Fender SR-6520, SR-6520P, SR-8520P, SR-6520PD, SR-8520PD Service Manual

SR-6520 / SR-8520 /P / PD

POWERED MIXERS w/ DIGITAL EFFECTS

SR-6520P p/n 071-6521-100 (120V)

SR-8520P p/n 071-8521-100 (120V)
SR-6520PD p/n 071-6521-200 (120V)
SR-8520PD p/n 071-8521-200 (120V)

SERVICE MANUAL

CONTENTS:

Notices

Specifications

Circuit description

Parts lists:

Preamp PCB assy

Front panel assy

Pwr amp PCB assy

Rear panel assy

Cabinet assembly

Miscellaneous

Dig rev cntrl

Schematics

Service diagram

Fender Musical Instruments Corp.

7975 North Hayden Road Scottsdale, AZ 85258

SR-6520P/PD SR-8520P/PD

(These are the model names for warranty claims)

MARCH 1998

IMPORTANT NOTICE:

The information contained herein is CONFIDENTIAL and PROPRIETARY to Fender Musical Instrum ents Corp. It is
disclosed solely for use by qualified technicians for purposes of equipment m aintenance and service. It is not to be
disclosed to others without the expressed permission of Fender Musical Ins truments Co. All specifications subjec t to
change without notice.

For warranty repair service, only Fender specified part numbers are to be used. It is recom mended 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 nec essary for RELIABILITY

and SAFETY requirements. DO NOT USE A SUBSTITUTE!

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

CAPACITOR CODES HARDWARE CODES

CAP AE = Aluminum Electrolytic BLX = Black Oxide
CAP CA = Ceramic Axial CR = Chrome Plated
CAP CD = Ceramic Disk HW H = Hex Washer Head
CAP MPF = Metalized Polyester Film M = Machine Screw
CAP MY = Mylar NI = Nickel Plated
CAP PFF = Polyester Film/Foil OHP = Oval Head Phillips

PB = Particle Board

RESISTOR CODES

PHP = Pan Head Phillips

PHPS = Pan Head Phillips Sems
RES CC = Carbon Comp SMA = Sheet Metal "A" Point
RES CF = Carbon Film SMB = Sheet Metal "B" Point
RES FP = Flame Proof SS = Stainless Steel
RES MF = Metal Film TF = Thread Forming
RES WW = Wire Wound ZI = Zinc Plated

1

SPECIFICATIONS

Type: PR 189

POWER AMPLIFIER SECTION:

Power output: 520 Watts R.M.S. @ 2 Ohms

350 Watts R.M.S. @ 4 Ohms

Distortion at 520 watts: Less than .2% @ 1 kHz into 2 Ohms

Minimum load impedance: 2 Ohms

Sensitivity: 1.28V R.M.S.

Input impedance: 22K ohm

DELTACOMP range: 20 dB

PREAMP SECTION:

Input impedance: LOW-Z 1.82k

HI-Z 18.2k

Hi-Z Input Sensitivity for 520 watts: 55 mV

(MAIN MASTER and CHANNEL
Main at maximum,all tone controls
and GEQ at "0" detent).

Channel tone controls: LOW +/- 15 dB at 100 Hz

MID +/- 15 dB at 750 Hz
HIGH +/- 15 dB at 10 kHz

Graphic equalizer: +/- 12 dB at 63, 125, 250, 500, 1k,

2k, 4k, 8k, and 16kHz.

Dimensions:

SR 6520PD SR 8520PD

Height: 10-7/16 inches (26.5 cm) 10-7/16 inches (26.5 cm)
Width: 23-5/8 inches (60.0 cm) 27-5/8 inches (70.2 cm)
Depth: 12 inches (30.5 cm) 12 inches (30.5 cm)
Weight: 40.0 lbs. (18.2 kg) 42 lbs. (19.1 kg)

CIRCUIT DESCRIPTION

POWER AMPLIFIER:

The signal from the preamp feeds from P1B pin 4, through a ribbon cable, to P1A on the

power amplifier PCB. The signal couples (via C1) to the unity gain amplifier U1B. U1B contains a c losed loop (gain of

1) negative feedback path, along with positive feedback through the inverting O perational Transconductance Amplifier
(OTA) U2. U1B and U2 make up the gain reduction circuit for the DELT ACOMP clip protection feature.

