You might have fried a power supply. Take off some gpus and just try with 1 psu. A 750w can only handle maybe 2-3 cards at full load. Good luck.
i m only connecting 1 8pin to power supply & not using 6pin as i don't want to oc?
That you're problem. It has an 8-pin and a 6-pin for a reason.
negative, for mining on amd gpus like the rx 500 series gpus you only need to use the 8pin, I thought the same thing but its not true for some amd gpus, now NVidia on the other hand you HAVE to use both lol