JBL SG520 - Page 16

Trouble Symptom
A. Pop or thump
during turn on or off.
B. Low output level from
low-level portion (Q101-
Q103).
C. Low output level from
line-amplifier portion
(Q104-Q110).
D. No output.
FIGURE 4-4 POWER SUPPLY
Remedies
1. Adjust A-C switch and output shorting switch contacts. Short on
SW 501 ab. should be maintained while switch SW 501 C in either
direction (see figure 4-8).
1. Voltage gain at 1 kc should be 42-44. If less, suspect open
bypass capacitors.
1. Voltage gain at 1 kc should be 3-12. If less, suspect open
bypass capacitors.
1. Using a 20,000 ohms-per-volt meter, verify that at least 1 volt
dc exists between the collector and emitter of each transistor.
lf the voltage is near zero, transistor is shorted and must be
replaced.
2. Check each coupling capacitor by measuring ac signal level at
each side of the capacitor. A greatly reduced signal indicates an
open capacitor.
NOTE When applying a test signal, make sure that level does not exceed 0.25 volt.
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