JBL SG520 - Page 15

Step 3. Using an accurate electronic ac voltmeter (HP Model 400 recommended), trace the
signal level at each stage of the amplifier. Refer to figure 4-9 which shows the component loca-
tions.
Step 4. Refer to the troubleshooting chart, table 4-1, when a signal fault is encountered.
Step 5. When a faulty component is found, refer to paragraph 4.4 for replacement recommen-
dations and to Section 5 for parts list.
4.2.3 Channel B Service.—-The channel B amplifier must be removed for service. Turn ac power off.
Proceed as follows:
Step 1. Using a #2 (or #1) Phillips screwdriver, remove six 6-32 screws which secure the chan-
nel B circuit board to its chassis.
Step 2. Lift three power leads (red, grey, black twisted together) out of the way. Then lift the
channel B circuit board up.
Step 3. Tilt the circuit board up to a vertical position and tuck a few strips of paper under
the edge to insulate the edge connections from the chassis.
Step 4. In this position, apply power and test the circuits as explained in paragraph 4.2.2, for
the A channel. Refer to table 4-2 for troubleshooting.
4.3 TEST TONE OSCILLATOR DISASSEMBLY AND SERVICE
When the test tone oscillator is known to be inoperative, service it as follows (it is not neces-
sary to remove the front panel):
Step 1. Lay the chassis on its side panel, with the oscillator board components facing up.
Step 2. Using a #2 (or #1) Phillips screwdriver, remove four 6-32 screws which secure the oscil-
lator circuit board to its subpanel.
Step 3. Lift the circuit board out into the clear. The leads are long enough to allow the board to
be placed on the bench for testing.
Step 4. Refer to the troubleshooting suggestions in table 4-3, and to figure 4-3 for component
locations and test point.
NOTE lf the transistor and voltages appear OK, but the circuit does not oscillate, the trou-
ble is probably one of the three capacitors in the circuit.
FIGURE 4-3 TEST TONE OSCILLATOR
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