Bose L1 classic model tower speaker repair.

[Yamin]

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 minimum 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.

I have tried attaching the pdf of the service manual in this post but it doesn't seem to upload. Here is the link for the file its in the first post:
 https://music-electronics-forum.com/showthread.php?t=41558


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/

[Renate]

Well, that's an unusual topology on the power supply.
You've got 4 transistors and 2 freewheeling diodes, any of which could blow out the rest.
Did you check all six?

I really wouldn't run anything on a light bulb. I hope that you took it out of circuit.
Ok, it's fine for a quick check to see if something is totally shorted.
The problem is, everything today is constant power, decrease the voltage (drop across bulb) and the current (tries to) go up.
That kind of thing was ok with old tube equipment with unregulated power supplies.

[Yamin]

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

[Renate]

The zeners Z501, Z502, Z503, Z504 are only there to make sure that you don't blow up the gates on the MOSFETS.
In normal operation they are effectively an open circuit.
They only way that they would blow up dramatically would be if there were an internal or external short between drain and gate.

Did you check D503 and D504 too? (With the amplifier disconnected).

I would have thought that D502 would be able to take anything that the MOSFETS could dump into the transformer.
But you seem to be saying that it takes a while for them to blow?
If they are overheating and blowing there must be some serious current.

I would try to test things using the shortest possible power on to get a reading off it.
Check the voltage on C516 on a good supply with no amplifier connected.
Check it on your bad supply. Don't go for accuracy, just really quickly.

Did you say already whether the bad supply actually generates +/- 27V?

[Yamin]

Thanks @Renate

I did check D503 and D504 both of them were ok.

D502 blows if I keep the power on, even with the 100W bulb connected. The unit cycles for sometime and then the D502 blows at that instance the bulb also remains on at bull brightness.

I wasn't able to check the outputs of the power supply for the +/- 27V as the unit shuts down too soon. However I checked the outputs of the other power supply with the faulty one removed and they register as around 25V but its very unstable. I am assuming that's because all the power supply aren't connected.

What else should I check? :S

[Renate]

Is D502 hot after it blows up?
Is it the voltage or the current?
Are you sure that your replacements are the right ones?
Can you repeatedly cycle without blowing if you do it for short enough each time?

Hook up a 5K resistor (ok, 4.7k, 5.1k, whatever) to a LED on each of the 27V outputs.
Do the LEDs go on?

It could be that the transformer is shorted (but I'd expect the MOSFETs to go first).
Check the transformer winding resistances against the good supplies.
Is the snubber resistor R511 ok?

[Yamin]

Yes the I replaced with the exact ones. I have included the picture of the last time it has blown. Notice the carbonized PCB  at where the heat sink is suppose to be soldered. Is this a problem?

Yes I could repeatedly cycle without blowing the diode if its done for short time. This time  Q501 and Q502 drain to source is  shorted. I would have to replace them too.

Nice idea with  the leds, I shall try it. I will have a look at the transformer wingdings too.

The resistor R511 checks out ok. I would like to ask what the use of this snubber resistor?

Thanks so much for the help

[Renate]

Hmm, this is all so wrong.
Are you triple sure that you have D502 in the right direction?
If it were backwards, it would blow sky high and also take out Q501, Q502.

When you started out was it all so burnt?
Are you cleaning all that carbon burn out?

[Yamin]

Yes I did make sure and checked the direction before I powered on. I had taken pictures before I took them out and also checked against other supplies as well.

The very first time as I mentioned in my first post the zeners and the mosfets were faulty. I didn't realize then the diode was also shorted out. Its only when I was replacing the mosfets I noticed the shorted diode. I replaced it before powering up.

Yes I am cleaning out the carbon. I'll report back . Thanks.

[Dacke]

Just curious but did you check R501 and R502?  Possibly gate discharge resistors... I didn't see them mentioned.

[Yamin]

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
PS: R501 and R502 are okay

[andy2000]

A few shorted turns wouldn't change the DC resistance noticeably.  You could try swapping in one of the known good transformers.  It's unlikely to be the problem, but it would take something off the list. 

Since you have a good identical power supply, you might try comparing some resistance measurements between the two after you think you've replaced all the bad parts.  Any discrepancies likely means you've missed something. 

[Yamin]

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