MOTU 2408 mk3 blown capacitor

[kosty]

Hey guys !

I am trying to fix a MOTU 2408 mk3 that has been powered on with 230V while switched to 110V.
The device wasn't powering up so I opened the lid to check for blown fuses or other components.
There was no burn marks on the PCB (top and bottom side) or no blown capacitors or resistors after visual inspection.
I deduced that the power transformer might have taken damage since the fuse was still good.
I reassembled the device and gave it one more try. You never know....

Luckily and I really have no idea why, but the device powered on and seemed to work !
I left the top lid open and the interface powered on to see if anything started burning.

After 5 min the large 1500uF cap exploded. I guess thats because of overvoltage.
edit: It's a 15000uF / 10V cap.

My questions to you now :

- Is it possible that the cap was already damaged when the device was powered with the wrong voltage and only exploded now because of it's precondition ? In that case I can try to replace the cap.

- Can the transformer be damaged when fed with wrong voltage ?

- Why didn't the slow blow 250 mA fuse blow ?

Hope you can give me some assistance
Thanks !

 

[mikeyB]

Which cap is this? - i've not been in a 2408 but i'm sure it'll have a 5V rail for dsp and logic and a +/- 15 to 18v for audio.
If you can - isolate the psu output from the cct - check the psu output is good (with new cap) and check the cct resistance V to ground.
Let me know - take some pics - whats the voltage rating of the blown cap? If it's low - like 16V then i'm sure it be the logic rail - but 1500uF seems stingy for logic current requirements/smoothing. If it's like 25-35 or higher it probably is one of the audio rails.
Either way - looks like overvoltage has slapped it!
NOTE - if it is one of the audio rails - then isolate BOTH +/- rails before testing!

 

[CJ]

failure mode for pwr transformers is usually a 15 minute meltdown followed by smoke and burned up insulation, so if it is still working, there is no reason to worry.

they saturate at higher voltages which increases current thru the wires, then it becomes a small space heater, but it takes quite a while for self destruction,

they hi-pot the pri and sec to 1500 volts usually, so 120 to 240 is a walk in the park as far as punching thru insulation.

you established a leakage point between foils on that cap, usually they self repair, but if it a big hole, then you have a resistor inside the can which causes heat which causes fluid expansion in a sealed container which causes an explosion.

they pick fuse size by shorting the output leads of the transformer and measuring the current thru the pri, then they pick a fuse at maybe half of this shorted condition current value. since normal current will be 10 times lower than the short circuit current (in a well designed transformer), the fuse will not pop. when you applied 220, there was no short yet and the cap leaking current was way under the value needed to trip the fuse.

 

[kosty]

Which cap is this? - i've not been in a 2408 but i'm sure it'll have a 5V rail for dsp and logic and a +/- 15 to 18v for audio.
If you can - isolate the psu output from the cct - check the psu output is good (with new cap) and check the cct resistance V to ground.
Let me know - take some pics - whats the voltage rating of the blown cap? If it's low - like 16V then i'm sure it be the logic rail - but 1500uF seems stingy for logic current requirements/smoothing. If it's like 25-35 or higher it probably is one of the audio rails.
Either way - looks like overvoltage has slapped it!
NOTE - if it is one of the audio rails - then isolate BOTH +/- rails before testing!

Thanks for your help. I made a mistake in my posting before. The specs for the blown cap are 15000uF / 10V.
I'm not familiar with your abreviation "cct". Does it have something to do with center tap of the power supply ?
I will get the measurements as soon as I replaced the cap ! 

failure mode for pwr transformers is usually a 15 minute meltdown followed by smoke and burned up insulation, so if it is still working, there is no reason to worry.

they saturate at higher voltages which increases current thru the wires, then it becomes a small space heater, but it takes quite a while for self destruction,

they hi-pot the pri and sec to 1500 volts usually, so 120 to 240 is a walk in the park as far as punching thru insulation.

you established a leakage point between foils on that cap, usually they self repair, but if it a big hole, then you have a resistor inside the can which causes heat which causes fluid expansion in a sealed container which causes an explosion.

they pick fuse size by shorting the output leads of the transformer and measuring the current thru the pri, then they pick a fuse at maybe half of this shorted condition current value. since normal current will be 10 times lower than the short circuit current (in a well designed transformer), the fuse will not pop. when you applied 220, there was no short yet and the cap leaking current was way under the value needed to trip the fuse.

Thank you very much for these information CJ ! I'm happy that the transformer is ok Will try to replace the cap and see if I can close this thread

 

[mikeyB]

Hi Kosty - yeah - 15000uF 10V is definitely the logic/dsp supply.
cct is abbreviation for circuit.
If you can isolate the transformer output - please do and measure the voltage - hopefully the transformer hasn't broke down and passing mains to the output! Before you power the circuit - check with continuity meter for a short between the primary (230V) and the secondary side(low voltage) NOTE - UNIT SWITCHED OFF!!!
Replace the cap and check the voltage of the regulator.
Check voltage from the transformer (ac) at the bridge rectifier input, then the bridge output -then the regulator output.
Be quick to measure - switch off between measurements to give you time to adjust your probes/meter scale.
If luck is on your side - the regulator and the dsp may have survived the overvoltage!
Good luck and fortune!!