Bass Box Only Test Procedures - Bose Loewe Concertos Acoustimass 61247 Service Manual

2.2 Check for AC mains voltage, 120 or 240 VAC at the transformer primary. If there is voltage at
the transformer primary, remove AC mains power and check the thermal fuse in the transformer
primary. Replace the transformer if the internal fuse is open.
If there is no voltage at the transformer primary, proceed as follows.
2.2.1 Check the fuse located on the PCB.
2.2.2 Ensure that the 18 conductor ribbon cable is fully seated.
2.2.3 With the line cord connected, connect the 10V turn-on signal to the 3.5 mm mono plug
located on the test cable. Check the voltage at U300 pin 1.
If it is ~1.25 VDC, U300 should be on, turning triac D302 on. Go to procedure 4.
If it is ~0.8 VDC, the opto-coupler U300 is crowbarred off keeping triac D302 off, which connects
AC mains voltage to the transformer primary. Disconnect power to the bass module and go to
procedure 3.
3. U300 Crowbarred Off, Triac D302 Off
Note: Refer to Figure 50. Protection Circuit Simplified Schematic Diagram in the Appendix for an
overview of the protection circuit.
3.1 If the bass module crowbars off (shuts off) immediately (<50 msec) after applying the 10V
turn-on signal, check the "protect" signal at J8 pin 6. If the voltage on J8 pin 6 is greater than
approximately +0.7 VDC, it will turn on Q202 and if the voltage is less than approximately -0.7V,
it will turn on Q203. Turning on either Q202 or Q203 causes Q202 and Q203 to latch and this will
activate the crowbar. If there is a fault condition at J8 pin 6, check the ±12 VDC, ±17 VDC and ±34
VDC power supplies. If the ±12 VDC power supply is not working and the ±34 VDC power supply
is working, check resistive fuses R714 and R713 located on the amplifier PCB. A failed component
could be causing one or more of these supplies to sag, causing the unit to crowbar off. If the DC
power supplies are working, continue onto step 3.2.
3.2 DC offset at the output of the bass amplifier J703 pin 2 (+) and pin 1 (-) will produce a fault
condition 'protect" voltage (greater than approximately ± 0.7 VDC) at J8 pin 6. Check the compo-
nents in the bass amplifier with an Ohmmeter.
3.3 If the bass module crowbars off after approximately 3 seconds, there could be DC offset on
one or more satellite outputs. The satellite outputs are summed through the "speaker output DC
offset" circuit (SD251571, grid location A7). The output of this circuit, P66, is sent to the
microcontroller U202 pin 25. In normal operation, the voltage at U202 pin 25 should be 2.5 VDC.
If the voltage strays more than 1 VDC from the normal 2.5 VDC, microcontroller U202 pulls pin 39
(P22) to ground (normally high impedance) pulling R308 to ground latching the circuit consisting of
Q302 and Q303 (SD251571, grid location B4). This latch circuit, when latched, will shut off the
opto-isolator U300, which will shut off the triac D302 disconnecting AC from the primary of the
transformer.
Note: U202 pin 26 is connected to U202 pin 39. The head unit does not send an "ON" command
to the bass module. The uC knows when the power has just come up by watching the status of the
10V turn-on voltage via pin 26, which is connected to the same circuit mentioned above that the
uC uses to shut down the triac.
3.4 If everything checks out up to this point and the bass module is not crowbarred off, then go to
procedure 4.

Bass Box Only Test Procedures

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