JBL Incorporated 250 Crossways Park Dr. Woodbury, New York 11797 (516) 496-3400
Service Bulletin JBL9901 - January 1999
Warranty labor rate: MINOR repair
All JBL Service Centers
Model: Sub125 (SCS125 Powered Subwoofer)
Subject: Amplifier plate overheating; Blown fuses
In the event you receive a Sub125 (SCS125 Powered Subwoofer) with the complaint “amplifier
plate is getting too hot”, or “the unit is blowing fuses”, perform the necessary steps listed below
to change the bias current of the amplifier. This procedure should be performed on all Sub125
amplifiers being serviced for any reason:
1. Set the subwoofer cabinet on a padded surface with amplifier facing upwards.
2. Remove the (14) Philips screws holding the amplifier and inner plastic cover to the cabinet. Four of these 14
screws are in each corner, slightly larger and just inside the smaller corner screws.
3. Remove amplifier assembly carefully . Do not pull the amplifier out of the cabinet too much; it can disconnect the
speaker wires. (The amp assembly can be laid on one corner of the cabinet for the remainder of the procedure
without disconnecting the speaker wires).
4. Carefully pry plastic cover off amplifier assembly. If you are too aggressive Molex connector P104 may be
damaged; speaker wire inside the cover has little extra length. Lift the left side of the cover first to avoid
disconnecting the cable at P104; this connector is located on the right side of the plate inside the cover.
5. Once the cover is pulled back, locate VR101. See Figure 1.
6. Using a thin Phillips screwdriver 5 or more inches long, set trim pot VR101 to Full CCW position (Minimum level).
Be gentle to avoid damage to the part.
7. If necessary, replace blown line fuse with h/k part# FS1067, (2A Slo-Blo 5 x 20mm GMC type).
8. Replace the amplifier in the cabinet; replace all (14) screws to secure the amplifier plate and plastic cover.*
9. Functional Test - Set the LEVEL control to minimum (Full CCW). Connect an AC Wattmeter to the AC input line
and turn the unit ON; maximum AC measured power should not be more than 12.5 watts.
10. Connect music signal from a CD source and increase volume to a medium level to verify sound from the
subwoofer. Do not increase volume to a point where the unit is driven into clipping. No evidence of distortion
and/or rattle inside the box should occur.
11. Conduct warm-up for 10 minutes; turn LEVEL control Full CCW or remove signal. Input power as measured by AC
Power meter should not exceed 13 watts. If value exceeds this, re-check pot. VR101 position if necessary.
12. Add an adhesive label (dot) to the amplifier plate with the letter “a” written on it; if the customer’s outer carton is
available, add an “a” with a marker to the “SCS125” printing on the carton in all places where it appears.
Orange Label on
amplifier plate: This
area is designed to
become quite warm
during normal operation
* Alternately, the amplifier may be driven out of
the cabinet by attaching a 4 ohm 25 watt load
resistor to the output terminals, and using a 50
Hz sine wave input signal adjusted to achieve 7
volts at the load resistor for the warm-up test.