JBL SG520 - Power Supply Details; System Troubleshooting

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ESR
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A
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The oscillator frequency is determined by toroid L401 and capacitor. C402 which tune to approx-
imately 1000 cps. Because transistor Q401 has no ac voltage gain, the inductor L401 is driven
by the emitter and tapped (an autotransformer) to provide in-phase feedback to the base which
is a larger voltage than is supplied by emitter. The circuit therefore depends on the transistor’s
current gain to sustain oscillation. The output is taken from the emitter.
Potentiometer R401 (TEST TONE LEVEL) is used to set the output sine-wave to a level of 0.25
volts rms. Diodes D401 and D402 are clippers which limit the oscillator drive.
3.4 POWER SUPPLY DETAILS a
The power supply has two outputs: +24 volts, which is unregulated, and —21 volts regulated. ‘
Bridge rectifier D502-D505 supplies +24 volts through an RC ripple filter. Bridge rectifier ie
D506-D509 supplies approximately —-24 volts which is dropped to —21 volts by the series
regulator. Power transistor Q501 (PNP) is the pass regulator. The output remains constant
since the normal base emitter drop is nearly constant over a wide current range. The emit-
ter follows the base which in turn follows the base of driver Q502. The base of Q502 is ref-
erenced to —-22 volts across zener diode D501. So, if the line voltage (or load current) varies,
the Q501 emitter remains at —21 volts.
Section 4
REPAIR AND ADJUSTMENT
4.1 SYSTEM TROUBLESHOOTING el
Amplifier problems reported by the owner of an elaborate stereo system can often be traced to
factors outside the amplifier. Unlike vacuum tube amplifiers, the SG520 Graphic Controller |
is normally not subject to gradual deterioration or intermittent hum and noise problems. In fact,
the solid state circuits have proven so reliable that the entire stereo system should be checked
before bringing the SG520 into the shop. Table 4-1 gives some hints for troubleshooting the 'B
system. These checks require only a few minutes to perform. r
4.2 AMPLIFIER DISASSEMBLY AND SERVICE i
After isolating any malfunctions to either of the amplifier circuit boards, disassemble the unit
as explained below. Note that the channel A amplifier is on top (figure 4-1) and may not have
to be removed.
4.2.1 Top and Bottom Cover Removal a
Step 1. Using #2 (or #1) Phillips screwdriver, remove five screws which secure top cover to
cabinet. Lift cover away. a
Step 2. Repeat for bottom cover. The unit is now accessible for service to the units shown in ey
figure
4-1.
:
TABLE
4-1
TROUBLESHOOTING
Trouble Symptom Remedies
A. Entire graphic 1. Check fuse under hinged panel. Replace with correct rating,
controller inoperative. slo-blo.
Pushbuttons do not 2. Suspect wiring of power switch and power transformer.
glow. 3. Export model: check voltage selection.
B. Pushbuttons glow, 1. Press stereo (S) MODE pushbutton and set TEST TONE switch
but unit has no pro- ON. Is 1000-cycle tone heard? If not, refer to symptom C. If only
gram output on either in one channel, trouble is in line amplifier portion of faulty channel.
channel (no noise is If tone is heard in both channels, refer to symptom D.
heard with maximum
VOLUME on MIC
channel).
Faulty —-21-volt power supply.
Faulty line amplifier in both channels.
Faulty MODE switch or wiring.
Faulty low-level amplifiers plus faulty oscillator (symptom E).
C. Pushbuttons glow,
but unit has neither
program audio or test
tone output.
Pina tat
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