QSC III Series Operating manual

Series III
Power Amplifiers
TECHNICAL SERVICE MANUAL
 3800
REV. B
*TD-300001-AX*
 3500
 3350

3200

2
SERVICE MANUAL
Series Three
Power Amplifiers
QSC Audio Products Inc.
Technical Services Group
Toll Free: 800-772-2834
Local: 714-957-7150
Fax: [phone]
QSC Audio Products, Inc.
1675 MacArthur Blvd.
Costa Mesa, CA 92626
www.qscaudio.com
[email protected]

3
SERIES THREE POWER AMPLIFIERS

4
Table of Contents
Product Specifications 5
Model 3200 5
Model 3350 6
Model 3500 7
Model 3800 8
Series Three Parts Lists 9
3200 PCB ASSEMBLY 9
3350 PCB ASSEMBLY 11
3500 PCB ASSEMBLY 13
3800 MAIN NPN PCB ASSEMBLY 15
3800 MAIN PNP PCB ASSEMBLY 16
3800 GAIN PCB ASSEMBLY 17
Series Three Power Amplifiers 18
Circuit Description 18
Block Diagram 18
Detailed Explanations 18
Troubleshooting Chart 23
Introduction 23
1. Current Draw Section 23
2. Protection Circuit Problems 24
3. Faults With Signal Present 25
Series Three Test Procedures: 29
Model 3200 29
Model 3350 31
Model 3500 33
Model 3800 35
Series Three Schematics 37
Model3200 37
Model 3350 38
Model 3500 Early Version 39
Model 3500 Late Version 40
Model 3800 41
Series Three PCB Drawings 43
Model 3200 PCB 43
Model 3350 PCB 44
Model 3800 46A PCB 45
Model 3800 47A PCB 46
Model 3800 48B PCB 47
Model 3800 Gain PCB 48

5
Product Specifications
Model 3200
OUTPUT POWER (per channel)
Continuous Average Output Power both channels
driven:
8 ohms, 20 Hz-20 kHz 0.1% THD 110 W
1 kHz 1% THD 135 W
4 ohms, 20 Hz-20 kHz 0.1% THD 140 W
1 kHz 1% THD 200 W
2 ohms, 1 kHz 1% THD NR
Bridged Mono operation:
16 ohms, 20 Hz-20 kHz 0.1% THD 220 W
1 kHz 1% THD 270 W
8 ohms, 20 Hz-20 kHz 0.1% THD 280 W
1 kHz 1% THD 400 W
4 ohms, 1 kHz 1% THD NR
Distortion: THD – 20 Hz-20 kHz from 250 milliwatts to rated
power shall be less than 0.1%, 0.015% typical. SMPTE-IMD less
than 0.020% 250 milliwatts to rated power.
Frequency Response: 20 Hz-10 kHz ±0.1 dB
8 Hz-300 kHz +0/-3 dB
Damping Factor: Greater than 200
Dynamic Headroom: 3.0 dB at 8 Ω
Noise: -100 dB 20 Hz-20 kHz
Sensitivity: 1V RMS for rated power (8 Ω)
Input Impedance: 20 kΩ
Controls: Front – Flush detented gain control, recessed
AC switch and AC circuit breaker for each
channel.
Rear – Mono-bridging and accessory module
switches.
Indicators: Bi-color LED indicating DC power –
OK/Protect mode, LED clip indicator, -30 dB
and -6 dB signal level indicators, flashing over-
temp indicators.
Connectors: XLR, ¼” (ring, tip, sleeve) and 3 terminal
barrier strip inputs wired in parallel, 2-terminal
barrier strip and 5-way binding post outputs
wired in parallel, octal input sockets provided
for input transformers or active accessories,
ground lift terminal block.
Cooling: Passive cooling combined with high efficiency
output stage for reduced operating
temperatures. Unique circuit configuration
allows direct metal mounting of output devices
for reduced thermal stress from short-term
peaks.
Amplifier Protection: Indefinite short circuit*, open circuit,
over-temp, ultrasonic and RF protection.
Stable into reactive and mismatched loads.
Inputs protected from overload. All protection
completely independent on each channel.
*Output Averagingshort circuit protection
(U.S. Pat. 4,321.554)
Load Protection: Individual channel Load Grounding
output relays provide DC Fault, 3 second
delayed turn on (transient protection), and
excessive low frequency protection, Instant
turn-off, pop suppression and power interrupt
protection is also provided.
Output Circuit Type: Full complementary two-level high
efficiency.
Output Devices (total): 8
Power Supply: Two completely separate power supplies
including AC switches and AC circuit breakers;
only AC cord is common.
Power Requirements: 120 Volts, 60 Hz, 4.4 A
Dimensions: Faceplate 19” × 1.75”
Depth (behind mtg,
with rear support) 16.5”
Depth (chassis) 14.6”
Weight: Shipping (lbs.) 31
Net (lbs.) 26

6
Model 3350
OUTPUT POWER (per channel)
Continuous Average Output Power both channels
driven:
8 ohms, 20 Hz-20 kHz 0.1% THD 200 W
1 kHz 1% THD 260 W
4 ohms, 20 Hz-20 kHz 0.1% THD 300 W
1 kHz 1% THD 420 W
2 ohms, 1 kHz 1% THD 450 W
Bridged Mono operation:
16 ohms, 20 Hz-20 kHz 0.1% THD 400 W
1 kHz 1% THD 520 W
8 ohms, 20 Hz-20 kHz 0.1% THD 600 W
1 kHz 1% THD 840 W
4 ohms, 1 kHz 1% THD 900 W
Distortion: THD – 20 Hz-20 kHz from 250 milliwatts to rated
power shall be less than 0.1%, 0.015% typical. SMPTE-IMD less
than 0.020% 250 milliwatts to rated power.
Frequency Response: 20 Hz-10 kHz ±0.1 dB
8 Hz-300 kHz +0/-3 dB
Damping Factor: Greater than 200
Dynamic Headroom: 3.0 dB at 4 ohms
Noise: -100 dB 20 Hz-20 kHz
Sensitivity: 1 V RMS for rated power (8 Ω)
Input Impedance: 20 kΩ balanced or unbalanced
Controls: Front – Flush detented gain control, recessed
AC switch and AC circuit breaker for each
channel.
Rear – Mono-bridging and accessory module
switches.
Indicators: Bi-color LED indicating DC power –
OK/Protect mode, LED clip indicator, -30 dB
and -6 dB signal level indicators, flashing over-
temp indicators.
Connectors: XLR, ¼” (ring, tip, sleeve) and 3 terminal
barrier strip inputs wired in parallel, 2-terminal
barrier strip and 5-way binding post outputs
wired in parallel, octal input sockets provided
for input transformers or active accessories,
ground lift terminal block.
Cooling: Passive cooling combined with high efficiency
output stage for reduced operating
temperatures. Unique circuit configuration
allows direct metal mounting of output devices
for reduced thermal stress from short-term
peaks.
Amplifier Protection: Indefinite short circuit*, open circuit,
over-temp, ultrasonic and RF protection.
Stable into reactive and mismatched loads.
Inputs protected from overload. All protection
completely independent on each channel.
*Output Averagingshort circuit protection
(U.S. Pat. 4,321.554)
Load Protection: Individual channel Load Grounding
output relays provide DC Fault, 3 second
delayed turn on (transient protection), and
excessive low frequency protection, Instant
turn-off, pop suppression and power interrupt
protection is also provided.
Output Circuit Type: Full complementary two-level high
efficiency.
Output Devices (total): 16
Power Supply: Two completely separate power supplies
including AC switches and AC circuit breakers;
only AC cord is common.
Power Requirements: 120, 220, or 240 Volts, 50 Hz-60 Hz,
8 A (each channel)
Dimensions: Faceplate 19” × 3.5”
Depth (behind mtg,
with rear support) 17.9”
Depth (chassis) 15.9”
Weight: Shipping (lbs.) 46
Net (lbs.) 41

