Repair of Mordaunt Short MS409W, Active Powered Subwoofer with 8" Aluminium Driver




Fault - no output  but the Leds light up on back panel 

The customer had taken the active amplifier chassis and back panel out of the main speaker cabinet to save on shipping,  as in our conversations he had proved the speaker to be ok.



Removed the back cover from the  amplifier assembly , on inspection found  the cause of the problem after some fault finding to be the blue relay on the auxiliary PCB (see picture), the relay was working ie going click, but was not switching, so no voltage was getting to the main PCB hence no sound. 

Unstuck then unsoldered the relay tested it and confirmed that it was not working, this relay is rated at 16amp  240v with a 24v DC coil

Fitted a 5amp  24v relay that had the same pin footprint from our stock, the sub immediately started working, so ordered a replacement OMRON 24v 16amp relay which was fitted to the PCB, as can be seen it has a lower profile than the original, see below the black relay bonded to the transformer on the refitted PCB.

Tested the amplifier, then refitted the heat shrink sleeve and bonded covering back in position


Refitted the back cover and tested unit for 4 hours all ok 

Unit ready for shipping back to the customer


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