JBL SG520 - Bottom View

FIGURE 4-2 BOTTOM VIEW
3. If program source is at fault, look for wrong stylus pressure,
improper tape player head alignment or improper multiplex.
F-M tuning.
G. Hum in system. 1. Check interconnecting cables for proper shield bonding to plugs.
. 2. Connect #18 ground wire between the amplifer GROUND post
and the system tape deck or turntable motor frame..
3. Move SG520 away from power amp, which may have stray
magnetic field.
4. Check SW307, F-22 switch and fuse F301.
4. Check —21-volt regulator.
5. Suspect ripple in amplifier power supply due to faulty electrolytic
filter capacitor. ;
6. If hum still exists in a complex system installation, connect the
system GROUND to a true earth ground (cold water pipe, etc.).
4.2.2 Channel A Service.—The channel A amplifier circuit board is accessible from the top of the cabi-
net. To test channel A, proceed as follows:
Step 1. Connect an audio signal generator to the A PHONO 1 jack. Set the level to approxi-
mately 10.0 millivolts @ 1 kc.
Step 2. Press the PH1 button and apply power.
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