OP, since you like fixing stuff, the issue with that bad hashboard is kind of straight forward, the kernel log states
driver-btm-c5.c:13522:bitmain_c5_init: retry Chain[J7] has 15 asic
This means that the test signal stops at chip 15, it either reaches there and doesn't move to the next one, or simply just doesn't get there, so you are going to need to check chips 15 and 14, here is a good guide on how to proceed.
https://www.zeusbtc.com/manuals/Antminer-S9-Hash-Board-Repair-Guide.aspThank you very much, I'll have a closer look at it when I take the miner apart the next time. At the moment the broken hashboard is in the running miner (not powered or connected to the control board, just for stopping air from finding an easier way past the 2 running hashboards).