7
Model 3500
OUTPUT POWER (per channel)
Continuous Average Output Power both channels
driven:
8 ohms, 20 Hz-20 kHz 0.1% THD 300 W
1 kHz 1% THD 350 W
4 ohms, 20 Hz-20 kHz 0.1% THD 450 W
1 kHz 1% THD 560 W
2 ohms, 1 kHz 1% THD 700 W
Bridged Mono operation:
16 ohms, 20 Hz-20 kHz 0.1% THD 600 W
1 kHz 1% THD 700 W
8 ohms, 20 Hz-20 kHz 0.1% THD 900 W
1 kHz 1% THD 1120 W
4 ohms, 1 kHz 1% THD 1400 W
Distortion: THD – 20 Hz-20 kHz from 250 milliwatts to rated
power shall be less than 0.1%, 0.015% typical. SMPTE-IMD less
than 0.020% 250 milliwatts to rated power.
Frequency Response: 20 Hz-10 kHz ±0.1 dB
8 Hz-300 kHz +0/-3 dB
Damping Factor: Greater than 200
Dynamic Headroom: 3.0 dB at 4 Ω
Noise: -100 dB 20 Hz-20 kHz
Sensitivity: 1 V RMS for rated power (8 Ω)
Input Impedance: 20 kΩ balanced or unbalanced
Controls: Front – Flush detented gain control, recessed
AC switch and AC circuit breaker for each
channel.
Rear – Mono-bridging and accessory module
switches.
Indicators: Bi-color LED indicating DC power –
OK/Protect mode, LED clip indicator, -30 dB
and -6 dB signal level indicators, flashing over-
temp indicators.
Connectors: XLR, ¼” (ring, tip, sleeve) and 3 terminal
barrier strip inputs wired in parallel, 2-terminal
barrier strip and 5-way binding post outputs
wired in parallel, octal input sockets provided
for input transformers or active accessories,
ground lift terminal block.
Cooling: Passive cooling combined with high efficiency
output stage for reduced operating
temperatures. Unique circuit configuration
allows direct metal mounting of output devices
for reduced thermal stress from short-term
peaks.
Amplifier Protection: Indefinite short circuit*, open circuit,
over-temp, ultrasonic and RF protection.
Stable into reactive and mismatched loads.
Inputs protected from overload. All protection
completely independent on each channel.
*Output Averagingshort circuit protection
(U.S. Pat. 4,321.554)
Load Protection: Individual channel Load Grounding
output relays provide DC Fault, 3 second
delayed turn on (transient protection), and
excessive low frequency protection, Instant
turn-off, pop suppression and power interrupt
protection is also provided.
Output Circuit Type: Full complementary two-level high
efficiency.
Output Devices (total): 24
Power Supply: Two completely separate power supplies
including AC switches and AC circuit breakers;
only AC cord is common.
Power Requirements: 120 Volts, 60Hz, 12 A
Dimensions: Faceplate 19” × 3.5”
Depth (behind mtg,
with rear support) 17.9”
Depth (chassis) 15.9”
Weight: Shipping (lbs.) 55
Net (lbs.) 50

8
Model 3800
OUTPUT POWER (per channel)
Continuous Average Output Power both channels
driven:
8 ohms, 20 Hz-20 kHz 0.1% THD 375 W
1 kHz 1% THD 440 W
4 ohms, 20 Hz-20 kHz 0.1% THD 600 W
1 kHz 1% THD 750 W
2 ohms, 20 Hz-20 kHz 0.2% THD 850 W
1 kHz 1% THD 1100 W
Bridged Mono operation:
16 ohms, 20 Hz-20 kHz 0.1% THD 720 W
1 kHz 1% THD 880 W
8 ohms, 20 Hz-20 kHz 0.1% THD 1200 W
1 kHz 1% THD 1500 W
4 ohms, 20 Hz-20 kHz 0.2% THD 1700 W
1 kHz 1% THD 2200 W
Distortion: THD – 20 Hz-20 kHz at rated power shall be less than
0.1%. SMPTE-IMD less than 0.025% at rated power.
Frequency Response: 20 Hz-10 kHz ±0.1 dB
8 Hz-300 kHz +0/-3 dB
Damping Factor: Greater than 200
Dynamic Headroom: 2 dB at 4 Ω
Noise: -100 dB 20 Hz-20 kHz
Sensitivity: 1 V RMS for rated power (8 Ω)
Input Impedance: 20 kΩ balanced or 10 kΩ
unbalanced
Controls: Front – Flush detented gain control, recessed
AC switch and AC circuit breaker for each
channel.
Rear – Mono-bridging and accessory module
switches.
Indicators: Bi-color LED indicating DC power –
OK/Protect mode, LED clip indicator, -30 dB
and –6 dB signal level indicators, flashing
over-temp indicators.
Cooling: Passive cooling combined with high efficiency
output stage for 50% reduction in dissipated
heat. Unique circuit configuration provides
direct-metal mounting of output devices to
minimize short-term thermal excursions of
power transistors. Fan-assisted cooling
recommended for high duty cycles at 2 ohms.
Amplifier Protection: Indefinite short circuit*, open circuit,
over-temp, ultrasonic and RF protection.
Stable into reactive and mismatched loads.
Inputs protected from overload.
*Output Averagingshort circuit protection
(U.S. Pat. 4,321.554)
Load Protection: Individual channel output relays
provide DC Fault, 3 second delayed turn-on
(transient protection), and excessive low-
frequency protection. Instant turn-off, pop
suppression and power interrupt protect is also
provided..
Output Circuit Type: Full complementary two-level high
efficiency.
Output Devices (total): 40
Power Supply: A complete separate power supply for each
channel including AC switch/circuit breaker
and AC cord.
Power Requirements: 120, 220, or 240 Volts, 50 Hz-60 Hz,
13 A (each channel)
Dimensions: Faceplate 19” × 5.25”
Depth (behind mtg,
with rear support) 17.9”
Depth (chassis) 15.9”
Weight: Shipping (lbs.) 75
Net (lbs.) 83

