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OXAUDIO 12th January 2012 10:43 AM

Schematic diagram for Mordaunt-Short MS309-W
Can anyone supply, or tell me where I can get, a schematic diagram or Service manual for the above, please. A U.K. source for spares would also be helpful.
The 1.6AT fuse in the mains input blows in one I'm trying to repair, and none of the usual suspects(Rectifier, Filter Capacitors, etc.,)are faulty. Prior to the fuse blowing, the amp. would cut out if the volume was advanced more than about 60%, with only a 'signal' from my finger on the line level i/p present
Could the mains transformer have shorted turns??

jaycee 12th January 2012 12:00 PM

More likely output transistors failed. The back of the sub mentions "digital" so its possibly a class D amp.

OXAUDIO 12th January 2012 06:17 PM

Didn't think of that possibility, jaycee. Thanks for the tip. I'll check them ASAP. The O/P
Transistors are mounted on a Heatsink/PCB assembly attached to the main board so not too difficult to get to. Not sure what type they are, though. Hope they're easily sourced.

SonicAssault 25th July 2012 07:02 PM

Bit late I know, but I recently resolved "The amp cutting out if the volume is advanced above a certain level" problem on my own 309i.

Under the heavy duty heat shrink covering the power PCB which powers the audio input detection and the switching relay for the toroidal you’ll find a small transformer, and in close proximity four 85C electrolytic caps - two 1000uf 25v and two 22uf 16v.

Obviously the heat shrink covering the board is due to the board having live mains on it, but the problem having the board totally encased is the transformer, which is permanently powered when the unit is connected to the mains, runs incredibly hot (and I mean ‘incredibly’ hot) and appears to be cooking the electrolytic caps.

To solve the problem I replaced all four caps with 105C rated, and cut away a square section of the heat shrink on the component side. Once replaced, the heat shrink still entirely covers the printed side of the board, the transformer, and the mains powered components – but now there’s a gaping hole at the low voltage end of the board from just below the transformer so the caps are exposed and transformer can ‘breathe’.

From what I've discovered since my own started doing it a few months back this cutting out problem has been widely reported, so I hope this may help those who have had this problem - and not resorted to taking theirs to the tip.

As to the schematics. I too was unable to find any for the Mordaunt Short subs, but what I did discover is the Digital Amp Module (DS-150) used in the 409/309 subs (and possibly others) is also used by Harman Kardon and JBL and turns up in a number of subwoofer model ranges.

Here’s a link to the two service manuals I found with some useful info on the module:

JBL SCS150SI/160SI/180.6S Service Manual free download,schematics,datasheets,eeprom bins,pcb,repair info for test equipment and electronics
Harman/Kardon SUB500 Service Manual free download,schematics,datasheets,eeprom bins,pcb,repair info for test equipment and electronics

All the best.

OXAUDIO 25th August 2012 05:25 PM

I've only just seen your reply. As the MS309W is still in my 'jobs to do' pile, I will check this out. I've downloaded both of those manuals, so, if, as you say, the module is identical with that used by Mordaunt-Short, I should be able to repair the faulty Sub. (M-S incidentally will not supply any spares or service information-they refer you to their authorized service company-in the U.K. Richer Sounds) Having, some months ago, extracted the power PCB from the heatshrink sleeving, I will replace those capacitors as well as the two power Mosfets, now I know what they are. There were no markings at all on the two devices on the DS100 PCB, but I should be able to obtain IRF640 & IRF9640 from one of the UK-based component suppliers. If so, then hopefully the SUB will at last be functional again!
Incidentally, on the PSU diagram of the JBL SCS150SI, those Capacitors are shown as being 2 x 100uf 35v(C507 & C508), and 2 x 22uf 50v(C505 & C506)Just thought I'd mention that. I don't have the MS309W in front of me at the moment, but that power supply circuit should be similar.

