Read JBL BASS 20 Service Manual online
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Powered Subwoofer
BASS20
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.
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
Ω
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.
IMPORTANT SERVICE NOTE:
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