9
Series Three Parts Lists
3200 PCB ASSEMBLY
WP-320038-PO 3200 MAIN PCB ASSY WP-320038-PO 3200 MAIN PCB ASSY
Part Number Description Reference Part Number Description Reference
SC-062050-PP #6 X 5/16" P/P ZINC Q5, Q6, PL-903125-SP SPACER, ROUND, NYLON 0.047 R14
Q7, Q8 PL-000220-MI Q1, Q2,
SW-000010-RY MX1500/3500 24V OUTPUT RELAY D19, D20
WC-0.6022-JW .6" JUMPER, WHITE J3, J6, PL-905385-SP SPACER RND, NYL #6 0.385"L B LD1, LD2,
J7, J10 LD3, LD4,
WC-0.8022-JW .8" JUMPER, WHITE J2, J11 LD5
WC-001102-SQ INSUL.SLEEVE QSC 1102,.6" CL
R
PTC PL-909235-SP HOLDER, LED, 90 DEGREES, NY LD1, LD2,
WC-1.1022-JW 1.1 JUMPER WHITE 22GA J4, J5 LD3, LD4,
XF-200010-RL INDUCTOR 2UH W/10 OHM AXIL L1, L2 LD5
IC-005532-OP OP DUAL NE5532 PT-110000-AT TRIM 100 OHM TR1
LB-100003-SR ON SUBPANEL PT-210000-AT 1K OHM TRIMPOT TR2, TR3
LB-376002-MP ON SUBPANEL QD-000134-LR LED RED T-1 3/4 LD1, LD2
MS-320310-HS HEAT SINK QD-000134-LX LED R/G T-1 3/4 LD5
MS-320372-SP SUBPANEL QD-000134-LY LED YEL T-1 3/4 LD3, LD4,
PL-090751-RV RIVET, FASTEX ON HEAT SINKS QD-0003.9-ZT ZENER 3.9V .25W DZ84073ID Z25
PT-350000-AT 50K DET GAIN POT QD-0004.7-ZT ZENER 4.7V .25W DZ840731E Z24
QD-050200-BX 5A 200V BRIDGE RECTIFIER B3, B4 QD-0007.5-ZT ZENER 7.5V .25W DZ840731F Z26
SC-061041-SP #6-32 X 1/4" SCREW SUBPANEL MTG. QD-000424-TX 2SD424 XSTOR NPN TO-3/TO-204
A
Q6, Q8
SC-062050-PP #6 X 5/16" P/P ZINC SUPBANEL MTG QD-000554-TX 2SB554 XSTOR PNP TO-3/TO-204
A
Q5, Q7
SC-062081-PO #6X1/2 B P/P OVAL BLACK HEAT SINKS MTG QD-000810-DX DIOD 100V 8A 35NS MUR810 D19, D20
CA-033100-CM 33PF, 100V, 5%, MICA C2 QD-001340-LR LED RED T-1 3/4 VREF LD6
CA-147100-BD 470PF, 100V, 10%, DISK C11, C12, QD-004004-DX DIODE RECT DO41 400V 1A D4, D7,
C13, C14 D8,
CA-210100-BP .001UF, 100V, 10%, MYLAR C1, C9, D9, D18
C10, C15 QD-004410-TX BJTN 80V .25A 1.5W 2N4410 Q9, Q13,
CA-222100-BP .0022UF,100V,10%,MYLAR C3 Q14
CA-347050-BP .047UF, 50V, 10%, MYLAR C7, C8 QD-004744-ZA ZENER 15V, 1W, 5%, 1N4744A Z21, Z22
CA-410100-BP .1UF, 100V, 10%, MYLAR C5 QD-004934-DX DIODE100V 1A 200NS VP 1N493 D1, D2,
CA-422100-CP .22UF, 100V, 5%, MYLAR C4 D3, D5,
CA-610035-BE 10UF, 35V, ELEC., RADIAL E7 D6, D10,
CA-647010-BN 47UF, 10V, 10% RADIAL,CAPLYT
I
E4 D11, D13,
CA-647035-BE E5 D14. D16,
CA-722010-BN 220UF 10V CAPACITOR E6 D17
CA-722025-BE 220UF, 25V, ELECT., RADIA E1, E2 QD-008599-TX BJTP 60V .5A .625W MPS8599 Q10 Q11,
CA-747010-BE 470UF, 16V, ELEC., RADIA E3 Q12, Q15
CA-822080-BE 2200UF 80V ELEC. CAPACITOR E8, E9, QD-015030-TM Q1, Q3
E10, E11 QD-015031-TM Q2, Q4
CA-833050-BE 3300UF 50V CAPACITOR E13, E14 QD-1.5200-BX DIODE BRIDGE RECT 200V, 1.5A B1, B2
CO-000008-IC SOCKET 100 8-PIN DIP RE-.27005-EM 2.7 2W 5% MOFP R32, R33
CO-520258-MJ 6 COND. MOD JACK RE-.33005-DM 3.3 1W 5% MOFP R12, R13
CO-641119-AH HDR 156 8-PIN RAMP LOCK PREF RE-.56005-BC 5.6 1/4W 5% CF R10
HW-060080-PS #6-32 X 8 PEM STUD RE-000000-PT 90 C PTC RESISTOR PTC
MS-000006-TG IN ALL LOCATIONS SHOWN AS GREASE RE-000050-NR NTC THERMISTOR, 50 OHM R14
MS-320311-HT RE-002205-DM 22 1W 5% MOFP R27, R28,
NW-000220-RW TO-220 RECT. WASHER Q1, Q2, R29, R30
D19, D20 RE-010005-BC 100 1/4W 5% CF R11, R23,
NW-060010-SL #6 SPLIT TOOTH LOCK WASHER Q1, Q2, R24, R47
Q5, Q6, RE-015005-BC 150 1/4W 5% CF R43, R44
Q7, D19, RE-033201-BM 332 1/4W 1% MF R8
D20, PTC RE-030005-NW R36
NW-060400-HN #6-32 BY 1/4" HEX NUT Q1, Q2, RE-068005-EM 680 OHM,2 W,MET OX,5%,RES R34
Q5, Q6, RE-082005-BC 820 1/4W 5% CF R21, R22
Q7, D19, RE-110005-BC 1K 1/4W 5% CF R5, R16
D20, PTC, RE-115005-BC 1.5K 1/4 W 5% CF R25, R26
PEM STD(2) RE-115005-EM 1.5K 2W 5% MOFP R35
NW-060410-ST LCKWSH ON HEAT XFER BRKT PEMS NEAR POT RE-125005-HW 2.5K OHM, 5W, WIREWOUND 10% R37, R38
PC-000038-EM RE-127005-BC 2.7K 1/4 W 5%WW R41, R42

10
3200 PCB ASSEMBLY, Cont'd
WP-320038-PO 3200 MAIN PCB ASSY
Part Number Description Reference
RE-139005-CC R31
RE-147005-DM 4.7K 1W 5% MOFP R15
RE-151005-BC 5.1K 1/4W 5% CF R6
RE-210001-BM RESISTOR, MF 10. OK 1% 1/4 R1, R2,
R3, R4
RE-212101-CM 12.1K 1/2W 1% MF R7
RE-210005-BC 10K OHM, 1/4W, CARBON 5% R45
RE-230005-BC 30K 1/4W 5% CF R46
RE-242201-BM RESISTOR,42.2K,1/4W,MET FM 1
%
R9
RE-239005-BC 39K,1/4W,CARBON 5% R20
RE-310005-BC 100K 1/4W 5% CF R48
RE-315005-BC 150K 1/4W 5% CF R17
RE-333005-BC 330K 1/4W 5% CF R18, R19

