JBL LSR28P, Техническое и сервисное руководство

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4. Lower Class A drive
Verify that voltage across R21 equals 870mV +/- 20%. If incorrect, check Q13 for open or
shorted elements.
Check the voltage at the base of Q11. This should be half way between the negative supply
voltage and the output node. From the formula:
V(base)=(+36+V)(output))/2.
For example, if the output is offset to +10V the voltage at the base of Q11 will equal (-
36+10)/2= -13V. Under normal conditions when the output is at zero volts the base of Q11 is at -
18 volts. If incorrect, check values of R20 and R19 (63K4). If R20 and R19 are correct, replace
Q11.
Remove power. Remove jumper across bases of Q18 and Q16. Re-solder lifted end R44 back
into the circuit board. Turn generator A on and power up board. Verify that output is centered
and undistorted. Turn down oscillators and reconnect 2ohm load. Bring up oscillator level and
verify that the output is centered and undistorted.