JBL LSR28P, Техническое и сервисное руководство

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Voltage Amplifier Test.
Turn the signal generator off. Disconnect 2-ohm load. Lift one side of R44. Connect base of Q16
to base of Q18. Power up amplifier. Verify that the voltage at U1B pin 7 is 15 volts. If
not, refer to “Amplifier Muting Test.”
1. Front end and Current Source
Verify 1.2V across anode of D17 to cathode of D16. If incorrect check D17, D16, R46, R45 and
Q19. Verify voltage across R45 (600mV). Verify resistance of R45 (620 ohms).
The voltage at the collector of Q19 should be -650mV. If voltage is substantially lower, look for
a short from collector to emitter of Q19.
The voltage at both bases of Q18 and Q16 should be very close to ground potential (the base of
Q18 will always be at a slightly negative voltage) and the emitters should be at -650mV. The
voltage drop across R40 and R41 should be around 30mV. Both drops should match /-
20%. If all above is not correct check Q16, Q18, R40 and R41.
The voltage drop across R30 and R28 should be 1.6V for both d/-20%. If voltages are
not matched, or if they are higher or lower, verify values of R30 and R28 (3K24), R29 (464ohm)
and R11 (150 ohm), and check for open or shorted elements in Q18, Q16, Q14 or Q12
. 2. Upper Class A drive
Verify that voltage across R11 equals 870mV +/- 20%. If incorrect, check Q12 for open or
shorted elements.
Check the voltage at the base of Q10. This should be half way between the positive supply
voltage and the output node. V(base Q10) = (36+V(output))/2. For example, if the output is
offset to -10V the voltage at the base of Q10 will equal (36-10)/2 = 13V. Under normal
conditions when the output is at zero volts the base of Q10 is at +18 volts. If incorrect check
values of R9 and R10 (63K4). If R9 and R10 are correct, replace Q10.
3. Level shifter
Verify that voltage across R29 equals 870mV +/- 20%. If incorrect, check Q14 for open or
shorted elements.
Verify that the voltage at the emitter of Q15 is +600mV. If incorrect check Q15 and Q14 for
shorts and opens
Verify that the voltages across R29 and R22 are the same (+/- 10%) If not check values of R22,
and R29 (464 ohms). Verify that D3 isn’t open, shorted or reversed. If the above checked okay,
verify value of R21 (150 ohms), and check Q13.