How to Save 200 Quid (or Repairing Standby Fault on Mordaunt Short MSW20 Subwoofer)

Having moved three times in two years I have only just last week bothered to set up my hifi for the first time since 2010. Unfortunately, this period in storage and bouncing around in removal vans doesn't seem to have been good for my decade-old system and amongst other issues I found that my MSW20 sub woofer had stopped working. I didn't fancy spending hundreds of pounds replacing kit I was perfectly happy with so I decided to have a go at fixing it myself, despite component-level fault finding not being a forte of mine.

I wasn't sure whether the problem lay with my surround amp (which was also suffering from other faults), the line cable, or my subwoofer. I thought I had proved the sub because when I connected the line in cable with the sub's volume on full while music was playing from the surround amp, it gave a good "thump" from the small surge generated on connection. This was misleading. After confirming the amp line out and the cable with my multimeter (volts fluctuating during music play) I came across this post at AV Forums and realised that my sub appeared to be suffering from the exact same problem - it was stuck in standby.

Dismantling the sub was a little fiddly, but as always, it's easy when you know how; I will explain that in this post. Finding the fault was a LOT harder and, frankly, the fault was discovered more by luck than anything, especially as there were no obvious signs of damage to any of the components.

Idiot Disclaimer: It should go without saying (as should most safety warnings) that you shouldn't attempt the below unless you are confident with electronics, electrical safety, and safe use of soldering irons. You do this work entirely at your own risk and if you break anything, hurt yourself or someone else, set anything on fire, or generally do anything stupid, you're the one responsible, not me or anyone else. If you're not sure then seek professional advice, etc etc.

To remove the woofer amplifier, first switch it off, remove the power cable and wait a while until the capacitors inside have discharged. Then unscrew the outer ten screws from the back panel:

Gently begin to remove the plastic box containing the electronics. Before removing it entirely you will need to put your hand in the speaker box and disconnect the red and black speaker wires by unplugging two spade connectors at the back of the speaker. Wobble them while pulling them gently but firmly and they should come free.

Then remove the next four screws in:

Now, gently remove the plastic housing from the metal chassis with a flat-head screwdriver. I'm not sure if they're glued together or if the rubber has just gone sticky with age but they will come apart with a bit of persistence.

Unclip and unplug the speaker wire from the PCB so that you can completely separate the two parts of the housing without breaking the glue sealing the hole for the speaker cable. After you've done this be careful where you rest the metal chassis as the edges are sharp and scratchy. (Antique wooden tables are best avoided - I'm still in the doghouse for that one!!)

This is where things got interesting. After a lot of staring blankly at the circuitry, some further fruitless research online and some poking around with the voltmeter while the circuit was powered resulting in a massive spark when my probe slipped while testing the power supply, I nearly gave up. Fortunately, my stepfather was visiting so we decided to have a bit of fun with some father-son teamwork. We identified an area of PCB discoloured by excessive heat as a likely candidate for the fault. We didn't think the fault lay with a capacitor as they usually show visual signs of damage and we thought a resistor fault was unlikely as they rarely go faulty at all and, when they do, are likely to show some sort of burning damage. My dad therefore took an educated guess that we should begin by investigating the transistors, as they both don't tend to show visual signs of being faulty and are capable of generating sufficiently large amounts of heat to create the PCB discolouration we could see. We needed (or thought we needed) to find out the exact type of transistors used, but couldn't see the markings - so I used my iPhone to get the camera where I couldn't get my eye. Note how the plastic of the PCB in the surrounding area is darkened:

We then wanted to see behind the nearest transistor (circled red in the above image) to see the markings on the second one, so my dad bent the near transistor a bit to move it out of the way. While he was doing this he realised that it was wobbly - and, sure enough, the solder joints had cracked.

The solder joint in question is here:

It's a little hard to demonstrate with photos so here's a video of the solder joint wobbling. Make sure you switch to HD so you can see it properly:

I'm not sure whether it is only this particular transistor that suffers from cracked solder joints or whether you might have a problem with the neighbouring transistor or one elsewhere, so you'll need to ensure that the fault does indeed lie here.

Also, take care when re-soldering the joints - my first attempt resulted in a flawed join that cracked again as soon as I touched the transistor, and I've had professional training and experience at this sort of thing. It may be that the surface of the solder point on the PCB is flawed or damaged with heat, or it may be that the transistor acts as a heatsink making it harder to get the solder sufficiently heated through for a good join. Either way, your first join of the three is likely to be a bad one as it's pretty much impossible to stop the transistor moving at all while the solder cools - so once you've soldered all three joins go back and re-do the first one. When soldering, make sure you apply enough heat to thoroughly melt all the solder in the join without over-heating it. As always, don't forget to add a touch of fresh solder just before you remove the iron to prevent a dry (cloudy) joint forming.

(If you need help with good soldering practice I'm sure there are tutorials online somewhere).

I hope this post is helpful. Please comment if you've anything to add or just if this has helped you fix your sub. I was pleased to have saved the £200 cost of a replacement - I'll be even more chuffed to know if, by taking the time to write this, I've saved you some cash as well as helping to avoid perfectly good hifi gear going into the skip!