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Old 19th December 2011, 06:32 PM   #1
RS232 is offline RS232  United Kingdom
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Default Mission "M-Cube" help

Hi all
I have 1 Mission "M-Cube" subwoofer and I need some help...
The problem is that the primary switching mosfets (IRF740) gone up in smoke and I whant to fix it.
My problem is that there is some damage (melted film) on the transformer and I don't know if it is damage to the windings.
So before I smoke 2 new mosfets can someone give me some pointers ?
Personally I don't like switch mode power supplies in audio gear and I would prefer to build a normal one with a toroid but I dont know the voltage and VA output rating of the original supply.
Can someone help ?
The Mission site is nice but the contact page is not working.

Ric
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Old 21st December 2011, 01:29 PM   #2
Puffin is offline Puffin  United Kingdom
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Hi RS,

Could be a transformer malfunction. Pity you are not closer a I could have had a look at it.

Have a look at this link for some pointers.

Basic Amplifier Repair

Rob.
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Old 21st December 2011, 05:05 PM   #3
RS232 is offline RS232  United Kingdom
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Hi
Thanks for the answer
I'm very comfortable with electronics (I worked over 10 years with industrial electronics).
I only ask for help because I was hopping that someone allready fix one of these sub amps.
The other thing is I never seen a HF transformer fail in a power supply (and I repaired loads of then).
In my opinion the melted film on the transformer was due the close proximity of the transisfors to the transformer.
The 2 mosfets have a hole on them and there was lots of black scrap caused by smoke/sparks around the IRF740 / transformer and on the board.
Going to try to take some pictures for you to have a better idea.

Ric
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Old 23rd December 2011, 12:34 PM   #4
RS232 is offline RS232  United Kingdom
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Default M-Cube

Well this is the best I could do with my camera
As you can see the burn mark on the transformer was done from the out side by the mosfets when they went up in smoke.
Going to check if the gate resistor are damaged.

Ric
Attached Images
File Type: jpg fonte1.jpg (866.0 KB, 173 views)
File Type: jpg fonte2.jpg (490.3 KB, 154 views)
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Old 24th December 2011, 10:23 AM   #5
RS232 is offline RS232  United Kingdom
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Ok, small update
The thermistor (DSP104) in line with the 240V is blown/kaput (if you look closely you will see that there is a bit missing on the left side)
Gate resistors are fine and can't find more damage.
All point to a power line spike or someone put the selector for 120V.

Merry Christmas everybody
Ric
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Old 24th December 2011, 04:03 PM   #6
Puffin is offline Puffin  United Kingdom
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I have been looking at an SMPS for a friend with a very similar demise. There was an on-board fuse which blew but it has taken out the main chip and a thermistor.
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Old 5th February 2012, 01:43 PM   #7
RS232 is offline RS232  United Kingdom
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Ok
I have got all the parts together and hopping to fire it up today
Part count (damaged)
2 x IRF740
1 x DSP 104 10 Ohm 6Amp thermistor
1 x MUR420
2 x 235 Ohms 1% resistor
2 x 1n5345b 15V zener diode
Replaced the 235Ohm resistor with 240Ohms because I could not find the correct value but the schematic says 100Ohm...
Will report back as soon I put everything together
Finger crossed

Ric

Last edited by RS232; 5th February 2012 at 01:48 PM.
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Old 5th February 2012, 05:51 PM   #8
RS232 is offline RS232  United Kingdom
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IT WORKSSSSSS!!!!!!!!!!
I protected the damaged film on the transformer (picture on post 4) with a clear sealant used in aerospace.
I damaged a track but that was ease to fix.
If you are fixing one of these BASH power supplies please be carefull the heatsint on the primary side with the MosFets is live and there is no indication/warning of that in the board.
If someone needs the schematic for this amp (Mission M-Cube) the Klipsh RW10 is exactly the same and can be found on the net or send me a message.
Time to fix my XR-225 hibrid amp...

Ric

Last edited by RS232; 5th February 2012 at 05:55 PM.
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Old 30th October 2012, 11:46 AM   #9
john65b is offline john65b  United States
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Hey cool, the Martin Logan Dynamo (original) also has that same BASH power supply, and mine has one of the IRF740 reading resistance and the other completly open. I plan to replace both - is there a good substitute for these?

Looks like someone already replaced the two IRF740s, but it appeasr they did a crappy job as the mosfet tab was not isolated from the sink, and re-blew the mosfet(s).

You say the sink is live? But the two mosfets have isolator pads, just like yours shown...this sink should not be live, correct?

Anyway, I may take you up on the schematics...I love this site!
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Last edited by john65b; 30th October 2012 at 11:48 AM.
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Old 30th October 2012, 03:18 PM   #10
RS232 is offline RS232  United Kingdom
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Hi joh65
The laptop with the schematic is dead and I need a USB adapter to get the stuff from the hard drive.
Look on the net for the part number on the transformer you get more hits that way.
This was the way I came across the schematic for the RW-10.
If the mosfets IRF740 are dead you going to have to remove the small board near the heatsink (be careful mainboard is double sided) and measure the zeners and gate resistors.
Check the 2 big diodes after the transformer I had 1 shorted.
Good luck
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