According to the schematic there are several fuses, the one that might have blown is a 125 V 1.6A fuse, did you check it?\n\nAlso look like you can buy parts from Sears, their parts list show only 1 fuse, referred to as fuse 6A, I am not sure that's an error, because the 125 V 1.6A one makes sense to me, and it is right from the schematics.\n\nhttps:\/\/elektrotanya.com\/yamaha_yst-sw216.pdf\/download.html\n\nhttps:\/\/www.searspartsdirect.com\/model-number\/ystsw216\/1217\/0311400.html\n\nPeng,thanks for digging up the service manual and circuit.\n\nThere is only 1 fuse, 1.6 amp 125 volt. It is not blown because the indicator light is on.\n\nInterestingly for a sub, this amp is not class D, but an A\/B biased amp with an IC chip power amp, driven from +15\/-15 volts. So that could produce 100 watts.\n\nSo there is an input circuit followed by the crossover ICs. Then IC1 is the power amp, there is a limiter circuit to protect against overload. This is discrete transistor. IC6 provides he servo feedback function.\n\nNow the output of IC 1 goes to a relay RY1 and then to the woofer. The relay is controlled by a protection circuit that is also analog and designed to open the relay from failure of IC 1 from DC offset. This would be he normal mode of failure of a power IC, and could cause a fire if it went to the woofer.\n\nNow there could be failure anywhere all the way from input, the crossover, to the power IC.\n\nHowever common things commonly happen. 100 watt power amp ICs are not known for longevity. The chip only costs $29 and currently is unavailable. This is how such a low unit cost was achieved with this sub.\n\nSo I suspect that relay RY1 is open, because at least one of the power transistors in IC have broken down and letting the DC rail voltage pass to the protection circuit which is doing its job and opening RY1 and not letting DC pass to the woofer. It also I'm sure, acted faster than the 1.6 amp fuse and prevented the DC going to ground through the woofer and preventing the 1.6 amp fuse blowing.\n\nSo actually this unit may not be so difficult to service.\n\nI would see if there is a voltage to RY1 keeping it open. DO NOT jump RY1. If there is voltage keeping RY1 open, then you can be certain that IC 1 is blown. Then it could be replaced with enough skill and the amp tested. IC 1 seems to be a proprietary chip of Yamaha's I think. The track record of chips becoming available after they disappear is not good. They should however be available for 7 years from last manufacture.\n\nI do note that failure of these subs seem pretty common, which given the low cost design is not surprising.