Bose Acoustimass 15

[kevinm34232]

Is anyone familiar with Bose systems, this is a subwoofer that is dead that I am trying to fix for a friend. I found two 20 ohm SMD resistors on the bottom of the board that were open. I replaced them with 2 from a scrap board and I heard it power up, but one resistor went open again. I'm not sure if it wattage was too small, or there is some other problem.

[kevinm34232]

Re: Bose Acoustimass 15

Also this leg of C23 connects to pin 1 of connector J2, which goes to large secondary transformer. If I connect the transformer to J2, all pins of J2 short to same leg of C23.

Does this mean the transformer is shorted??

[kevinm34232]

Re: Bose Acoustimass 15

Replaced the two 20 ohm resistors with one larger 10 ohm resistor fixed this.

[momaka]

Re: Bose Acoustimass 15

Replaced the two 20 ohm resistors with one larger 10 ohm resistor fixed this.

Interesting.
Looks like those two 20 Ohm resistors in parallelI are in series with C23. I wonder if C23 going bad/shorted. Failure mode of metal film caps is not always a short circuit (and is rare), but it does happen. Most ofthe circuitry in the area appears to be connected to line voltage, so it is possible that C23 could have gone bad.

It's a bit hard to understand what that circuit does, though. I can see line voltage going through the plug, then the switch, then the fuse, but after that it looses me. If C23 is used like a X2 safety-cap, it could fail due to a voltage spike on the line. And if C23 is used as a high impedance low-voltage signal source (read: cheap way to bring line voltage to IC supply voltage), then that could have caused it to fail as well. You might have to trace this on paper for us to get the fuller picture.

But I guess if it works now, no point in doing that, right .

[kevinm34232]

Re: Bose Acoustimass 15

Ya and it seems from what I could find, that this is a common issue, if it doesn't blow the fuse first.

[budm]

Re: Bose Acoustimass 15

Do you mean C24 (1.5uF) and the 10 Ohms R100 as shown on page 4 of the SCH?
That is the RC Snubber for the TRIAC which is used as A switch to turned on and supplies Voltage to the primary side of the transformer so it is powered up from STANDBY mode when audio signal is detected. That RC is large in capacitance because when the TRIAC is not on and the unit is in standby, the primary gets the current through the RC network just enough to run the standby/audio detection circuit.
The audio detector and rectifier (U9, U12) that convert AC to DC for the timing RC circuit R92/C22.
Seems to me that the power handling of the resistor is too small, the SCH is shown as 1/4 W, so when the resistor is open, there is no voltage applied to the primary side of the transformer to run the audio detector/Timer circuits to turn on the TRIAC.

[momaka]

Re: Bose Acoustimass 15

Oh, I see now.... O/P provided schematics

Yeah, looks like the primary side of the traffo is powered through that RC network (R100, C24) when in standby. Crappy design, IMO. I've seen a lot of double-insulated small kitchen appliances' logic boards use that RC kind of circuit with a Zener diode to bring AC voltage from the wall down to a lower voltage that the IC on the logic board can use. More often than not, the cap in the RC network either overheats and opens, or becomes shorted and then the Zener bites the dust.

But hey, it's cheap and it works! (for a little bit - just enough to pass warranty period)