The attack and release circuit for the DELT ACOMP is made up by the Diode, Resistor, and Capacitor network which
drives the Base of Q29. Comparator U1A senses the voltage from the Collectors of Q27 & Q28. W hen the power
amplifier approaches clipping on a positive swing, the collector of Q9 pulls up R50, which turns on Q28, tr ansferring ­15vdc to the collector, pulling down R52. This causes the output (pin 1) of U1A to switch from +15vdc to -15vdc,
lighting the DELTACOMP/Clip Led. Note when the DELTACOMP is disabled, the Led indicates clipping of the
output stage. With the DELTACOMP enabled, the Led indicates an ac tive DELTACOMP circuit. The negative
control voltage from R52 also feeds through P1A to P1B on the preamp PCB. The voltage is routed to the
DELTACOMP switch (S1), located on the front panel. W ith the switch engaged, the control voltage is sent back
through the ribbon cable (via P1 pin 6) to the attack/release circuit.

The negative voltage drives through Diodes CR34 - CR37, and charges Capacitors C26-C29 all at once in parallel as
a one pole filter through a single time constant of R52 X (C26+27+28+29) or 3.9K X 8.8µF = 34.3m s. As they charge,
Q29 turns on and provides current to pin 5 of the OTA (U2). The c urrent controls the output amplitude of U2. The
inverted signal from U2 mixes with the input to U1B providing cancellation which reduces the input to the power
amplifier. This prevents the amplifier from clipping. When the output of the power amplif ier is reduced, the output of
U1A toggles back to +15vdc. Due to the blocking action of Diodes CR34 - CR37, Capacitors C26-C29 are f orced to
discharge as a 4-pole filter with different time constants thr ough R58. R62 prevents Parasitic oscillation while Zener
CR38 provides 2 slopes which results in smoother limiting.

When the power amplifier approaches clipping on a negative swing, R46 is pulled low, turning on Q26 which pulls up
the Base of Q27 transferring -15 vdc to the collector.

JFET Q1 and associated components provide a 4-5 second tur n-on delay for the audio input to the power amplifier.
When the power is switched on, Capacitor C3 charges through Resis tor R4. The negative Gate voltage pinches off the
JFET, removing the ground from the input of the am plifier. When the power is switched off, C3 immediately discharges
through Diode CR1, grounding the input of the amplifier.

TROUBLESHOOTING TIP:

Check for proper operation of this circuit when experiencing exc essive turn-on or turn
off "Pops", or no output when signal applied to the input. Many times the JFET itself can be the culprit. NOTE:
Excessive turn-off "Pops" can also be caused by uneven discharge of the +/- power supplies. Usually a mismatch in
the Filter Capacitors will cause this problem. Its easy to look at both supplies on an oscilloscope. Invert one scope
input and check for even discharge to zero volts.

Q4 is the current source for the Differential Amplifier (Q2, Q3). For the Diff am p to work properly, one half of the
current from the current source must flow through each leg of the Diff amp. If the same amount of cur rent flows
through each leg of the Diff amp, the voltage drop across resistors R12 & R13 m ust be the same. If not, there will be a
DC offset at the output of the amplifier. The overall gain of the amplifier is set up around the Diff am p. R15 (27K) is the
feedback resistor and R14 (1K) is the pull-down resistor. Rfb + Rpd ÷ Rpd = Av. Therefore 27K + 1K ÷ 1K = 28K ÷ 1K
= 28 Av.

2

CIRCUIT DESCRIPTION Cont.

Q7 & Q11 make up a voltage amplification stage. Again, to operate properly, the same am ount of current must flow
through these transistors. Thus the voltage drop across R21 & R22 m ust be the same.

Adjustable bias is provided by Q8 & it's associated components. 1.2volts is provided to the Base of Driver Tr ansistors
Q14 & Q15, and -1.2 volts is provided to the base of the Driver Transis tors. This provides + or - 0.6 volts to the bases
of the output transistors (Q18-Q25) in parallel, biasing them on.

Transistors Q12, Q13, and associated components c omprise the Current Limiting or Short Proof protection circuit.
There are usually two conditions which demand excessive output current from the amplifier. A shorted speaker cable,
or a load impedance which is below the minimum rating of the amplifier. If these conditions occur, the voltage drop
across the Emitter Resistors (.33Ω 5 Watt) will dramatically increase. On the positive side of the amplifier, C11
charges through R25 & R27. On the negative side C12 charges through R31 & R32. This will c ause Q12 & Q13 to turn
on. This limits the amount of signal which is available to the Base of the Driver Transistors (Q14 - Q17).

