Protection Circuit Problems - QSC III Series Technical & Service Manual

2. Protection Circuit Problems

CIRCUIT OPERATION. The Speaker output relay is switched by Q17, which is driven by Q18. Diode D18 absorbs
inductive kickback from the relay when shut off. There are three distinct protect functions; turn-on/off muting,
thermal shutdown, and DC protection.
RELAY POWER. "Protect" power is supplied by R12 from the positive half-voltage supply, which passes about 7-8
mA to the red side of the protect/pilot LED, LD5, and also power the thermal sensing circuit. Relay power is
supplied through R32, D4 to E5. When the relay is switched on, 22mA flows to the pilot LED, overcoming the
"protect" current and leaving about 15mA to drive the green side of the LED.
MUTE. When first powered, the protect current drives the red side of the pilot LED and holds the base of Q18 off
through R30. Because Q18 has emitter in series with LD6, it takes about –2V at its base to turn on. Timing
capacitor E7 charges up in three seconds through R28, supplying enough drive through R29 to Q18 to turn on the
relay circuit. When the relay switches and reverses the LED, there is a latching effect through R30 to Q18's base
which ensures a "snap action" to the relay drive. When the circuit is turned off, the relay power capacitor E5 is small
enough to discharge quickly, for fast relay shutdown. D17 discharges the timing capacitor E7, ensuring a time delay
even if the circuit is restarted immediately. Note that the circuit can't tell if the relay actually trips; be sure the relay
itself is not defective.
THERMAL. Note that the "protect" power flows through the PTC resistor, which has a cold resistance of 100 ohms,
and thus a cold voltage drop of 0.7-0.8V. As temp exceeds 75C, PTC resistance starts rising; at about 85C the
voltage drop increases to 3-4 volts, which is enough to activate the flashing circuit, through R13; this was an
integrated flashing LED on early models, later revised to discrete circuit comprising E3, R59, R60, R61, C11, Q20,
Q21, driving the thermal LED, LD1; and at about 95C, voltage reaches 5-5.5V, which is enough to activate zener
diode Z17, which turns off the base of Q18 (which is at –2.2V when on) and shuts off the relay circuit. The latching
effect holds the circuit off until the PTC voltage has cooled off and fallen by several volts.
DC PROTECTION. Output voltage is sensed through R27, but all frequencies above about 10Hz are filtered by
nonpolar E6. Excess voltage at subaudio frequencies will charge E6 until rectifier B2 and Q19 are triggered (on
either polarity of DC fault). Q19 discharges the timing capacitor E7, shutting off the relay (and forcing a restart delay
to prevent chattering).
PROTECTION CIRCUIT TROUBLESHOOTING. Note: all voltages are with respect to the speaker bus. If amplifier
itself is o.k., you can conveniently measure voltages to ground by turning signal level off (so there is zero volts on
the speaker bus).
Relay won't come on (first test)
1. Check speaker bus for DC; if over 3V DC present, (1-V on 3800), the protect circuit is operating
normally to hold off operation. If no DC is present, continue onto the section below, Relay Faults with
Normally Operating Amplifier.
DC Fault Without Current Draw Section
In most cases, shorts in the output circuit will cause current draw, but certain shorts will only cauyse DC offset in the
output. IN both cases, measure all the voltages, and look for abnormal values to help trace the fault, which can be a
solder or component short (zero volts), reversed zener or diode (0.6V), reversed electrolytic (several volts), or
wrong value parts (abnormal voltage). Look at the following points in the circuit:
1. Shorted IC rail, sometimes both shorted together; including mod jack
Blown Z13, Z14 gives zero volts on the IC rail
2.
TR2 or TR3 turned all the way off or way too low in value
3.
Collector-base short on driver (this is rare without further damage)
4.
Sometimes IC forces the rest of the circuit into DC due to shorts in the feedback network, etc. To check
5.
this, remove the IC, check for +15V and –15V and see if adjusting current limit trimmers TR2 and TR3
will produce a zero volt output (there will be no signal, of course). If so, output stage is probably o.k.,
look for problems in the IC or its associated parts.
Defective or reversed IC (pull the IC and check voltages)
6.
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