Mordaunt Short ms309w Thunder rumble Repair attempt

Blacksubaru

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Hi all, I am a newbie on avforums, I have found some good advice on this forum regarding my Mordaunt Short Ms309w.
I bought this Sub on ebay from someone who said it was all working fine etc just a little dusty etc........ I picked it up quite late at night so i did the ultimate wrong thing and did not test it with the seller and drove 100 miles home with the sub and when i got home the sub had no power 😫.
So i checked the fuse above the kettle power lead socket at the back of the sub and it had blown so i changed it and i had power, then i placed and input and had sound (what a room shaker that ms309w is) but after about 10 mins i started to hear a constant thunder like rumble noise which got louder with the volume and was present even without input. So done some research and turns out its those 2 x 10uf 100v Bi Polar capacitors on the small pcb inside. I will be attempting to repair this myself as i enjoy this kind of thing as im a plumber/electrician by trade so solder and wires isnt strange to me lol.
I will be uploading my step by steps incase anybody wants to try themself or has the same problem. Wish me luck

I bought the capacitors from RS Components at around £1.24 for 5 with free delivery (Bargain😎)
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So i replaced the 2 10uf 100v caps and the issue was still there and found another issue with the power relay, it kept switching on and of and blowing fuse so i cut the black insulation surround off to reveal relay and it was discolured from heat on a small section of relay
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Hi Blacksubaru, did you manage to resolve the issue you describe?

I have two MS309W unit and they have both failed with the same fault. Either unit might power up if you're lucky but then will tend to power off after a while. The power LED will remain lit but you can hear the relay click open and the woofer stops working after a while. The power switch is not in the auto-off position.

It's clear that the actual amp circuit works just fine. The issue is with whatever is allowing the speaker power relay to open. Having wasted time pondering the circuit diagram and tracing the safety circuit to identify the fault, I have pretty much concluded that neither unit is worth more time and should be scrapped. The number of faulty units in eBay support this idea. I have not identified a suitable replacement amplifier plate either.

I'm wondering how you got on with yours or is it just gathering dust?

Rick.
 
Hi @rickspark ,
Yeah it did fix with no rumble noise worked perfectly until today (23 jan 2021) my missus left the sub on since christmas i believe it may have overheated or the 2 large capacitors need changing, i will do another tear down and fix and post up my findings.

Nate
 
I have a problem with my ms3339 it won't power up at all would you have any suggestions please I've seen a few posts about fuses above the kettle lead but I can't see one
 

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Hi supergav2021 Check below the kettle power lead a small panel with writing on it pop it out fuse is inside
 

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