DIFFERENCES IN VERSIONS:

In all versions (SR6520/8520, SR6520P/8520P, & SR6520PD/8520PD) the power
amp remained unchanged; normal schematic revisions were not related to version c hanges. The "P" version added
Phantom power, which caused the preamp PCB assy to change by adding traces to carry phantom power to the XLR
microphone connectors, and added a +15V regulator circuit to the preamp PCB as sy in order to provide the phantom
power. The "PD" version added digital effects (PCB assy 041875) and a ribbon cable to connect the PCB's together
(P2 to P3), along with some obvious cosmetic changes

WARRANTY PHILOSOPHY ON DIGITAL REVERB MODULE:

Replacement, rather than repair of the 041092

module is the recommended warranty procedure for failur es of the module.

PARTS LIST

NOTE: SHADED ITEMS ARE FOR REFERENCE ONLY

NOTICE:
Reference designators 100-199 refer to and are listed only for channel 1. Chan nel 2 designators are 200-299
etc. Part quantities are listed only for the SR-6520PD (6 channel version).

PREAMP

PRINTED CIRCUIT BOARD ASSEMBLY

QTY PART # DESCRIPTION REFERENCE DESIGNATION

1 037832 PCB. ASSY PREAMP SR6520P (STUFFED)
1 037875 PCB. ASSY PREAMP SR8520P (STUFFED)
1 049148 PCB ASSY PREAMP SR6520PD (STUFFED)
1 049271 PCB ASSY PREAMP SR8520PD (STUFFED)

1 028577 CABLE ASSY REVERB 30" (SR6520/8520/P)
1 038534 CABLE, JUMPER 3CKT RIBBON

P2 TO P3 (SR6520/8520/P TAPE IN)

1 048914 CABLE, RIBBON 8CKT RIBBON @ P1 (SR6520/8520PD)

1 028485 CAP AE RDL 16V 470uF 20% C11
1 028492 CAP AE RDL 16V 1000uF 20% C10

21 028465 CAP AE RDL 25V 22uF 20% C14,16,25,103,104,111

1 028474 CAP AE RDL 25V 100uF 20% C7
1 033605 CAP AE RDL 50V .15uF 20% C29
1 037943 CAP AE RDL 50V .33uF 10% C37

1 033606 CAP AE RDL 50V .47uF 20% C27
17 028459 CAP AE RDL 50V 2.2uF 20% C3,12,18-20,105,106
13 028460 CAP AE RDL 50V 4.7uF 20% C5,114,115

20 020909 CAP CD 1KV 47pF 10% C4,6,13,15,21-24,112,113
12 069724 CAP CD 1KV 100pF 10% C101,102
12 025984 CAP CD 1KV 180pF 10% C107,108

1 007029 CAP CD 1KV 220pF 10% C34

6 025986 CAP CD 1KV 390pF 10% C110

1 025777 CAP CD 1KV 470pF 10% C42

1 020842 CAP CD 1KV 500pF 10% C9

1 033588 CAP PF RDL 100V .01uF 10% C36

2 033578 CAP PF RDL 100V .001uF 10% C32,40

1 036234 CAP PF RDL 100V .015uF 10% C41

1 033590 CAP PF RDL 100V .022uF 10% C26

1 033591 CAP PF RDL 100V .033uF 10% C31
16 033596 CAP PF RDL 100V .068uF 10% C1,2,8,39,116,117

1 033579 CAP PF RDL 100V .0015uF 10% C30

1 033580 CAP PF RDL 100V .0022uF 10% C35

2 033581 CAP PF RDL 100V .0033uF 10% C17,38

3

PARTS LIST

PREAMP

PRINTED CIRCUIT BOARD ASSEMBLY (CONT)

QTY PART # DESCRIPTION REFERENCE DESIGNATION

1 033582 CAP PF RDL 100V .0039uF 10% C28

1 033583 CAP PF RDL 100V .0047uF 10% C43

7 033586 CAP PF RDL 100V .0068uF 10% C33,109

1 064089 DIODE 1N4003 CR2

1 006260 DIODE 1N4448 SIGNAL BENT CR1

1 027416 HEADER 8 PIN P1
21 016795 IC DUAL OP-AMP TLO72C U1-9,101,102

7 030771 JACK 1/4" PCB 2COND SCC J1,3-8

6 030987 JACK 1/4" PCB 3COND DOC J102

1 025933 JACK PHONO DUAL PC MOUNT J2 ( RCA) (SR6520/8520/P)

1 049295 JACK RCA QUAD VERT PC MOUNT J2A,B (SR6520/8520 PD)

6 037248 JACK XLR FEMALE PC MOUNT J101

1 028098 LED 5X5mm GREEN SLB-55MG3 LD3

1 028039 LED 5X5mm RED SLB-55VR3 LD1

1 028097 LED 5X5mm YELLOW SLB-55YY3 LD2

1 031879 POT RTRY 50K 30A TAPER R6
23 031266 POT RTRY 50K B TAPER R8,19,22,29,30,117,120,122
18 033002 POT RTRY 50KB TAPER CNTR DT R110-112

