JBL LSR28P, Техническое и сервисное руководство

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Check all components that affect high frequency stability and response; C11, R27
(output loading), C2, C5 (Current amp stabilizers), C30, R31 and C32.
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Is there crossover distortion in the output?
To verify this note the character of crossover distortion. This is caused by both halves
of the output stage (Q6, Q7 on positive side, and Q8, Q9 on the negative side) being
turned off right around crossover (crossover from positive drive to negative drive). This
actually takes a small chunck (sometimes only visible in distortion waveform) out of
the center of the output waveform. This also causes some harmonic distortion (Note the
curved portions of the “Distortion waveform’). Remove input signal to amplifier and
measure DC voltage across emitters of Q7 and Q8. Bias adjustment, R16, should allow
adjustment from zero volts to well over 10mV. If not proceed to “Bias Reference Test”.
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Is there excessive second harmonic distortion in output?
Note the character of second harmonic distortion. This is caused by an imbalance in
amplifying capability in either the current amplifier or the voltage amplifier. Note that
the “Distortion Waveform” could also be inverted in relation to above depending of the
polarity of the imbalance. Place a probe on the output of the amplifier and adjust for
around 40VP/P if the condition of the board allows this much voltage. Visually note the
characteristics of the waveform and then probe on both sides of C1. C1 is across the input
to the current amp. If the waveform visibly appears almost identical to the output
waveform (same P/P voltage, same waveform shape) then the problem lies in the voltage
amp. Proceed to “Voltage Amplifier Test”. If there is much difference between the output
waveform and the waveforms measured at C1 there is probably a breakdown in the
current amplifier.
Bias Reference Test
The purpose of the bias reference circuit is to provide bias to the driver and output
transistors to just barely turn them on. Q10 is a positive current source and Q11 is a
negative current source. The voltage between the collectors of Q10 and Q11 is
regulated by bias reference.
A voltage of around 0.6V is required across the base emitter junction of a transistor to
begin to turn it on. Since a total of 4 junctions are in the current amp (Q6, Q7, Q8 and
Q9) the bias reference needs to have around 2.4 V (4 drops) across it to turn on all four
transistors in the current amplifier.
The range of adjustment is very small (Between 1.7V and 2.8V when R16 is adjusted
from stop to stop). The reason the adjustment is small is because a small amount of
change in voltage across the reference produces a radical bias current change in the
output stage (due to the logarithmic relationship between base emitter voltage to