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Engineer: 
MikeCosta
Technical Robert
Writer: Smith
FRACAS
172
Number:
Product: |
AmPlusTM 50 and 100 amplifiers |
Bose Part Number: 194101-B1 |
Subject: |
Volume ramp-up |
|
Effective Date: 6/98 |
Serial Numbers: Affected Units |
|
Symptom: Volume ramps up to full when the amplifier is powered down.
Cause: The volume control voltage on the voltage controlled amplifiers discharges before they have powered down in the circuit.
Solution: Install a 22uF, 50V electrolytic cap and a 1N4148 diode in the locations shown in the figures, and replace R200 and R220 with 15.8k Ohm resistors.
Details: Note: This modification is only required for AmPlus 50 and 100 amplifiers that are exhibiting the symptom shown above. In many installations this modification will not be necessary.
Note: Refer to the AmPlus 50 and 100 Business Music Amplifier service manual, part number 194101, for the following procedures.
Note: If you are modifying an AmPlus 50 amplifier, you need only perform the operations that apply to channel 1.
1.Remove the AmPlus amplifier top cover using procedure 1.
2.Remove the Main PCB assembly from the chassis using procedure 3.
3.Using a multimeter, locate the tracks that run from S5, across the Main PCB, to the via holes shown in Figure 1. These via holes correspond to M32 and M33 as shown on Sheet 1 of the service manual schematic diagram, and as silkscreened on the bottom of the Main PCB near S5.
http://serviceops.bose.com/index.htm (1 of 4)6/26/2010 12:22:25 AM
Bose® Service Repair Information
4.Remove the solder in the via holes shown, so that you can install the cap leads through them.
5.Using an ExactoTM knife, cut the trace on the bottom of the PCB that runs between the via hole to M33 and the main portion of the trace (see Figure 2). Be sure to leave the portion of the trace that runs to R200 intact. Verify that the trace is open using the multimeter.
6.Cut the trace that runs from the via hole to M32 and the collector of Q221 (see Figure 2). Be sure to leave the portion of the trace that runs to R220 intact. Verify that the trace is open using the meter.
7.Install the 22uF, 50V caps (Bose P/N: 137126-220) into the locations shown. Be sure that the negative lead is installed into the via hole for M32 or M33 as applicable.
8.Pre-form the 1N4148 diode (Bose P/N: 121501) leads as shown in Figure 2, and solder them in place as shown.
9.Replace R200 and R220 with 15.8k Ohm resistors (Bose P/N: 1336251582). This is to keep the loading on the circuitry the same as before modification.
10.Replace the Main PCB assembly into the amplifier chassis using procedure 4.
11.Replace the AmPlus amplifier top cover using procedure 2.
http://serviceops.bose.com/index.htm (2 of 4)6/26/2010 12:22:25 AM
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