BOSE 181812 B2 Schematic
181812-B21800V AMPLIFIER
R4 Failing
Symptom: 1800V blows fuses.
Reason: R4, which is located on power supply PCB, was stressed to failure.
Action: Replace R4 with a 1/2 W carbon composition resistor.
1800V, 220V amplifiers have been experiencing failures due to R4, which is located on the
power supply PCB. Symptoms include the amplifier's fuse blowing, the mains blowing a
fuse, or excessive amplifier current draw.
When a 220V, 1800V comes in for service, R4 should be changed from an 18Ω, 5%, 1/2W
carbon film resistor to an 18Ω, 5%, 1/2W carbon composition resistor. Contact Bose corporation to order the carbon composition resistor (part number 187934-18R0). Refer to the
figures on the back of this page for the location of R4.
It is possible to replace this resistor without removing the PCB. Refer to the 1800V service
manual, part number 181812, for disassembly procedures. Refer to the procedures on the
back of this page for replacing R4.
Unplug the unit and remove the top cover. Unplug the spade connector WL7. Measure the
resistance of R4. If R4 measures 18ý ±5%, changing R4 may not fix the problem. If R4
measures open, out of tolerance or is a carbon film resistor (light brown body color), replace it with P/N 187934-18R0 carbon composition resistor (dark brown body color) as
described below.
1. De-solder the damaged R4 and discard it. Be sure to remove all the solder from the
PCB holes or the new resistor will not fit.
2. Dress the leads of R4 so that the body of R4 is 1/8" off of the surface of the PCB.
Ensure that the leads will not contact the heat sink behind the back side of the PCB.
3. Solder the new component in place and check to ensure that the leads of R4 are not
contacting the heat sink.
4. Replace the spade connector WL7 and the top cover. Replace the fuse if necessary.
Test the amplifier according to the test procedures in the 1800V Service Manual.
Location of the Power Supply PCB and R4
R101
R102
J1
6
7
8
9
10
11
2
1
5
3
4
LINE
LOAD
BLU
BRN
J1
Power Supply
PCB
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