the Power Amp in jack. The input signal is processed by JFET Q7, which is part of a muting and turn-on delay circuit for the power amp input. The signal remains muted until a plug is inserted into any of the input jacks.

The first stage, U2A, acts as a second-order high-pass filter with a 3dB down point at 60Hz and a gain of approximately 3.8. The output of U2A splits to feed both the clean and overdrive channels. The clean channel signal passes through Treble, Bass, and Mid controls, which provide boost and cut. The signal then goes to U7A, a buffer for an adjustable high-pass filter formed by C59, C60, R108, and R109. An inverted signal at U5A is fed back to C59 and C60. As the Volume control is adjusted, feedback from R109 provides -6dB attenuation at 30Hz, preventing speaker flapping at low frequencies. U2B is an auto-pad circuit for improved headroom and low noise.

The overdrive channel signal, originating from U1A, passes through a tone-shaping circuit that includes a third-order band-pass filter with a peak boost at 2.2kHz, optimizing the response for the distortion circuit. U2A provides variable gain adjustment for the distortion circuit and also functions as a band-pass filter with 3dB down points at 90Hz and 4kHz. U2B is the distortion circuit, utilizing LD1 and LD2 for diode distortion.

The Contour control adjusts the overdrive channel's tone by emphasizing or notching mid-range frequencies. In the fully counter-clockwise position, the response is relatively flat up to about 2kHz, with a gentle roll-off. In the fully clockwise position, a –20dB notch is introduced at approximately 1kHz. The signal then proceeds to the overdrive volume control and the treble and bass controls.

The signals from the Clean and Drive channels are summed at U5A via JFETs Q4 and Q5. The output of U5A feeds the reverb drive circuit (U6A) and summing amplifier U5B. U6B is the reverb sensing circuit. The output of U6B drives the Reverb volume control and is then summed with the dry signal at U5B. The output of U5B drives the preamp out jack, is normalled through the power amp in jack, and feeds the power amp input.

Channel Switching

AC voltage from the power transformer secondary is tapped off through resistor R82 and supplied to the footswitch jack (approx. 20 V RMS). By rectifying the positive or negative waveform, a DC control voltage is generated and sensed by comparators U4A and U4B. Diodes CR20 and CR21 set the reference voltages for the comparators, requiring the control voltage to toggle between +15Vdc and -15Vdc. The comparator output controls the JFETs and indicator LEDs. When the footswitch is not connected, CR15 rectifies the negative half of the waveform. U4B senses the DC control voltage and provides –15Vdc to the gate of Q6, enabling the reverb output.

Power Amp

U1 is a high-voltage op-amp providing voltage gain for the power amplifier. Diodes CR3 and CR4 act as differential protection diodes, preventing the positive and negative inputs of U1 from differing by more than 0.6 volts. Bootstrap capacitors C37 and C38 sense the output through R69, providing increased voltage swing for the op-amp relative to ground.

Diodes CR5-CR8 (BYV26D) form the Fixed Bias circuit for the output transistors. These Bias Diodes are mounted through a hole in the transistor heatsink to ensure proper tracking of the output transistors' temperature. These diodes exhibit a 2mV decrease in Knee Voltage for every 1-degree Celsius increase in temperature. Pin 6 of U1 is at 0 Vdc. The Bias Diodes provide two voltage drops (+/- 1.2 volts) to the base of Darlington transistors Q1, Q2, Q10, and Q11.

Diodes CR9-CR12 form a voltage clamp protection circuit. Q8 and Q9 form a current limiting protection circuit. If the output is shorted, the voltage at the emitters of Q10 and Q11 will increase. This voltage is sensed through R100 and R107 and presented to the bases of Q10 and Q11. When Q10 and Q11 turn on, the drive current and bias to the output transistors are removed, shutting down the output section of the amplifier. The output signal passes through the Headphone jack circuit before driving the speaker. With headphones connected, the speakers are muted.

Parts List

Printed Circuit Board Assembly

Chassis Assembly

Footswitch Assembly

Cabinet Assembly

Miscellaneous

Schematics/Diagrams

The document includes a service diagram for the Deluxe 112 Plus guitar amplifier, detailing the electronic components and their interconnections. This diagram is crucial for troubleshooting and repair.