11
3350 PCB ASSEMBLY
WP-335040-TC 3350 MAIN PCB ASSY WP-335040-TC 3350 MAIN PCB ASSY
Part Number Description Reference Part Number Description Reference
RE-230005-BC 30K 1/4W 5% CF R59 MS-000066-HS HEATSINK, TO-66 Q1, Q2
RE-260401-BM 60.4K 1/4W 1% MF R31 MS-000220-HS HEATSINK, TO-220 AAVID#5943B Q3, Q4
RE-310005-BC 100K 1/4W 5% CF R30, R62 MS-350411-HT HEAT TRANSFER BRKT.
RE-333005-BC 330K 1/4W 5% CF R28, R29 NW-000220-RW TO-220 RECT. WASHER D21, D22
SC-060060-PP #6-32 X 3/8" P/P ZINC Q1, Q2 NW-060010-SL #6 SPLIT TOOTH LOCK WASHER Q1, Q2,
SC-062050-PP #6 X 5/16" P/P ZINC Q5, Q6, D21, D22,
Q8, Q9, PTC
Q12, Q13, NW-060400-HN #6-32 BY 1/4" HEX NUT D21, D22,
Q15, Q16 PTC, PEM
SW-000010-RY MX1500/3500 24V OUTPUT RELAY NW-060410-ST LCKWSHR-PEM STUD-HEAT XFER BRKT MTG.
XF-200016-CR IND 2UH 16GA AIR L1, L2 PC-000040-FM MAIN PCB
WC-001102-SQ INSUL.SLV QSC 1102,.6", CLR PTC PL-000220-MI D21, D22
WC-003018-TQ BLUE, FROM HOLE POS. #1&2 TO B4 PL-903125-SP SPACER, ROUND, NYLON 0.047 L1, L2,
WC-003019-TQ BRN., FROM HOLE POS. #3&4 TO B3 R14
WC-003020-TQ RED, FROM HOLE POS. 'P HI' TO B4 'P HI' PL-905385-SP SPACER RND, NYL #6 0.385"L B LD1, LD2,
WC-003021-TQ GRN, FROM HOLE POS. 'N HI' TO B4 'N HI' LD3, LD4,
WC-003022-TQ ORG, FROM HOLE POS. 'P LO' TO B3 'P LO' LD5
WC-003023-TQ YEL, FROM HOLE POS. 'N LO' TO B3 'N LO' PL-909235-SP HOLDER, LED, 90 DEGREES, NY LD1, LD2,
WC-003024-TQ BLK, FROM HOLE POS #5 TO #5 NEXT 50K POT LD3, LD4,
PL-003109-SP OBS-NYLON SPACER D21, D22 LD5
NW-060010-SL #6 SPLIT TOOTH LOCK WSHR 25A BRIDGE RECT. PT-110000-AT TRIM 100 TR1
IC-005532-OP OP DUAL NE5532 PT-220000-AT TRIM 2K TR2, TR3
LB-100003-SR ON SUBPANEL QD-001340-LR LED RED T-1 3/4 VREF LD6
LB-376002-MP ON SUBPANEL QD-000134-LR LED RED T-1 3/4 LD1, LD2
MS-350410-HS HEAT SINK QD-000134-LX LED R/G T-1 3/4 LD5
MS-350472-SP SUBPANEL QD-000134-LY LED YEL T-1 3/4 LD3, LD4
SC-062120-PP #6 X 3/4" "B" P/P ZINC - 25A BRIDGE RECT QD-0003.9-ZT ZENER 3.9V .25W DZ84073ID Z15
PL-003.65-TW TIE WRAP, 4" BLACK ( SMALL) QD-0004.7-ZT ZENER 4.7V .25W DZ840731E Z16
PL-000011-TW TIE WRAP, 11" WHITE (LONG) QD-0007.5-ZT ZENER 7.5V .25W DZ840731F Z17
PL-090751-RV RIVET, FASTEX ON HEAT SINK QD-000424-TX 2SD424 NPN TO-3/TO-204AA Q12, Q13,
PT-350000-AT 50K DET GAIN POT Q15, Q16
QD-250400-BX 25A 400V BRIDGE RECTIFIER B3, B4 QD-000554-TX 2SB554 PNP TO-3/TO-204AA Q5, Q6,
SC-061041-SP #6-32 X 1/4" SCREW-SUBPANEL(3) 413 A/B(3) Q8, Q9
SC-062050-PP #6 X 5/16" P/P ZINC FOR SUBPANEL MTG. QD-000810-DX DIOD 100V 8A 35NS MUR810 D21, D22
SC-062081-PO #6X1/2 B P/P OVAL BLACK HEAT SINK MTG. QD-001847-TH SJ1847 PNP TO-66/TO-213AA Q2
CA-027100-BM OBS-27PF 10% 100V SM C2 QD-001848-TH SJ1848 NPN TO-66/TO-213AA Q1
CA-147100-BD 470PF, 100V, 10%, DISK C6, C8, QD-004004-DX DIODE RECT DO41 400V 1A D4, D7,
C12 D10, D17,
CA-210100-BP .001UF, 100V, 10%, MYLAR C1 D18
CA-215200-BP .0015UF, 200V, 10%, MYLAR C3, C7, QD-004410-TX BJTN 80V .25A 1.5W 2N4410 Q17, Q20,
C11 Q23
CA-322100-BP .022UF, 100V, 10%, MYLAR C13 QD-004744-ZA ZENER 15V, 1W, 5%, 1N4744A Z13, Z14
CA-368100-BP .068UF, 100V, 10%, MYLAR C5 QD-004934-DX DIODE100V 1A 200NS VP 1N493 D1, D2,
CA-410100-BP .1UF, 100V, 10%, MYLAR C9, C10 D3, D5,
CA-415100-CP .15UF, 100V, 5%, MYLAR C4 D6, D8,
CA-610035-BE 10UF, 35V, ELEC., RADIAL E7 D9, D11,
CA-647010-BN 47UF, 10V, 10% RAD,CAP LYTIC E4 D12, D14,
CA-647050-BE 47UF, 50V, ELECT., RADIAL E5 D15
CA-722010-BN 220UF 10V CAPACITOR E6 QD-008599-TX BJTP 60V .5A .625W MPS8599 Q18, Q19,
CA-722025-BE 220UF, 25V, ELECT., RADIA E1, E2 Q21, Q22
CA-747010-BE 470UF, 16V, ELEC., RADIA E3 QD-015030-TM Q3
CA-822100-BE 2200UF 100V CAPACITOR E8, E9, QD-015031-TM Q4
E11, E12 QD-1.5200-BX DIODE BRIDGE RECT 200V, 1.5A B1, B2
CA-847050-BE 4700UF 50V CAPACITOR E14, E15 RE-.01505-FW .15 OHM 3W 10% RESISTOR R35, R36,
CO-000008-IC SOCKET 100 8-PIN DIP R38, R39,
CO-520258-MJ 6 COND. MOD JACK R42, R43,
CO-641119-AH HDR 156 8-PIN RAMP LOCK PREF R45, R46
HW-060080-PS #6-32 X 8 PEM STUD RE-082005-BC 820 1/4W 5% CF R19, R22
MS-000006-TG IN ALL LOC SHOWN AS 'GREASE' RE-.27005-EM 2.7 2W 5% MOFP R49, R50

12
3350 PCB ASSEMBLY, Cont'd
WP-335040-TC 3350 MAIN PCB ASSY
Part Number Description Reference
RE-.33005-DM 3.3 1W 5% MOFP R25, R26
RE-.56005-BC 5.6 1/4W 5% CF R10
RE-.56005-EM 5.6 2W 5% MOFP R55
RE-000000-PT 90 C PTC RESISTOR PTC
RE-000050-NR NTC THERMISTOR, 50 OHM R14
RE-001005-EM 10 2W 5% MOFP R56
RE-002205-DM 22 1W 5% MOFP R15, R16,
R17, R18
RE-010005-BC 100 1/4W 5% CF R11, R20,
R21, R61
RE-010005-CC 100 1/2W 5% CF R32
RE-033201-BM 332 1/4W 1% MF R8
RE-033005-HW 330 OHM 5 WATT WW 10% R52
RE-015005-BC 150 1/4W 5% CF R57, R58
RE-110005-BC 1K 1/4W 5% CF R5
RE-110005-EM 1K 2W 5% MOFP R47
RE-115005-BC 1.5K 1/4 W 5% CF R13, R23,
R24
RE-120005-EM 2K 2W 5% MOFP R48
RE-127005-HW 2.7K 5W 5% WW R33. R34
RE-151005-BC 5.1K 1/4W 5% CF R6
RE-147005-CC 4.7K,1/2W CARBON 5% RES R12
RE-168005-DM 6.8 OHM, 1W, MET. OXIDE 5% R27
RE-210001-BM RESISTOR, MF 10. OK 1% 1/4 R1, R2,
R3, R4
RE-210005-BC 10K OHM, 1/4W, CARBON 5% R60
RE-216901-CM 16.9K, 1/2W MET FM 1% RES R7

