QSC ISA 280, Technical & Service Manual

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Technical Service Manual 21
ISA Series Power Amplifiers
4. ISA calibration procedures
resistor, adjust the trimpot to obtain the voltage listed in
Table 1.
4. Channel 2: Locate resistor R247 (except ISA1350: R199 on the
lower module) and trimpot R231 (except ISA1350: R171 on
the lower module). While measuring the DC voltage across
resistor, adjust the trimpot to obtain the voltage listed in
Table 1.
After setting the bias in any model except the ISA1350, calibrate
the positive and negative current limiting; instructions for the
procedure follow in part 4.2.
4.2 Setting positive and negative
current limits
All models except the ISA1350 have adjustable positive and
negative current limiting thresholds. To provide better access, the
adjustment trimpots for the lower module are located on a small
auxiliary circuit board. Current limiting in the ISA1350 is not
adjustable.
Tools and resources you will need
• Oscilloscope
• 2-ohm resistive load (rated for at least 1200 watts)
• Shorting connector for amplifier output
• Variable AC transformer (e.g., Variac, Powerstat, etc.) rated for
25A (120V) or 12A (230V). Make sure the AC supply is appropri-
ate for the amplifier.
• 1 kHz audio sine wave generator
• Digital multimeter
• Clamp-on digital current meter (e.g., Fluke 30 Clamp Meter)
• Small flat screwdriver (non-conductive) for adjusting trimpots
Procedure
1. Set the audio sine generator to 1 kHz at 1 volt RMS and
connect it to Channel 1's input. Connect a 2-ohm load and the
oscilloscope probe across Channel 1's output.
2. Turn up Channel 1's gain control partway. On the oscilloscope
you should see the amplitude of the sine wave increase
accordingly.
3. Turn the gain control back down and apply a short circuit
across the output terminals of Channel 1. Clamp a current
probe either onto one of the brown wires running to the AC
switch or onto the gray output wire from channel 1's module.
4. Turn the gain control all the way up. Adjust trimpots R139 and
R140 equally until the current measured falls within the range
shown in Table 1.
5. Turn the gain control all the way down and remove the short
circuit so the channel drives the 2-ohm load. Turn the gain control
4.1 Setting bias
Always set the bias
• after replacing any output or driver transistor.
• after replacing any diode or resistor in the driver/output
circuitry.
• if the amplifier seems to run too hot at idle.
• if the amplifier exhibits crossover distortion.
The bias network sets the quiescent base current in the NPN and
PNP driver transistors, which in turn sets the quiescent current in
the output transistors. The driver transistors should both be
slightly “on” at idle so that the transitions of the signal voltage
between positive and negative are smooth and free of gaps or
glitches. Too much bias current will cause the amplifier to run
hotter than it should, especially at idle, while too little will cause
noticeable crossover distortion, especially at low signal levels.
The amplifier circuitry must be cool, or at least within a couple
degrees of ambient air temperature, and the top cover must be
removed. If the driver and output transistors are significantly
warmer than the ambient air, leave the amplifier off and let it cool
before proceeding.
Before turning the amplifier on to set bias on one or both
channels, familiarize yourself with the locations of the trimpots
(R131 and R231, or R171 of the ISA1350) and the voltage measur-
ing points so you can work quickly but thoroughly (in all models
except the ISA1350, the trimpots for the lower module are located
on a small auxiliary circuit board). If the amplifier warms up before
you finish setting the bias, you will need to shut the amplifier off
and let it cool down before you resume.
To set bias on the ISA1350, first remove the screws holding the
channel module/heat sink assembly to the chassis. Lift the
assembly up from the chassis standoffs and set it down inside the
chassis resting on the heat sinks. Do not disconnect any wires. This
gives you equal access to both channels’ adjuments and measure-
ment points.
Tools and resources you will need:
• Small flat screwdriver (non-conductive) for adjusting trimpots
• DC voltmeter
• AC power
Procedure
1. Turn the amplifier’s gain controls all the way down. No test
signal is needed.
2. Plug the amplifier into an appropriate AC source. Turn the
amplifier on.
3. Channel 1: Locate resistor R147 (except ISA1350: R199 on the
upper module) and trimpot R131 (except ISA1350: R171 on
the upper module). While measuring the DC voltage across