I'd hate to bump an old thread but I've been dealing with this very same issue across 3 different S7s.
All 3 S7s have been voltage modded using sidehack's HEX files from the S7LN. I ran these miners a lot about 2-3 years ago then stopped and shlef'd them ever since. A few weeks ago i fired them up and it was like waking up zombies... they wouldn't even hash at all for the first few minutes.
I've tried multiple firmware types, 2fan and 1fan, same outcome.
I've tried clock speeds as low as 450m , same outcome.
The only common thing I have been able to find is that they need some heat in them to stabilize -- around 65-68 they hash OK for a while before the cards start to drop or reboot.
Sometimes the BBB is just unresponsive and needs a reboot... I've seen it where the BBB was hung but the cards looked like they were still hashing but weren't actually doing anything but wasting electricity.
Is there a way to test the power stage of the board to see if its failing? I gave all of the hashboards a dusting but the problem still is happening... my next step is an alcohol bath on the upper corner of the boards.
Which leads me to this :
https://bitcointalk.org/index.php?topic=2657162.msg27080552#msg27080552Added on external regulator modules?
The best I was able to get
each these S7 hash boards to do is about 1TH/s for roughly 250w using 670/650mV HEX file for the PIC. Anything less and the cards barely wake up, anything more and they just get too hot.
Follow up to my last post, I was able to figure out why this was going on.
I cleaned the upper corners of the hash boards but they continued to have the issue turning on and off while mining. I reflashed the PIC with stock firmware and its been stable for 12+ hrs now,.. i was barely able to get 1hr out of the hash boards before.
I'm noticing that anything less than 680mV flash on the PIC will lead to these boards becoming very unstable.