this is my result. an 13.5T Autotune Firmware flashed with 650M Fixed Firmware, Power Supply are Dell 2100W (first and second) and HP 2450W (third one)
no problem so far, tried to push to 800M with 9V but too much HW Error, some board disappear and reappear again, then disappear again
Cooling are SanAce 12V 4A Fan in Intake and a Antminer S9 Standard 2.7A Fan at Exhaust, using a Flex Duct to separate intake and exhaust
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So it looks like the temp bug affected your third board but it was still mining which is meh okay since it will still clearly mine fine. I have gotten a few reports now on the 13.5 and I am thinking the 13.5 s9 is a bit of a lottery on if it can run at 800Mhz stable or not. If not your best bet is to run it at 725-750 as you did as the hw errors are far less as you saw. You CAN get the 13.5th to run at 800Mhz or more but it would require more than 9volts and honestly anything beyond 9v of power makes the efficiency go to shit why mind 18th at 2.1kw when you can just mine 16.5 at 1600w you know in most cases unless your electricity is very cheap it would just cost more.
Luckily Darval contacted me and is going to assist me with making this have an autotune per each board. So you would set voltage and then it would tune to the best speed for the voltage so if you got a 1600w or 1800w psu you can get the maximum bang for your buck. But I am not C programmer and they are so they are going to be a godsend for me as I am more familiar with python java (meh) matlab r studio afni/
I would suggest trying to run it at 750Mhz.
Also if you would like to try this:
https://github.com/taserz/S9-Volt-Rocket-Ship/raw/master/bmminer890Is 8.9v and it might be worth trying at 750 if 8.8v is throwing to many errors or even beyong 750 since your psu can handle it.