It sounds like the -12V supply is used to generate the DAC low voltage
reference. If it is floating, the DAC output will be unstable. If it is at a
solid voltage, the DAC output will be stable. The standard EFC range for a
tbolt is -5V to +5V, but since almost all Tbolts are running the osc with a
positive EFC voltage, it will work with the -12V line grounded.
I suspect that although it may appear to be working, the DAC output may not
be as stable as with a negative rail available since whatever is generating the
negative DAC reference voltage will be well below its dropout voltage.
One thing to try is to do an oscillator autotune with the -12V line grounded
and at -12V. Your DAC gain will probably double (or half) since the DAC output
range is now 0V..5V instead of -5V to 5V. The tbolt firmware knows what the
expected dac voltages rails are (readable via an undocumented TSIP message) are
and also has settings for what range to allow the EFC to swing.
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