ouch.. what exactly did you change? pix?
link to that mod?
if the tone generator still works, i doubt you fried the whole chip.
first, make sure it's really dead - 100% sure.
for those of you that read schems:
service manual heaven..only schem i could find easily, but it's got YM1050.
so not sure what pin you LFO output is on.
still, there should only be a single elco in between the chip and the pot, so checking on the pot is your safest bet:
one side goes to ground, middle is to the VCO/VCF switch, other side should be full LFO signal.
LFO at the fastest rate is only 28Hz, so unless you have a subwoofer you won't be able to listen in, best to check visually.
do you have a scope, or one you could use? for signals like these, even a software scope would do.
worst case you could use a multimeter and check AC voltage with the speed cranked. should be around 200mV.