JBL LSR28P Technical & Service Manual

Also see for LSR28P: Owner's manual

46• Does amp oscillate?Verify correct values of C35, C25, C28, and C36. Is there an oscillation on the railsthat is not being filtered? If all this is okay and gain test above has been passed,replace U2• Is output distorted?Verify pin 8 of U2 measures approximately -3volts. If not refer to “AmplifierMute Test” above. If the input and mute circuits do measure okay replace U2anyway..Indicator LED control• Is the green LED not illuminated?Verify -15 volts at U3 pin 2. if not refer to “Amplifier Mute Test” above.Check for around 0 volts at pin 6 of U3. if not found check value of R38 and R57(both 10Kohms). If these are correct, replace U3.Verify -7.5 volts at U3 pin 10. if not found check value of R60 and R63 (both10kohms). If these are correct, replace U3.Check for 0 volts at U3 pin 11. If not check D.C. offsets at both power amplifieroutputs. If substantial negative offset detected refer to “Low Frequency PowerAmplifier Test” and to “High Frequency Power Amplifier Test”. If these arecorrect, replace U3.Verify around 5 volts at U3 pin 7. If not check value of R61, R58 (both 10K ohm)and R56 (22k ohm). If these are correct, replace U3.Check for approximately -12.5 volts at U3 pin 1. If voltage at U3 pin 1 is close to -15 volts replace U3. If the voltage at pin 1 of U3 is more than -15 volts but less than-12.5 volts check value of R50 (2K26). If U3 pin 1 is close to +15 volts replace theLED.• Is the red (or amber color) LED illuminated?Verify -7.5 volts at U3 pin 10. If not found check value of R60 and R63 9both10Kohms). If these are correct, replace U3.Check for 0 volts at U3 pin 11. If not check D.C. offsets at both power amplifieroutputs. If substantial negative offset is detected refer to “Low Frequency PowerAmplifier Test” and to “High frequency Power Amplifier Test.” If these measurecorrectly, replace U3.