13
3500 PCB ASSEMBLY
WP-350040-PC 3500 MAIN PCB ASSY REV Q WP-350040-PC 3500 MAIN PCB ASSY REV Q
Part Number Description Reference Part Number Description Reference
RE-230005-BC 30K 1/4W 5% CF R59 CA-847050-BE 4700UF 50V CAPACITOR E14, E15
RE-275001-BM 75.0K 1/4W 1% MF R9 CA-122100-BD CAP 220PF 10% 100V DISK C7
RE-247005-BC 47K 1/4 W 5% CF R31 CA-310100-BP .01UF, 100V, 10%, MYLAR C9
RE-310005-BC 100K 1/4W 5% CF R30, R62 CO-000008-IC SOCKET 100 8-PIN DIP
RE-333005-BC 330K 1/4W 5% CF R28, R29 CO-520258-MJ 6 COND. MOD JACK
SC-060060-PP #6-32 X 3/8" P/P ZINC Q1, Q2 CO-641119-AH HDR 156 8-PIN RAMP LOCK PREF
SC-062050-PP #6 X 5/16" P/P ZINC Q5, Q6, HW-060100-PS #6-32 X 20 PEM STUD P2, P3,
Q7, Q8, P4, P5
Q9, Q10, HW-060200-SP OBS-SPCR RND ALUM .25"ODX.1
Q11, Q12, HW-060080-PS #6-32 X 8 PEM STUD P1
Q13, Q14, MS-000006-TG IN ALL LOC SHOWN AS 'GREASE'
Q15, Q16 MS-000066-HS HEATSINK, TO-66 Q1, Q2
SW-000010-RY MX1500/3500 24V OUTPUT RELAY MS-000220-HS HEATSINK, TO-220 AAVID#5943B Q3, Q4
XF-200016-CR IND 2UH 16GA AIR L1, L2 CH-350208-GX HEAT TRANSFER BRKT.
WC-001102-SQ INSUL. SLV QSC 1102,.6", CLR PTC NW-000220-RW TO-220 RECT. WASHER D21, D22
WC-003018-TQ BLUE, FROM HOLE POS. #1&2 TO B4 NW-060010-SL #6 SPLIT TOOTH LOCK WASHER Q1, Q2,
WC-003019-TQ BRN., FROM HOLE POS. #3&4 TO B3 D21, D22,
WC-003020-TQ RED, FROM HOLE POS. 'P HI' TO B4 'P HI' PTC
WC-003021-TQ GRN, FROM HOLE POS. 'N HI' TO B4 'N HI' NW-060400-HN #6-32 BY 1/4" HEX NUT D21, D22,
WC-003022-TQ ORG, FROM HOLE POS. 'P LO' TO B3 'P LO' PTC, PEM
WC-003023-TQ YEL, FROM HOLE POS. 'N LO' TO B3 'N LO' NW-060410-ST LCKWSHR-PEM STUD-HEAT XFER BRKT MTG.
WC-003024-TQ BLK, FOM HOLE POS #5 TI #5 NEXT 50K POT NW-060410-ET #6 EXTERNAL TOOTH LOCKWASHER
PL-003109-SP OBS-NYLON SPACER D21, D22 PC-350040 MAIN PCB
NW-060010-SL #6 SPLIT TOOTH LOCK WSHR BRIDGE RECT. PL-000025-PL 9222-PH140 AMATOM SPACER
IC-005532-OP OP DUAL NE5532 PL-000026-PL TO-220 TEK-SIL INSULATOR D21, D22
LB-100003-SR ON SUBPANEL PL-903125-SP SPACER, ROUND, NYLON 0.047 L1, L2,
LB-376002-MP ON SUBPANEL R14
MS-350410-HS HEAT SINK PL-905385-SP SPACER RND, NYL #6 0.385"L B LD1, LD2,
MS-350472-SP SUBPANEL LD3, LD4,
SC-062120-PP #6 X 3/4" "B" P/P ZINC-25A BRIDGE RECT. LD5
PL-003.65-TW TIE WRAP, 4" BLACK ( SMALL) PL-909235-SP HOLDER, LED, 90 DEGREES, NY LD1, LD2,
PL-000011-TW TIE WRAP, 11" WHITE (LONG) LD3, LD4,
PL-090751-RV RIVET, FASTEX ON HEAT SINK LD5
PT-350000-AT 50K DET GAIN POT PT-110000-AT TRIM 100 TR1
QD-250400-BX 25A 400V BRIDGE RECTIFIER B3, B4 PT-250000-AT TRIM 2K TR2, TR3
SC-061041-SP #6-32 X 1/4" SCREW-SUBPANEL(3) 413 A/B(3) QD-001340-LR LED RED T-1 3/4 VREF LD6
SC-062050-PP #6 X 5/16" P/P ZINC FOR SUBPANEL MTG. QD-000134-LR LED RED T-1 3/4 LD1, LD2
SC-062081-PO #6X1/2 B P/P OVAL BLACK HEAT SINK MTG. QD-000134-LX LED R/G T-1 3/4 LD5
CA-018100-CM CAP 18PF 100V 5% MICA C2 QD-000134-LY LED YEL T-1 3/4 LD3, LD4
CA-147100-BD 470PF, 100V, 10%, DISK C6, C8, QD-0003.9-ZT ZENER 3.9V .25W DZ84073ID Z15
C12 QD-0004.7-ZT ZENER 4.7V .25W DZ840731E Z16
CA-168100-BD 680PF, 100V, 10%, DISK C14, C15 QD-0007.5-ZT ZENER 7.5V .25W DZ840731F Z17
CA-210100-BP .001UF, 100V, 10%, MYLAR C1 QD-000424-TX 2SD424 NPN TO-3/TO-204AA Q11, Q12,
CA-215200-BP .0015UF, 200V, 10%, MYLAR C3, C11 Q13, Q14,
CA-322100-BP .022UF, 100V, 10%, MYLAR C13, C16 Q15, Q16
CA-368100-BP .068UF, 100V, 10%, MYLAR C5 QD-000554-TX 2SB554 PNP TO-3/TO-204AA Q5, Q6,
CA-268100-CP OBS-.0068UF 100V 5% MYL C10 Q7, Q8,
CA-412100-CP .12UF, 100V, 5%, MYLAR C4 Q9, Q10
CA-610035-BE 10UF, 35V, ELEC., RADIAL E7 QD-000810-DX DIOD 100V 8A 35NS MUR810 D21, D22
CA-647010-BN 47UF, 10V, 10% RAD,CAP LYTIC E4 QD-002763-TM 170-220V, 2A, TO-220, PNP Q2, Q4
CA-647050-BE 47UF, 50V, ELECT., RADIAL E5 QD-002762-TM 170V -200V, 2A TO-220, NPN Q1, Q3
CA-722010-BN 220UF 10V CAPACITOR E6 QD-004004-DX DIODE RECT DO41 400V 1A D4, D7,
CA-722025-BE 220UF, 25V, ELECT., RADIA E1, E2 D10, D17,
CA-747010-BE 470UF, 16V, ELEC., RADIA E3 D18
CA-822100-BE 2200UF 100V CAPACITOR E8, E9, QD-004410-TX BJTN 80V .25A 1.5W 2N4410 Q17, Q20,
E10, E11, Q23
E12, E13 QD-004744-ZA DIODE 15V, 1W, 5%, 1N4744A Z13, Z14

14
3500 PCB ASSEMBLY, Cont'd
WP-350040-PC 3500 MAIN PCB ASSY REV Q
Part Number Description Reference
QD-004934-DX DIODE100V 1A 200NS VP 1N493 D1, D2,
D3, D5,
D6, D8,
D9, D11,
D12, D14,
D15
QD-008599-TX BJTP 60V .5A .625W MPS8599 Q18, Q19,
Q21, Q22
QD-1.5200-BX DIODE BRIDGE RECT 200V, 1.5A B1, B2
RE-.01505-FW .15 OHM 3W 10% RESISTOR R35, R36,
R37, R38,
R39, R40,
R41, R42,
R43, R44,
R45, R46
RE-.39005-EM 3.9 2W 5% MOFP R49
RE-.33005-DM 3.3 1W 5% MOFP R25, R26
RE-.56005-BC 5.6 1/4W 5% CF R10
RE-.56005-EM 5.6 2W 5% MOFP R55, R50
RE-000000-PT 90 C PTC RESISTOR PTC
RE-000050-NR NTC THERMISTOR, 50 OHM R14
RE-001005-EM 10 2W 5% MOFP R56
RE-002205-DM 22 1W 5% MOFP R15, R16,
R17, R18
RE-010005-BC 100 1/4W 5% CF R11, R20,
R21, R61
RE-018005-CC 180 1/2W 5% CF R32
RE-033201-BM 332 1/4W 1% MF R8
RE-045005-HW 450 OHM, 5 W, WIREWOUND 10% R52
RE-015005-BC 150 1/4W 5% CF R57, R58
RE-082005-BC 820 1/4W 5% CF R19, R22
RE-110005-BC 1K 1/4W 5% CF R5
RE-113005-EM 1.3K 2W 5% MOFP R47
RE-115005-BC 1.5K 1/4 W 5% CF R13, R23,
R24
RE-124005-EM 2.4K 2W 5% MOFP R48
RE-133005-HW 3.3K 5W 10% WW R33. R34
RE-151005-BC 5.1K 1/4W 5% CF R6
RE-156005-CC 5.6K 1/2W 5% RESISTOR R12
RE-168005-DM 6.8 OHM, 1W, MET. OXIDE 5% R27
RE-210001-BM RESISTOR, MF 10. OK 1% 1/4 R1, R2,
R3, R4
RE-210005-BC 10K OHM, 1/4W, CARBON 5% R60
RE-221501-CM 21.5K 1/2 WATT 1% MF R7

