QSC III Series, Техническое и сервисное руководство

Страница: 26 / 48

26
Unstable
Signal is usually distorted, fuzzy or shows a smeared scope trace, usually worse on some part of the waveform. Do
not confuse this with hum in the signal, which evenly spreads trace vertically. Distinguish between the two by
reducing the sweep rate to a 10ms range, use an AC/line trigger, and look for 60-120 Hz hum frequencies.
1. Possible causes of severe instability, often with current draw:
a. Slew rate capacitor, C3, and R10 are missing or measure too low
b. Feedback capacitor, C2, is missing or measures too low
2. Possible causes of medium instability, especially with a light load:
a. The stability capacitor value is too high; check feedback capacitor C2, secondary stability
capacitors, C9 and C10
b. R10 is the wrong value
3. If gain is very low, suspect open circuit in feedback shunt, R8 and E4, or broken circuit trace.
4. Also, substitute the IC and check the IC socket for contamination
Wrong Current Limits
1. If the current is too high into a shorted output and the IC rails measure normal (5-6 volts), check for
a. A reversed or shorted 3.9V or 4.7V zener diode (Z15 or Z16)
b. Shorted bias diode, D5 or D6 (shows severe crossover)
2. If the current is too high into a shorted output and the IC rail measures too high during the short, some
possible causes are:
a. A bad IC (has weak output current)
b. Does the clip LED work? If not, the clip LED, LD2, could be open, the clip circuit part, R11
could be missing or measure too high or B1 could be defective
c. Check for short circuit current balance by raising the scope gain during a short and looking at
the small voltage across the residual short resistance. Concentrate on parts and circuits
connected to the side with the high current if unbalanced (remember the reverse polarity of the
QSC circuit, see below).
3. If the current is too low into a shorted output as well as a 2 ohm load, check:
a. If the IC rails are o.k. some possible causes are:
i. Bias resistor R19 or R22 measures too high
ii. Very low gain resistors (see below)
4. If the current is too low into a short only and is normal into typical loads:
a. Check whether the clip LED, LD2 is shorted, 1.5A rectifier, B1, is shorted or R11 is too low
b. Check whether the 3.9V or 4.7V zeners, Z15 or Z16, are too high (i.e. 7.5V or 15V respectively)
5. If the current is normal into a short but low into 2 ohms (usually on one side only), check
a. If IC rail is too low (driving two ohms even before clipping,
i. Check for high or missing chargeback resistor, R52, or missing or reversed chargeback
diode, D8 or D9 (usually causes premature clipping at 4 ohms as well)
b. If IC rail is o.k. (at least until clipping starts),
i. This could indicate low gain above or below the step (see Driver Gain Section below)
ii. Check if all the high voltage outputs (Q8, Q9, Q10 and Q14, Q15, Q16) and emitter
resistors (R38, R39, R40, and R44, R45, R46) are good; otherwise, it’s possible that
the upper driver has very low gain (see next section)
Step Problems
These are troubles associated with the circuit switching from the 50% rail to the 100% rail. Normal performance
allows about 0.03-0.05% extra distortion which can be seen when measuring into 4 or 8 ohm loads at 20kHz. Watch
for instability or spikes at the step, or weak output above the step.
1. If the output stops at the step, even into no load, check for
a. Missing or reversed D12 or D15
2. If the output steps correctly with an 8 ohm load but is weak or hesitant with a 2 or 4 ohm load, check for
a. A missing Q23, D11, R57 or Q22, D14, R58
b. A reversed Q22 or Q23
3. To test the driver gain and step circuit:
a. Turn the gain knobs up until the output of the amp is at the threshold of clipping.
b. If one side clips too early, the circuit of the opposite polarity has the problem (this is a usual
feature of QSC amplifier designs).