[sdiy] SSM2164 protection

[David G. Dixon]

> Be careful there, David. The diode needs to go from -15 to 
> ground not between -15 and the chip's supply pin.

Yeah, I realized I'd misspoken as soon as I logged off.  Lastnight I went
through all my 2164-based layouts and stuck Schottky diodes in convenient
places between -15V and GND.  I'll be able to kludge the diodes into my
existing modules with little difficulty, and the layouts now include
provision for the diodes, in case anybody else wants to use my boards (I do
sell them fairly frequently).