JBL SG520 - Page 21

Trouble Symptom Remedies
A. Low 1-kc tone 1. Faulty coupling capacitor C404 or bypass capacitor C401. Check
amplitude (at input and replace if necessary.
to Q104).
B. No Oscillation. 1. Faulty transistor, Q401.
2. Open.capacitors C402 or C403.
3. No power. Make certain that —-21-volt power is supplied through
TEST switch contacts.
4. Open potentiometer R504.
5. As a last resort, suspect toroid L401 and check it on a Q meter
*or inductance bridge. If it has a shorted turn, the inductance and
Q will be low.
4.4 POWER SUPPLY SERVICE
To test the power supply, lay the SG520 chassis on its top. All components of the power supply
are readily accessible and are shown in figure 4-4. With power applied, measure all of the volt-
ages and compare them with the voltages shown in figure 4-4.
4.5 FRONT PANEL DISASSEMBLY
To remove the front panel, lay the SG520 chassis on its side and remove three screws on each
side of the panel (at rear of trim strip). Carefully lower the front panel to a near horizontal posi-
tion. The wires are not jong enough to allow the panel to be pulled clear of the chassis so all
panel repairs must be carefully made to prevent wire breakage. Pane! components are identified
in figures 4-5, 4-6, 4-7.
4.6 PUSHBUTTON SERVICE
The pushbutton banks can be subject to intermittent contact closure. This condition may be due
to either (1) an accumulation of oxide on the contacts or (2), loss of contact wiping pressure.
To correct condition (1), spray on a contact cleaner such as G-C type 8666 Spra-Kleen. To in-
crease wiping pressure, use a pair of needle-nose pliers to bend the stationary contacts together.
This can be done by twisting each contact half inward.
4.7 Replacement of Resistors and Capacitors.—Resistors and capacitors on the circuit boards must be
replaced as follows:
Step
1.
When
a
component
is
suspected,
clip
one
of
the
leads
at
midpoint
to
remove
it
from
the
circuit.
Step 2. Check the component.
Step 3. If faulty, clip the other component lead at midpoint.
Step 4. Loop the new component leads over the old stub leader and solder.
CAUTION
DO NOT APPLY HEAT DIRECTLY TO THE PRINTED CIRCUIT BOARD. THIS MAY
CAUSE CIRCUIT FOIL TO LIFT FROM THE BOARD.
F22 RELAY—CIRCUIT DESCRIPTION AND OPERATION
20
The DC activating signal for the F22 relay is applied between the chassis ground of the SG520
preamplifier and the shield connection of the channel A output cable. Although the shield of
the channel A cable operates at ground potential for the AC signal, a DC potential of about
+20 volts between the CHA shield and the chassis ground. The polarity of the DC signal deter-
mines the mode of operation of the two relays.
The AC power relay, Ryl, is connected through a full wave bridge, Dz, so that a DC signal of
either polarity will energize the relay. Th 500 MFD capacitor, Cl, supplies enough delay so
that any momentary interruption in voltage as the polarity is reversed will not cause the relay
to drop out.
The stereo balance relay is connected through a single diode, D1, so that it is polarity-sensitive.
The relay operates only if there is a positive DC voltage at the channel A ground connection.