JBL BASS 20 Powered Subwoofer – Circuit diagram – Exploded view - Modifications
The Bass20 powered subwoofer is part
of the SCS120 or MUSIC 20 System. User friendly “auto on” circuit.
Signal sensing automatically turns the subwoofer on so you don't have
to; the subwoofer is meant to be left “on” without continual use of the
power switch.
Low Frequency Woofer (2) . . . . 8 1/2"
Crossover Frequency . . . . . . . . 120Hz
Frequency Response . . . . . . . . 40Hz - 120Hz (+/-3dB)
External Dimensions (mms)
Height . . . . . . . . . . 451
Width . . . . . . . . . . .254
Depth . . . . . . . . . . .610
Weight. . . . . . . . . . 42lbs (19.1kg)
Exploded view
Circuit Diagram (Schematic)
Amplifier
assemblies with serial numbers less than 11757 should be modified for safe
operation. Examine amplifier as follows (serial #11756 or lower) to determine
if a factory modification has taken place:
1. Unplug and remove amp assembly; (12) #8 x 3/4" screws.
2. Lift and examine the main (large) amp board, solder (green) side of the board.
3. Look for the presence of two large resistors (1k, 3 Watt or 470 ohm, 1Watt) directly underneath the two large filter caps on the opposite side of the board.
4. If these resistors are present, the unit has been modified.
5. If they are missing, the unit should be modified before future use, regardless of what the initial service concern was. To continue without modification may result in a hazardous condition.
1. Unplug and remove amp assembly; (12) #8 x 3/4" screws.
2. Lift and examine the main (large) amp board, solder (green) side of the board.
3. Look for the presence of two large resistors (1k, 3 Watt or 470 ohm, 1Watt) directly underneath the two large filter caps on the opposite side of the board.
4. If these resistors are present, the unit has been modified.
5. If they are missing, the unit should be modified before future use, regardless of what the initial service concern was. To continue without modification may result in a hazardous condition.
MODIFICATION
1. Cut tie wraps to bundled wire assemblies for easier access. Circuit board does not have to be removed from heatsink.
2. R36: See drawing: cut trace connecting Q4 collector and main C1 (+) pad.
3. Create solderable surface on PCB, (on Q4 side of cut) by scraping the green mask from board with Exacto knife or similar tool.
4. Attach R36, 470 1W resistor (JBL #RS2180) to new surface and cathode side of CR1. Resistor should "stand off" from PCB; secure with RTV or epoxy if desired.
5. R41: See drawing; cut trace connecting Q1 collector and main C2 (-) pad.
6. Create solderable surface on PCB, (just above R49) by scaping the green mask from the board with Exacto knife or similar tool.
7. Attach R41, 470W 1W resistor (JBL #RS2180) to new surface and C1 (-) pad. Resistor should "stand off" from PCB; secure with RTV or epoxy if desired.
8. Retie cables with tie wraps.
9. Cut and insert two in-line fuse holders (JBL #155020A) with 4A fuses (JBL #312004); one in each positive wire leading to woofer terminal, close to the amplifier assembly.
10. Replace amp assembly in cabinet
1. Cut tie wraps to bundled wire assemblies for easier access. Circuit board does not have to be removed from heatsink.
2. R36: See drawing: cut trace connecting Q4 collector and main C1 (+) pad.
3. Create solderable surface on PCB, (on Q4 side of cut) by scraping the green mask from board with Exacto knife or similar tool.
4. Attach R36, 470 1W resistor (JBL #RS2180) to new surface and cathode side of CR1. Resistor should "stand off" from PCB; secure with RTV or epoxy if desired.
5. R41: See drawing; cut trace connecting Q1 collector and main C2 (-) pad.
6. Create solderable surface on PCB, (just above R49) by scaping the green mask from the board with Exacto knife or similar tool.
7. Attach R41, 470W 1W resistor (JBL #RS2180) to new surface and C1 (-) pad. Resistor should "stand off" from PCB; secure with RTV or epoxy if desired.
8. Retie cables with tie wraps.
9. Cut and insert two in-line fuse holders (JBL #155020A) with 4A fuses (JBL #312004); one in each positive wire leading to woofer terminal, close to the amplifier assembly.
10. Replace amp assembly in cabinet
