Well, I have tried with all of them and changing frequencies but nothing improve, always same problem with high temps. Answering to your question, yes, I tried flashing 650 fixed frequency firm first and from there just swapping bmminer file for your file. For example, I have tried with bmminer 880 and frecuency 650 onwards, like 656, 662, etc till 762. With all of them I get same problem. Thanks for your help !
seem like fan setting problem in bmminer.
with my S9 13.5 other firmware's plus bmminers and tricks all over the internet the custom fan speed not work my miner always follow auto fan. But custom fan speed only work with LPM firmware and the bmminer inside LPM.
my controller board is c41 v1.20