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Diode D502 is a Schottky commutation diode. It plays no role in regulating the bias voltage/current. Neither does C504 when functioning properly. Regulation is done under MCU control by monitoring ADC_BIAS and adjusting PWM_BIAS as needed.

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Loss of regulation or erratic behavior might be due to:

- erratic, failed C504,

- an erratic short in the PIN diode array in the LPFs,

- a damaged Q502 (BSS84),

- a damaged MCU input (ADC_BIAS).

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You are right! Symbol on Hans' schematic shows as Zener. Anyway, the QMX+ goes on the 'scope this afternoon to see what I can see. SOMETHING is driving the bias nuts and I dearly want to find it!

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