Trying to revive a Bose L1 classic model tower speaker.

Thread Starter

Yami

Joined Jan 18, 2016
282
Hi guys, I have got a Bose L1 classic model in for repair. There was some saltwater splashed on to the unit. Previous tech had a look and gave up saying there was too much corrosion. I had a look inside and there was very mimum to no corrosion. I noticed that there was a short between two pins near a connector with carbonized PCB(CN120A Page #26 between pin 4 and 5). I used a dremel to remove the carbon and measured the resistance the short was now removed. I cleaned the connectors. This was on the front panel board, only place where the salt water could have effected.

After powering on, the unit keeps shutting down and then starts back up(keeps cycling) due to high current draw. I should mention I have got a 100W bulb in series with the input. I removed the power supplies leads to the amp section and only had the preamp/microcontroller board attached (the board which supposedly was splased with salt). All the voltages measurements taken seemed okay and the unit did not shut down. The current draw from the mains were a bit high still.

I had a look at the other power supply boards which powers up the amplifiers (page 20 of the pdf). All these were connected at the time just not connected to the amplifier section. I noticed charring of R507 and R508 both have failed open. Z501 and Z502 have shorted out. The two mosfets Q501 and Q502 have both failed open. (I have marked in the photo). What could be the cause of this? I checked the amplifier sections thinking something might have shorted out. Everything seemed to look in order. Could the high current draw be caused by these components being failed? I noticed that all the boards needs to be connected for stable voltages. I noticed by removing this power supply board the voltage ouputs of the aux power supply outs have gone slightly up.

I hope some one could help me understand this power supply, and what might have caused this components to fail. Any idea on what to look for next.

Thanks in advance.

p.s I came across this blog which has got some good pictures of the unit https://lensprojects.com/2016/08/25/bose-l1-repair/
 

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Thread Starter

Yami

Joined Jan 18, 2016
282
Update:
I replaced the blown out components and powered up the device.

When all the boards are connected (amplifier, power supplies etc). The units starts shutting down after turn on. I noticed that after a while D502 keeps shorting out. Changed it powered it up again and yet the same thing.
Then what I tried was replaced the shorted out diode powered the unit up but this time I didn't connect the amplifier board. The same thing happens the unit keeps shutting down. This time I turned off the unit before the diode shorted out. I removed this particular power rupply (there are 3 identical power supplies in this unit) and powered up the unit again. This time it didn't shut down.

I should also mention there was some sparked marks from the heat sink where the mosfets are mounted to the chassis of the unit.

After troubleshooting for sometime I came across bulged capacitors(but this was from other power supply boards). I couldn't find any obvious issues. After replacing these components I came across I checked each power supply separately and everything seemed to be ok. The output should be +/- 27V but it wasn't but I assume that's because the load wasn't connected.

Connected all the boards together - same thing! Unit keeps shutting down. So I assumed it might be the 100W bulb which I had in series with the unit. So I removed it placed the 10A fuse provided and powered it up. This time D502 blowed up! I replaced the fuse and took out the problematic power supply out. Powered on the unit, the current draw was unusually high I thought, so I turned of the unit. Turned it back on again this time it took out the NTC thermistor (page #19).

I am pretty much confused on what the issue could be. Why is it D502 which always blows, and could you please explain what the use of the zeners Z501 AND Z502.

Any more tips on this would be greatly appreciated
 

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Thread Starter

Yami

Joined Jan 18, 2016
282
Make sure there is a very high resistance between the ends of D502 and the supply safety earth.
I shall check this, also one other thing I remarked was the heat sink was connected to the positive end on the input supply :S . I am just waiting for the components I have ordered.
 

Thread Starter

Yami

Joined Jan 18, 2016
282
I have included a photo of the diode which has blown. Please note the carbonized PCB at where the heat sink is suppose to be soldered, could this be an issue? This time Q501 and Q502's drain to source has shorted out as well.
@AlbertHall yep the mica strips are intact.

Thanks again.
 

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Thread Starter

Yami

Joined Jan 18, 2016
282
I just figured out why D502 keeps shorting out. The moment the power is on Q501 and Q502's drain and source shorts out instantly. If I let the unit cycle for a couple of times then D502 blows up. I checked each power supply separately used a LED to check the outputs. This only happens with the faulty power supply. I assume the protection works as the supply shuts down.
I have checked ohms readings of the winding of the transformer against a good supply and it's identical.
What I can't figure out is why the mosfets keeps shorting out. Any idea on what to look for next?
Interesting fact - The heatsink which the mosfets are mounted on are actually used to connect the positive input to one of the transformers terminal. Eventhough there are mica strips there is a connection between the heatsink and the tabs of the mosfet. Which is the case with all the power supplies.
Any idea on what to look for next?
Thanks in advance
 

AlbertHall

Joined Jun 4, 2014
9,889
The heatsink which the mosfets are mounted on are actually used to connect the positive input to one of the transformers terminal.
Please clarify that.
By positive input do you mean the drain of the MOSFETs Q501/Q502?
That point should not be connected to the transformer at all.
 

Thread Starter

Yami

Joined Jan 18, 2016
282
@AlbertHall yes it turns out that it was two Mosfets drain(tab) connecting with the heat sink Q503 and Q504. But I can't figure out how this is happening. Its only when I put in the screw it shows a connection. The mica strip is intact, there were no rubber spacers used with the screws. I cross checked with the other power supplies and its only happening with this faulty one. The screws are quite tight, with the other power supplies. Finally what I did was I had the screws of the tab Q503 and Q504 slightly loose with the faulty supply.
The unit is working now with stable power supply.
I have attached a photo of the power supply, as you can see the heat sinks are connected to the positive input of the supply. Q501 and Q502's drains shows a connection with the heat sink but that's from the drain being tied to the positive supply and not from actually touching the heat sink.
I am still wondering how the two transistors are connecting with the heat sink because visually there is no connection.
Thanks
 

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