15
3800 MAIN NPN PCB ASSEMBLY
WP-380046-NP 3800 QSC46A MAIN PCB ASSY WP-380046-NP 3800 QSC46A MAIN PCB ASSY
Part Number Description Reference Part Number Description Reference
CA-147100-BD 470PF, 100V, 10%, DISK C4 WC-0.6022-JW .6" JUMPER, WHITE J1, J2,
CA-210100-BP .001UF, 100V, 10%, MYLAR C3 J8
CA-368100-BP .068UF, 100V, 10%, MYLAR C2 WC-0.9022-JW JMPR INS 0.9" 22AWG SLD WHT J3, J4
CA-410160-BP CAP 0.1UF 160V 10% MYLAR C1 PL-003109-SP SLEEVE D5, D6
CA-822100-BE 2200UF 100V CAPACITOR E3, E4, WC-2.5018-JW JMPR INS 2.5" 18AWG SLD WHT J5, J6,
E5, E6 J7
CA-833080-BE 3300UF 80V 20% LYTIC E1, E2 XF-200014-CR INDUCTOR,ZOBEL 14 AWG,2UH L1, L2
HW-060080-PS #6-32 X 8 PEM STUD "P", D5,
D6
MS-000066-HS HEATSINK, TO-66 Q1, Q2
MS-350411-HT HEAT TRANSFER BRKT.
NW-000220-RW TO-220 RECT. WASHER D5, D6
NW-060010-SL #6 SPLIT TOOTH LOCK WASHER Q1, Q2,
D5, D6
NW-060410-ST A-4 SPRING TOOTH LOCKWASHER "P"
NW-060400-HN #6-32 BY 1/4" HEX NUT "P", D5,
D6
PC-000046-AM MAIN NPN BOARD
PL-000220-MI D5, D6
PL-905100-SP SPACER, ROUND, NYLON, #6,0 L1, L2
PT-250000-AT RES VAR IT 5K 20% 0.15W CARB TR1
QD-0004.7-ZT ZENER 4.7V .25W DZ840731E Z1
QD-000424-TX 2SD424 NPN TO-3/TO-204AA Q3, Q4,
Q5, Q6,
Q7, Q8,
Q9, Q10,
Q11, Q12
QD-001847-TH SJ1847 PNP TO-66/TO-213AA Q1, Q2
QD-004934-DX DIODE100V 1A 200NS VP 1N493 D1, D2,
D3
QD-005402-DX DIODE RECT. 200V, 1N5402 D4
QD-008599-TX BJTP 60V .5A .625W MPS8599 Q13
QD-000810-DX DIOD 100V 8A 35NS MUR810 D5, D6
RE-.02205-FW 0.22, 3W, 10%, RES WW R23, R24,
R25, R26,
R27, R28,
R29, R30,
R31, R32
RE-.33005-DM 3.3 1W 5% MOFP R1
RE-001005-EM 10 2W 5% MOFP R12, R13,
R14, R15,
R16, R17,
R18, R19
RE-001505-DM 15 1W 5% MOFP R2, R3
RE-010005-BC 100 1/4W 5% CF R8
RE-015005-BC 150 1/4W 5% CF R6
RE-082005-BC 820 1/4W 5% CF R5
RE-115005-BC 1.5K 1/4 W 5% CF R7
RE-130005-HW 3K OHM, 5WATT, WIREWOUND R20
SC-060060-PP #6-32 X 3/8" P/P ZINC Q1, Q2
SC-062050-PP #6 X 5/16" P/P ZINC Q3, Q4,
Q5, Q6,
Q7, Q8,
Q9, Q10,
Q11, Q12

16
3800 MAIN PNP PCB ASSEMBLY
WP-380047-PN 3800 QSC47A MAIN PCB ASSY WP-380047-PN 3800 QSC47A MAIN PCB ASSY
Part Number Description Reference Part Number Description Reference
CA-147100-BD 470PF, 100V, 10%, DISK C5 RE-.02205-FW 0.22, 3W, 10%, RES WW R23, R24,
CA-210100-BP .001UF, 100V, 10%, MYLAR C2, C6 R25, R26,
CA-333100-BP .033UF, 100V, 1-%, MYLAR C4 R27, R28,
CA-347100-BP CAP 0.047UF 100V 10% MYLAR C1 R29, R30,
CA-222100-BP .0022UF,100V,10%,MYLAR C3 R31, R32
CA-610035-BE 10UF, 35V, ELEC., RADIAL E1 RE-.33005-DM 3.3 1W 5% MOFP R15
CA-722010-BN 220UF 10V CAPACITOR E2 RE-000000-PT 90 C PTC RESISTOR PTC
CA-822100-BE 2200UF 100V CAPACITOR E6, E7, RE-000050-NR NTC THERMISTOR, 50 OHM R22
E8, E9 RE-001505-DM 13 1W 5% MOFP R20, R21
CA-833080-BE 3300UF 80V 20% LYTIC E4, E5 RE-010005-BC 100 1/4W 5% CF R4, R5,
CA-647050-BE 47UF, 50V, ELECT., RADIAL E3 R18
HW-060080-PS #6-32 X 8 PEM STUD "P" RE-015005-BC 150 1/4W 5% CF R19
MS-000066-HS HEATSINK, TO-66 Q1, Q2 RE-012005-DM 120 1W 5% MOFP R14
MS-350411-HT HEAT TRANSFER BRKT. RE-156005-CC 5.6K 1/2W 5% RESISTOR R9
NW-000220-RW TO-220 RECT. WASHER D8, D9 RE-082005-BC 820 1/4W 5% CF R16
NW-060010-SL #6 SPLIT TOOTH LOCK WASHER Q1, Q2, RE-115005-BC 1.5K 1/4 W 5% CF R1, R17
D8, D9, RE-130005-HW 3K OHM, 5WATT, WIREWOUND R33, R34
PTC RE-168005-DM 6.8 OHM, 1W, MET. OXIDE 5% R13
NW-060410-ST LCKWSHR ON HEAT XFER BRKT MTG PEMS RE-247005-BC 47K 1/4 W 5% CF R11
NW-060400-HN #6-32 BY 1/4" HEX NUT PEMS RE-210005-BC 10K OHM, 1/4W, CARBON 5% R2, R12
PC-000047-BM MAIN PCB BOARD RE-230005-BC 30K 1/4W 5% CF R3
PL-000220-MI MICA WSHR D8, D9 RE-310005-BC 100K 1/4W 5% CF R8, R10
PL-903125-SP SPACER, ROUND, NYLON 0.047 R22 RE-333005-BC 330K 1/4W 5% CF R6, R7
PT-250000-AT RES VAR IT 5K 20% 0.15W CARB TR2 SC-060061-PP #6-32 X 3/8" P/P BLACK Q1, Q2
PT-110000-AT TRIM 100 TR1 SC-062050-PP #6 X 5/16" P/P ZINC Q3, Q4,
QD-001340-LR LED RED T-1 3/4 VREF LD1 Q5, Q6,
QD-000004-TX BJTN 60V 0.5A 1W MPSW05 Q18 Q7, Q8,
QD-0003.9-ZT ZENER 3.9V .25W DZ84073ID Z2 Q9, Q10,
QD-0007.5-ZT ZENER 7.5V .25W DZ840731F Z1 Q11, Q12
QD-000554-TX 2SB554 PNP TO-3/TO-204AA Q3, Q4, SW-000013-RY RLY SPDT 24VDC, 15A, 125VDC K1
Q5, Q6, WC-0.6022-JW .6" JUMPER, WHITE J1, J2,
Q7, Q8, J3, J4,
Q9, Q10, J4, J6,
Q11, Q12 J7, J8
QD-000810-DX DIOD 100V 8A 35NS MUR809 D8, D9 WC-0.9022-JW JMPR INS 0.9" 22AWG SLD WHT J10
QD-001848-TH SJ1848 NPN TO-66/TO-213AA Q1, Q2 WC-001102-SQ INSUL. SLV QSC 1102,.6",CLR PTC
QD-004004-DX DIODE RECT DO41 400V 1A D1, D2, PL-003109-SP SLEEVE D8, D9
D3, D4 WC-2.5018-JW JMPR INS 2.5" 18AWG SLD WHT J9, J11,
QD-004410-TX BJTN 80V .25A 1.5W 2N4410 Q15, Q16 J12
QD-005402-DX DIODE RECT. 200V, 1N5402 D10
QD-1.5200-BX DIODE BRIDGE RECT 200V, 1.5A B3
QD-004934-DX DIODE100V 1A 200NS VP 1N493 D5, D6,
D7
QD-008599-TX BJTP 60V .5A .625W MPS8599 Q13, Q14,
Q17