9 031267 POT SLIDE 50KB 45mm CNTR DT R33-37,40-43 (SR6520/8520/P)

9 040534 CONTROL SL 60MM CSE 50K PANASONIC R33-37, 40-43 (SR6520/8520PD)

1 024942 RES CF 1/4W 22ohm 5% R2

1 024947 RES CF 1/4W 47ohm 5% R14

6 024965 RES CF 1/4W 1K 5% R11,15,21,24,28,46

2 024969 RES CF 1/4W 1.5K 5% R1,10
34 024971 RES CF 1/4W 2.2K 5% R25,48,50,52,54,56,58,60,62,64,

R109,115,116,119
2 024980 RES CF 1/4W 8.2K 5% R39,44
7 024981 RES CF 1/4W 10K 5% R3-5,12,13,32,38
1 024985 RES CF 1/4W 15K 5% R27
1 024987 RES CF 1/4W 22K 5% R31

23 024993 RES CF 1/4W 47K 5% R7,16-18,26,118,121,123

2 029613 RES CF 1/4W 91K 5% R55,63

10 024997 RES CF 1/4W 100K 5% R9,20,23,45,113

1 028549 RES CF 1/4W 110K 5% R53
6 024999 RES CF 1/4W 150K 5% R114
1 025058 RES CF 1/4W 180K 5% R61
2 025059 RES CF 1/4W 220K 5% R51,59

PARTS LIST

PREAMP

PRINTED CIRCUIT BOARD ASSEMBLY (CONT)

QTY PART # DESCRIPTION REFERENCE DESIGNATION

2 029450 RES CF 1/4W 240K 5% R47,57
1 025061 RES CF 1/4W 330K 5% R49

24 031819 RES MF 1/4W 1.82K 1% R101-104
24 031818 RES MF 1/4W 18.2K 1% R105-108

3 036375 SPACER RND NYL .147X.250X.720 (AT LD1-3)
5 033574 STANDOFF SWEDGE 4-40X.937
2 029904 SWITCH PUSHBUTTON PC MOUNT S1,2
1 027113 TERMINATION RING #6 18-14GA (CRIMPED ON GND WIRE)

.5ft 027182 WIRE 18GA BONDED BLACK (GND TO FOIL SHIELD)

1 016739 XSTR NPN 2N4401 TO-92 Q1

FRONT PANEL PREAMP ASSEMBLY

42 9904814100 BUSHING SNAP-IN (POT SHAFT)

1 033326 CABLE RIBBON 8 CIRCUIT (INTERCONNECT) (SR6520/8520/P

9 028939 KNOB,GRAPHIC EQUALIZER (SR6520/8520/P)
9 041109 KNOB EQ BXR SERIES (SR6520/8520PD)
2 028841 KNOB PUSHBUTTON

18 028797 KNOB RTRY DARK BLUE (LOW/MID/HIGH)

7 028800 KNOB RTRY LIGHT GRAY (LEVEL)
2 038511 KNOB RTRY SALMON 177C (MAIN REV,MON REV)
7 038513 KNOB RTRY RED 192C (MONITOR)
7 038514 KNOB RTRY PURPLE 265C (EFF/REV/SEND)
1 038515 KNOB RTRY GREEN BASIC (AUXILIARY)

18 041835 KNOB CTL DBL GRY/BLUE (

LOW/MID/HIGH) SR6520P/8520P/PD

7 041833 KNOB CTL DBL GRY/WHT (LEVEL) SR6520P/8520P/PD
9 041836 KNOB CTL DBL GRY/BLK (MAIN REV, MON REV, MONITOR)

SR6520P/8520P/PD

7 041834 KNOB CTL DBL GRY/GREEN

(EFF/REV/SEND) SR6520P/8520P/PD

1 041837 KNOB CTL DBL GRY/DK GRY (AUXILIARY) SR6520P/8520P/PD

13 038537 NUT JAM 3/8-32 BLK NYLON (1/4" INPUT JACKS)

1 049153 PANEL FRONT SR6520PD 037786 = SR6520P
1 049154 PANEL FRONT SR8520PD 037826 = SR8520P
5 014172 SCREW M 4-40X1/4 PHP BLX (PCB T O PANEL MTG)

13 037990 SCREW PLASTITE 6X3/8 PHP BLX (XLR/RCA J ACKS)
13 038538 WASHER FW 3/8X5/8X3/32 BLK NYLON (1/4" JACKS)

4