i had same problem with my magaworks 5.1 thx550.
i have tried to find solution in web but it was only one which voltage sense resistor 2Mohm track in PCboard was damaged.
after check PCboard in my megaworks thx 550 i found that tracks are quite well and voltage sense resistor is well too.
REPAIRING PROCEDUREin below items there are repairing procedures that i did with my THX550.to access to BCboards it is required to remove electronic boards from woofer case.
1- remove cover of woofer.2- remove woofer speaker from case by cutting and marking connected wires.
3- remove all Philips screws around cover and remove electronic parts smoothly, care about glues around power cord.4- remove and disconnect power supply board from other parts.
5- replaced switcher TOP234Y and capacitors C102, C206, C205 and D10 as below photos.
6 check all of related tracks in repairing area.
7- now it is time to re assemble electronic parts very carefully and prepare for test. it is better to use isolated transformer and put a lamp in serial to protect PCboard against short circuit. if green LED (POWER LED) is on it means that every thing is O.K.
8- in this step all of electronic parts should return back to case and reconnect sub woofer connections.
after replacing damaged parts my THX 550 is working well maybe for next 10 years.
i hope above mentioned instruction can be useful and helpful.