17
3800 GAIN PCB ASSEMBLY
WP-380048-GN GAIN BOARD ASSY
Part Number Description Reference
CA-018100-CM (REPLACED WITH CA-018001-10) C4
CA-047100-BD 47PF, 100V, 10%, DISK C5
CA-210100-BP .001UF, 100V, 10%, MYLAR C1
CA-215100-BP .0015UF, 100V, 10%, MYLAR C2
CA-410100-BP .1UF, 100V, 10%, MYLAR C6
CA-412100-CP .12UF, 100V, 5%, MYLAR C3
CA-547016-BN 4.7UF, 16V, 20% LYTIC NP E1
CA-647010-BN 47UF, 10V, 10% RAD, CAP LYTI
C
E4
CA-722010-BN 220UF 10V CAPACITOR E6
CA-722025-BE 220UF, 25V, ELECT., RADIA E2, E3
CA-747010-BE 470UF, 16V, ELEC., RADIA E5
CO-000008-IC SOCKET 100 8-PIN DIP IC1
CO-520258-MJ 6 COND. MOD JACK
PC-000048-BG GAIN PCB BOARD
PL-905385-SP SPACER RND, NYL #6 0.385"L B LD1, LD2,
LD3, LD4,
LD5
PL-909235-SP HOLDER, LED, 90 DEGREES, NY LD1, LD2,
LD3, LD4,
LD5
QD-000134-LR LED RED T-1 3/4 LD1, LD2
QD-000134-LX LED R/G T-1 3/4 LD5
QD-000134-LY LED YEL T-1 3/4 LD3, LD4,
QD-0003.9-ZT ZENER 3.9V .25W DZ84073ID Z3
QD-004744-ZA DIODE 15V, 1W, 5%, 1N4744A Z1, Z2
QD-004934-DX DIODE100V 1A 200NS VP 1N493 D1, D2,
D3, D4
QD-1.5200-BX DIODE BRIDGE RECT 200V, 1.5A B1
RE-.56005-BC 5.6 1/4W 5% CF R7
RE-006805-BC 68 OHM 1/4W CARBON 5% RES R12
RE-010005-BC 100 1/4W 5% CF R11
RE-033201-BM 332 1/4W 1% MF R10
RE-045005-HW 450 OHM, 5 W, WIREWOUND 10% R13
RE-110005-BC 1K 1/4W 5% CF R5
RE-151005-BC 5.1K 1/4W 5% CF R6
RE-210001-BM RESISTOR, MF 10. OK 1% 1/4 R1, R2,
R3, R4
RE-221501-CM 21.5K 1/2 WATT 1% MF R9
RE-275001-BM 75.0K 1/4W 1% MF R8

18
Series Three Power Amplifiers
Circuit Description
(9-1-1983)
Block Diagram
All Series Three amplifiers share the same basic circuit, with different size power supplies and different numbers of
output devices. We will use the 3500 as the basis for explanation. The circuit breaks down into the following block
diagram:
1. Power Supply, two-level main rails and op-amp rails
2. Balanced input circuit, gain control, and frequency limits
3. Amplifier feedback network, gain stage, distortion LED, and short circuit protection
4. Complementary driver transistors, crossover bias, and high/low driver sections
5. Complementary output transistors and output filters
6. Protection and signal readout circuits
7. Input/output functions (jack plane)
Detailed Explanations
Please refer to the 3500 schematic. Minor improvements made to the circuit will be explained as we go along.
1. Power Supply
AC power comes in through a thermal (slow-blow) circuit breaker and AC switch. A row of terminals on a small
circuit board allows the transformer primary to be wired for 120V, 220V, or 240V, 50-60 Hz operation, (120V only on
the 3200). There is a separate transformer for each channel, so the Series Three is a true dual-monaural design.
The transformer secondary windings are connected to the channel module through an 8-pin connector. The high-
voltage windings are rectified by B4, and filtered by E8, 9, 10 (positive) and E11, 12, 13 (negative). Half-voltage
windings are rectified by B3 and filtered by E14 (positive) and E15 (negative).
There are two unusual features of this power supply circuit. First, in addition to the usual full-voltage positive and
negative rails, we have secondary rails at half the voltage. These are used in the high-efficiency improvement
discussed in sections 4 and 5. Unlike our Series One amplifiers, the power supplies are DC-coupled with the usual
transformer center-tap. We use a speaker relay for DC protection as discussed in section 6.
The second unusual feature is that we reverse the usual location of the speaker and ground terminals. For reasons
explained in sections 4 and 5, we ground the common collectors of the output transistors, and take the audio output
from the midpoint of the power supply capacitors. This requires separate filters, rectifiers, and transformer
secondaries for each channel, but results in minimal cross talk, and permits on channel to fail without affecting the
other.
The bipolar 15V rails for the op-amp are derived using dropping resistors (R33 and R34) and zener stabilizers (Z13
and Z14) from the main rails. The op-amp supply rails are involved in our short-circuit protection scheme, as
described in section 3.
2. Balanced Input Circuit, Gain Control and Frequency Limits
The first stage of the dual op-amp is used as a differential input. Matched precision resistive dividers, R1/R3 and
R2/R4 are arranged so that any signal appearing equally on both the positive and negative inputs results in no
voltage across the op-amp input terminals and no voltage at the output of the op-amp. In other words, noise signals,
which normally occur equally on both sides of the balanced line, are rejected. The audio signal appears as a
difference between the balanced line conductors and these signals are picked up and appear at the same gain at
the output of the op-amp. For unbalanced inputs, either input line may be grounded and the circuit will respond at
unity gain to the other line. For reasons of overall stability in actual use, QSC uses the inverting input for
unbalanced signals.
After balanced-to-unbalanced conversion, the audio signal flows through R5 and C1, which roll of high frequency
response (less than -0.2dB at 20kHz, and –3dB at 200kHz). The 31-step gain control requires R6 for proper taper.
The output from the gain goes to the actual power amp circuit.

