SetPower(..) has no effect on RCX2

I'm testing my RCX2 using NQC and sample program 1. The motors reverse just fine, but SetPower(OUT_A, 3) has no effect. I've tried
all three motor ports and all 8 power levels. The speed is always max. In fact, the voltage is always ~7.6VDC.
I've reloaded firmware v3.68. No help.
Has anyone seen this before, or have a clue what might be causing it? Does the RCX2 generate this power level with a DAC or PWM? How easy would it be for me to open it up and track it down?
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