Introduction; Service Bulletins; The Well-Equipped Service Bench; Working With Surface-Mount Components - OSC RMX 850 Technical & Service Manual

1. Introduction

1.1 Service bulletins

Contact QSC Technical Services to make sure you have the most up-to-date service bulletins for RMX Series amplifiers. Service bulletins
may be distributed in hard copy, via fax, and electronically (Adobe Acrobat PDF) via CD-ROMs, FTP from the QSC web site
(www.qscaudio.com), and e-mail.
These service bulletins had been issued at the time this manual was printed: RMX0001, "Q205 Lead Stress" (RMX 2450 only); RMX0002,
"RMX Turn-on Delay" (all RMX models); RMX0003, "RMX 2450 Turn-off Mute" (RMX 2450); RMX0004, "RMX 2450 AC Wire Routing"
(RMX 2450); RMX0005, "RMX 2450 AC Wire Replacement" (RMX 2450); and RMX0006, "RMX 2450 IRFZ44N Field Effect Transistors (RMX 2450).

1.2 The well-equipped service bench

To properly service RMX amplifiers, a technician needs the right tools. The technician's service bench should have the following equipment:
• Digital multimeter with RMS AC voltage and current
• Digital clamp-on ammeter
• Dual-trace oscilloscope
• Audio distortion analyzer
• Non-inductive load resistors, configurable as 8 ohms (min. 500 watts capacity), as 4 ohms (min. 750 watts capacity), and 2 ohms (min.
1200 watts capacity)
• Variable AC voltage source, such as a Variac or Powerstat variable transformer, with a rated current capacity of up to 25A (for 120V
models) or 12A (for 230V models)
• Low-distortion audio sine wave generator
• Philips and flat screwdrivers
• Soldering iron with a fine tip (25–60W recommended)
• Rosin-core solder (60/40 or 63/37)
• Long-nose pliers
• Diagonal cutters
• Wire strippers
Automated test equipment, such as an Audio Precision workstation, is very useful for servicing RMX amplifiers. Contact QSC Technical
Services to obtain applicable AP test files.

1.3 Working with surface-mount components

RMX amplifiers, like many modern electronic products, use surface-mount technology (SMT) components where appropriate in order to
make high-density circuitry that is reliable and economical to manufacture.
SMT components in the RMX amps are used in the small-signal and control circuits, so they do not handle significant amounts of power;
therefore, they are subject to very little stress and should seldom fail. Sometimes they do fail, or they require replacement for a performance
upgrade or modification. Thus, it is important to know how to work with SMT components.
Specialized tools and equipment exist for soldering, unsoldering, and removing SMT components quickly and
efficiently, but they are often expensive. Most SMT repairs, though, can be handled reasonably well with common
tools and equipment, such as tweezers, solder braid, and fine-tip soldering irons. The original factory components
are tacked to the board with a spot of glue, so you might have to apply some force to break the adhesive.
Two-terminal components (resistors, capacitors, diodes, etc.)
Removal
1 Use two soldering irons, preferably about 25 to 40 watts, with fine tips.
2 With a soldering iron in each hand, hold one tip on the solder at one end of the component and the other
tip on the other end (Figure 1.1).
3 Once the solder melts on both ends, grip the component between the two tips and lift it from the circuit
board.
4 Use solder braid and a soldering iron to remove the solder from the two pads (Figure 1.2).
4
Figure 1.1.
Solder braid
Figure 1.2.
QSC Audio Products, Inc.