If your Bose SoundTouch 300 soundbar has suddenly gone completely dead, no lights, no signs of life don’t panic just yet. This is a widespread issue, and the fault almost always points to the power supply board.
WARNING- this work requires knowledge of electricity, safety and the dangers presented on mains-powered equipment. DO NOT TAKE THIS ON if you don't have the basic knowledge on safety.
Over the years, I’ve repaired a few of these, and the pattern is always the same: the SMPS (switch-mode power supply) fails, usually taking out the UCC28630 driver IC, the main switching MOSFET, and a couple of nearby resistors in the process. Sometimes on the mainboard, there are 5V regulators that short out, easy to identify by part number ( they are not op-amps 😉). There are some SMD capacitors around the chips. If you find a short on any of those caps, you remove the IC and the short goes away... the chip is dead.
What actually happens
The SoundTouch 300 uses a fairly standard UCC28630 primary-side controller, a great little IC, but when it goes, it tends to go spectacularly. When the MOSFET shorts, it blows the start-up and current-sense resistors as well. This is a smart driver |IC with load regulation on the primary side, there is no secondary feedback, but this can create other issues. Please see below.
One resistor worth mentioning is the bigger SMD 0.2 Ω current-sense resistor sitting close to the MOSFET — this almost always goes open-circuit when the IC fails.
The good news is that the component values follow almost exactly the reference circuit from the UCC28630 datasheet, so once you identify the chip, you can cross-check the parts easily. Get the Datasheet, and towards the end, you will have a full power supply schematic as a typical application.
Step-by-step checklist
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Disconnect and pull out the PSU board.
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Check the main fuse, sometimes is fine, sometimes is blown, so don’t stop there.
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Test the primary MOSFET ; 99% of the time it’ll be shorted.
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Inspect the UCC28630 driver IC, if it’s cracked or has a hole in it, it’s done, but it can be done without any signs, if the MOSFET is shorted, the driver is probably gone
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Measure the resistors around the IC and MOSFET, especially that 0.2 Ω SMD resistor.
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Replace all damaged parts with quality components (not random eBay stuff, get your parts from Digi Key, Mouser, for example).
Use a bulb current limiter instead of the fuse. If you don't have one/don't know what it is, YouTube is full of examples and explanations, learn about it and build one. It will save you time and money.
Due to the power supply design and the primary regulation with no secondary feedback, this PS, after being repaired, might still not start properly, or it starts and turns off. One fix I found to work, is to add a small load resistor on the power supply output. I don't remember the value but a 1W resistor to load the power rail a bit without getting too hot. You can experiment with values, start high, like 1k and keep going down until the PS starts and remains on, and the resistor does not cook itself.
Don’t forget the 5V rail
When the PSU fails violently, it sometimes takes out the 5 V voltage regulators on the main logic board. Before you power it back up, check the 5 V line for shorts. A failed regulator can pull the rail down and make you think your power supply is still faulty.
Wrapping up
The Bose SoundTouch 300 power supply repair isn’t as scary as it looks. With the right parts, and a careful inspection, most dead units can be revived for very little cost.
If you enjoy repairing audio gear, this is a perfect example of how a bit of patience and some datasheet reading can save perfectly good equipment from the bin.
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