SonicAssault 26th August 2012 05:20 PM

Too be honest, it wasn’t such a simple fix as my post suggests. I’m not an electronics expert, so was totally reliant on suggested advice I found on the net to the cutting out problem, and went through a raft of component replacements before resolving the problem – the power transistors, TL072 on the digital amp board, main power caps, speaker and power relays - none of which worked. Although, having said that, upgrading the original components with higher quality bits along the way has had some beneficial results to the sound now I’ve actually got it working again. :cool:

The other things you might want to check before reassembling.
Solder joints on the pots on the main board – mine were dry and cracked and the volume pot made a rumbling noise though the speaker when rotated.
The connector on the speaker cables – you might find wiggling it about causes the speaker to cut out. A quick spray of Deoxit would probably do the job, but as I’d already gone down the tweaking path whilst replacing the possibly suspect components, I decided to replace the original cables with two slightly longer unbroken runs of Furukawa I had laying around.

I should mention though, that even after addressing these specific issues, they both still failed to resolve the main cutting out problem.

OXAUDIO 27th August 2012 08:02 PM

Although I have been repairing audio gear for 40 years, and have a City & Guilds qualification, I'm not familiar with these Sub-Woofers. Sounds as if cutting out is a common fault, but finding the cause isn't easy. I may continue along the lines suggested, in the hope that i can fix the ****thing. The main power supply/output PCB had a substance sometimes referred to as 'Sony Bond' on it, which can seemingly become conductive when heated. I'll make sure I've removed all of that, since it could obviously cause all sorts of problems, check, and if necessary, replace the speaker cables. In short - go over it with a fine toothed comb!!(not literally of course!!!)

SonicAssault 28th August 2012 02:08 AM


Originally Posted by OXAUDIO (
The main power supply/output PCB had a substance sometimes referred to as 'Sony Bond'

Oh, is that what that mustard coloured stuff is called?
What I wanna know is, who thought it would be a good idea to put it on with a 4" paintbrush? Some of the components on my boards were practically drowning in it. :rolleyes:

OXAUDIO 28th August 2012 09:48 AM

It's meant to securely hold components in place since vibrations due to heavy bass notes might otherwise cause problems due to dry joints, etc., but if it is 'Sony Bond' it does become conductive with heat. I first came across the problem in an internet radio about two years ago, so, if, and when I succeed in getting the MS309W working again, I will reseal the components, and the speaker cable, where it exits the plastic case, but will use something that is definetively
NON-CONDUCTIVE! Any suggestions, anyone?

SonicAssault 30th August 2012 02:50 AM

I used a hot glue gun on the plastic enclosure speaker cable outlet – reused the original through-panel cable clamp and then just smothered the entire assembly with glue. This works well as it’s fairly easy to peel the glue off again if needed.
For the large components I used a few blobs of Araldite Precision as its fairly heat resistant once set – didn’t bother with the smaller stuff.

I’ve compiled a list of the replacement/substitute components I used, and where I got them:

Main Board
Power Amp Caps – Nichicon KG 4700uf 80v x2 (Parts Connexion) – these are 5mm wider than the original Su’scon items, but interestingly the screen printing on the main board has a placement outline exactly the same diameter as the Nichicons so they clip in without fouling any adjacent components.

Digital Amp Board (DS-150)
Power Transistors – Vishay IRF9640PBF / IRF640PBF (Mouser)
Electrolytic Caps - Nichicon EP (Bipolar) 10uF 100v 105C (Mouser)
Opamp - Texas Instruments TL072BCD (Mouser) – has a lower input offset voltage (3mv) than the original ST Microelectronics TL072CDT (10mv)

Relays – Omron G2RL-1-E-24VDC 16A x2 (Rapid) - The original speaker relay is a Goodsky 5A DPDT with the two poles paralleled on the circuit board input/output tracks, the Omron 16A SPDT has the same terminal layout and works fine. The original power relay is a Goodsky 16A SPST, but the power board has the extra through holes for a SPDT layout blanked off with solder, so you can open up the holes on the board to take the Omron. The Omrons are approximately 2/3 the height of the Goodskys as well.

Power Board Caps (Farnell originally, I think) - Panasonic FC 1000uF 25v 105C x2, and 22uF 50v 105C x2 – I had these lying around.

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