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3. Amplifier Feedback Network, Gain Stage, Distortion LED and Short Circuit Protection
The actual power amplifier begins at the input to the second stage of the op-amp. Audio feedback is established
across R7 and R8. A 47µF non-polar capacitor, E4, in series with R8 ensures that 100% of the DC feedback goes
to the op-amp. This has the effect of rolling off low frequency response below 20Hz, and ensures only a few
millivolts of DC offset at the amplifier output. The added feedback elements R9 and C4 reduce feedback slightly
below about 50 Hz, which counteracts the roll off of E4 in the region of 20-30Hz. Thus, we keep audio response
very flat without unnecessary sub-audio bandwidth, which could lead to unwanted speaker excursions due to breath
pops, etc. The resultant low-frequency response is down .2dB at 20Hz, and 3dB at 8 Hz.
High frequency stability is established by C3 and C2. C3 is the primary “phase lab” capacitor, which sets slew rate
and thus the overall “speed limit” of the circuit. C2 establishes “phase lead” in the speaker feedback network,
increases feedback at very high frequencies and thus improves control at frequencies where oscillations might
occur. C9 and C10, around the emitter resistors of the driver transistors, add a little more phase lead margin. A
speaker output filter, L2, R49, R50, and C5 isolates the amplifier from reactive loads at very high frequencies where
instability might occur.
Most of the gain in the amplifier is contributed by the second stage of the op-amp. This stage’s primary job is to feed
the driver transistors, but is also used to drive the distortion LED (LD1) and is tied in to the short circuit protection
scheme.
As long as the output of the amp is not clipping, the drive voltage to the bases of the driver transistors, Q1 and Q2,
remains below 1.5 volts peak. If the amp clips, the op-amp will attempt to overcome this by instantly delivering a
higher than normal voltage to the drivers. This excess voltage is used to drive the distortion LED (LD2) through
small bridge rectifier, B1, which requires a voltage in excess of about 2.6 volts to operate. Thus any form of clipping
immediately results in illumination of the distortion LED.
One of the featured advantages of the QSC circuit is our effective short circuit protection. In order to maintain good
audio performance into low impedance loads, it is necessary to maintain a high output current limit. The normal
current limit is determined by the base current to each driver through 1K resistors and 3.9V zener diodes (R19 and
R22, Z15 and Z16). When the op-amp rails are at their normal 15V, this current is about 9ma, and results in about a
25A current from the output transistors. This is sufficient to deliver 625 watts RMS to a two-ohm load. The output
transistors can withstand this much current into a short for a few seconds, but would overheat and fail before
thermal shutdown can occur. Therefore, we need a method of reducing current into shorts without affecting normal
operation.
The normal range of currents from the op-amp is less than the current supplied from the main power supply rails by
R33 and R34. However, as full power is reached, and especially if the amp is clipping and the op-amp has to
deliver extra current to the distortion LED, more current is drawn than these resistors can supply, and the op-amp
rail voltage would normally be drawn down. To prevent this, we have a replenishing circuit, from the speaker output
voltage, through R52 (three parallel resistors in early versions) and rectifiers D8 and D9. As long as the load
impedance is above two ohms, there will be enough output voltage to recharge the op-amp rails through this
network. If the output is shorted, however, the output voltage will be clamped to a low value, the recharge will not
occur, and the op-amp will deplete its supply rails to about 6 volts. This will cut the current to the drivers drastically
(through R19 and R22), and reduces output current to a safe level which the amp can withstand until thermal
shutdown occurs. The exact current limits are adjusted using 1k trimmers T2 and T3. (Certain second-generation
revisions will be discussed in section 4 below.)
4. Complementary Driver Transistors, Crossover Bias and High/Low Driver Circuit
The basic circuit design for Series Three is the same as Series One. We only need two stages of discrete devices
to amplify output of the op-amp to full rated power. However, with a little added complexity, we can double the
efficiency of the normal Class B output stage.
First let us examine the lower voltage section of the circuit, which can be regarded as a complete amplifier in itself,
operating from half the main rail voltage. Complementary drivers Q1 and Q2 are connected to the op-amp through
bias diodes D5 and D6. The forward voltage drop of the diodes matches the forward base voltage of Q1 and Q2, so
that as the op-amp swings above and below zero, it immediately drives Q1 or Q2. This eliminates any “dead zone”
which would cause crossover distortion. Trimmer T1 in parallel with R14 permits the bias to be fine-tuned. Emitter

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resistors R25 and R26 are used to stabilize the gain of Q1 and Q2, and to minimize any tendency to draw more idle
current as they heat up. The final collector current from Q1 and Q2 is about 500 mA, and goes to the bases of the
lower voltage output transistors Q5, Q6, Q7 and Q11, Q12, Q13.
These parts handle the first 50% of the output voltage, but then approach saturation. In order to deliver the
remaining 50% of the output swing, we need to transfer to the high-voltage drivers Q3 and Q4, which drive the full-
voltage output transistors Q8, Q9, Q10 and Q14, Q15, Q16. This is done by putting Q3 and Q4 in series with Q1
and Q2, and using 3.9 V Zeners Z15 and Z16 for turn-on threshold.
To explain this fully, we will look at just the positive half of the circuit, keeping in mind that the same thing will
happen on the negative half-cycle. Remembering that the speaker voltage is superimposed on the main power
supply rails (see section 1), you will note that as the positive driver Q1 is driven harder, its collector current,
amplified by its bank of outputs, will pull the positive half-voltage power supply closer to ground. Since the entire
supply is coupled together as a unit, this means all voltages move negative together. The speaker is connected to
the common midpoint, so this results in a negative speaker output. As we approach the point of 50% output, the
positive half-voltage supply will come within a few volts of ground, This carries the emitter of Q3within a few volts of
ground, until its base voltage drops below the voltage established by the 3.9V zener Z15. At this time, D12 becomes
forward biased and begins to drive Q3. Q3 takes over the current coming from Q1, thus cutting off the low-voltage
outputs and starting to drive the high-voltage outputs. These then pull the full voltage supply the rest of the way to
ground, resulting in 100% output to the speakers. Since the half-voltage supply is carried below ground during this
period, high-current diode D11 protects the low-voltage outputs from reverse polarity.
This reduces waste heat considerably, compared to the normal single-stage circuit. A power amp with a single level
DC supply has a lot of voltage across the power transistors for all levels up to full o0utput. This means that more
power is wasted at moderate power levels than at full output. By having a lower level DC supply for lower levels of
output voltages (up to 50%), we greatly reduce waste voltage and power for lower level signals.
Of course, the big problem is to switch over smoothly between high an low power stages. To assist this, especially
at high frequencies, we have a speed-up network, R23 and C17, which makes Q3 switch sooner at very high
frequencies, and coil, L1, between the low and high voltage filter capacitors to slightly “cushion” the transition.
One problem with the original scheme is that the zener voltage (about 3.9V) is added to the overall saturation loss,
which reduces peak output voltage by this amount. A second-generation circuit uses a small transistor after the
zener to isolate the loading effect of Q3 and Q4. This permits full signal swing, which improves dynamic headroom
somewhat.
5. Complementary Output Transistors
The emitters of the positive and negative output transistors are connected to their respective power supply rails
through load-sharing emitter resistors. The parallel bases of each bank are bypassed with 22 ohm resistors, R15,
R16, R17 and R18, which assure positive shut-off of the outputs. The 500 mA current from the collectors of the
driver transistors are connected to the output transistor bases and amplified to about 25 A peak.
Here you will see another unique feature of the QSC circuit. Note that the collectors of all output transistors meet at
a common ground. (This is possible because of the special power supply arrangement, as described in Section 1.)
This means we can simply screw the cases of all four banks to a grounded heat sink, with no need for the usual
insulating mica. This saves money and improves reliability through better cooling.
As mentioned in Sections 1 and 4, the speaker output is taken from the midpoint of the power supply capacitors.
This means that the audio output voltage is superimposed on top of the DC supply voltages, which must be kept in
mind when checking these voltages.
6. Protection and Signal Readout Circuits
A small, independent –24V power supply is tapped off the low voltage transformer secondary through diode D4 and
(in later versions) dropping resistor R32, and filtered by E5. A high current relay is put in series with the speaker
output, which is switched by small transistors Q17 and Q18. Note that the relay connects the load to ground when
not activated. This not only helps protect the load from DC fault, but enables the “bridged mono auto back-up”
feature (see Section 7).