Faults With Signal Present - QSC III Series Technical & Service Manual

Relay Faults with Normally Operating Amplifier
1. If the relay won't turn on,
a. Check the relay power voltage. With the relay off, voltage at E5 should measure –54V on a
3800, -47V on a 3500, -39V on a 3350, and –32V on a 3200. With the relay on, all amplifiers
should have about 27V across E5. If not, check R32, D4, D18 and E5 polarity.
b. If the power voltage is o.k., check the voltage on the timing capacitor, E7. This voltage should
rise to 12-15V in three seconds, which triggers Q18. If this doesn't happen, check R28, R29
and E7 polarity.
c. If the timing voltage is good, check the relay transistor, Q17. If this is good, a 47k resistor
between the base of Q17 and the speaker bus should activate the relay.
d. If the relay driver, Q17, is good, check driver Q18 and LED LD6.
2. If there is no red protect LED,
a. Check the LED voltage. If it is over +2V, the LED is defective. If it is 0V with the relay off (no
positive voltage to the LED), there is no protect power (missing R12), which defeats shut-down
circuits. Be sure to correct this before proceeding.
No Muting Delay (be sure the protect LED works, see above)
1. Possible causes of no muting delay without thermal or DC protect (relay or circuit is stuck on):
a. Relay driver Q17 is shorted or wrongly mounted. Check by jumping the base to emitter; this
should turn off the relay.
b. Driver Q18 is shorted or wrongly mounted. Check by jumping the base to emitter.
2. Possible causes of no muting delay only (the rest works normally):
a. D6 is reversed (charges timing capacitor E7 immediately)
b. R28 has too small of a resistance (E7 charges too fast)
c. R29 has too small of a resistance (a low voltage on E7 turns Q18 on too soon)
d. Missing or too large of a resistance at R30 (no off current)
3. Possible causes of an excess muting delay:
a. Check the voltage across the timing capacitor, E7. If it rises normally but the circuit is slow to
turn on, check Z17 (lift temporarily), LD6, whether bad or intermittent and whether R30 is too
low.
b. If the timing capacitor voltage is wrong, determine the cause. It may be that R28 or R29 are
bad, E7 is reversed, R62 is missing or Q19 is reversed or bad.
No Thermal Shutoff
Short the amplifier load with full signal input to raise the heat sink temperature and put a voltmeter across the PTC
(yellow sensor on heat sink bracket). Look for the voltage to rise from 0.7V when cold to 5.5V at shutdown.
1. If the red protect LED does not turn on and flash,
a. R12 may be missing which defeats the whole thermal circuit (no voltage across PTC).
2. If the red protect LED flashes but the amplifier doesn't shut down,
a. Check if Z17 is too high or missing, or R13 is too low, or the relay drive circuit is stuck on (see
above).
3. If the red protect LED comes on and R12 is good, but there is no voltage across the PTC,
a. The PTC may be defective or shorted (lift it temporarily; the amp should turn off immediately)
No DC Protect
1. Possible causes of the absence of DC protect
a. R27 is missing or has a very high resistance
b. B2 is defective or mounted wrong
c. Q19 is defective or mounted wrong
d. D17 is reversed (there would also be no muting delay)
e. Replace LD6 if voltage is less than 1.5V
The Amplifier Shuts Off with Signal
1. Possible causes of the amplifier shutting off with signal:
a. E6 is missing, defective or much too small

3. Faults With Signal Present

The amplifier passes a signal but is not running correctly.
25