After Dirk’s revelations, and my stupid denials of the problem, I finally pulled my head out of my ass and got to the bottom of this. Thanks for all the help.
The problem is simple rust from water trapped between two steel plates.
Way back in 2015, when we first launched BHK, the bottom plate for the amplifier was built in two pieces. There was the main steel bottom piece and attached to it, in the middle, another steel piece used as a stiffener so the big toroidal transformer didn’t deform the bottom plate during shipping.
To build this assembly, each piece was separately fabricated using laser cutters, then attached to each other with a spot welder. That assembly was then sent out to the platers where it was zinc plated. As part of that plating process the assemblies are washed and dried, then baked to remove any moisture that might eventually cause some rust.
For the first two months of production, the platers neglected to bake the assemblies. Our quality control people caught the issue and returned all the inventory to be rebuilt. Unfortunately, two months of production got out the door and Dirk’s BHKs were among those. He first noticed a discoloration around the outside edges and, when he drilled out the spot welds and separated the two plates he saw the rust inside.
We are still investigating how many amps were affected and then trying to figure out what to do about it. Of course, anyone with an affected amp can get a new bottom plate. It doesn’t look like there’s actually anything other than ugliness to be concerned with as the stiffeners we’ve looked at were more ugly than non functional. There’s likely not enough water inside to cause more ugliness than what we saw but I cannot be sure of that. They certainly aren’t right.
If you want to check your serial numbers, look at the last block. Dirk’s ended in 5G0032 so they were right in the first month. The way to read the serial number, BTW, is simple. The first is the year, so 2015. The G corresponds with the month in alphabetical order. G is the 7th month (counting up from A), and then the last two are the unit ID. So, Dirk’s was number 32, built in July of 2015.
Anything built starting in August of 2015 is fine.
As a side note, not long after this we changed entirely the way those plates are assembled. Instead of spot welding our metal vendor now separately plates each piece and then assembles them together using some sort of viciously strong glue.
My apologies, again, to anyone affected with this issue and thanks to our loyal PS customers for the help.