JBL LSR28P, Техническое и сервисное руководство

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collector current). Too great a range results in a control that is too sensitive and very
hard to adjust correctly.
As a transistor heats up it requires less and less base emitter voltage to obtain the same
collector current. If a bias reference voltage remains constant, and the output and the
driver transistors begin to heat up, the current amp will draw more and more current,
eventually destroying itself. This is why the reference transistor, Q3 is mounted to the
heatsink: as the outputs (Q4 through Q9) and drivers (Q1, Q5) heat up so does Q3. This
action regulates the output bias current and keeps it constant with temperature.
a. Excessive power pulled from AC line.
This is due to excessive voltage across the reference. The range (measure
across C1) should be 1.7V to 2.8V. If not check R12, R13, R14, R16 and Q3.
b. Crossover distortion in output waveform.
This is due to inadequate voltage across the reference. The range (measure
across C1) should be 1.7V to 2.8V. If not check R12, R13, R14, R16 and Q3.
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After amplifier heats up, bias current, and power pulled from AC line increases.
This is due to bias circuit not tracking the current amplifier with temperature. Make sure that all
of the transistors, Q3, Q6, Q7, Q8 and Q9 make good thermal contact with heatsink (hardware is
tight, adequate thermal grease is on devices, insulators and heatsink) If all of this is okay replace
Q3.
Bias Set Procedure
The following steps are necessary to set the bias on the LSR28P low frequency amplifier
module, PCB 510-00032-XX rev (any).
Connect a 2-Ohm load (250Watts0 to the red and the black leads of the amplifier. While
maintaining 120VAC, 60 Hz, turn the unit on.
Set the signal generator to 900Hz and to 0.2 Vrms approximately.
Using a distortion analyzer and an oscilloscope, turn the bias trim pot (R16) CW until distortion
spikes just disappear, and the THD+N reading is below 0.1 %
Turn oscillator off, while keeping the amplifier on.
Verify the power is between 10 and 20 watts and that D.C. voltage across TP3 is less than 5mV.