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schematic

simulate this circuit – Schematic created using CircuitLab

I am trying to repair my stove Vacuum Fluorescent Display circuit board. One or two segments light up and quickly fade away. The circuit board model is ORBITRON A-403 from Borg Instruments INC. Diehl timer control division. I cannot find a circuit diagram. By reverse engineering I found that xfmer secondary winding negative cycle side is rightly connected to the power supply bridge rectifier and also to PNP transistor emitter through a resistor, and its collector to the negative rail of the power supply.

Can someone explain to me what is happening? I appreciate your help. Thank you.

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    \$\begingroup\$ Can you draw a circuit diagram of what you're talking about? \$\endgroup\$
    – Hearth
    Jan 5, 2022 at 15:50
  • \$\begingroup\$ Thank you for your interest. I have added a schematic of the point in concern. Hope it is a bit clearer now. Regards. \$\endgroup\$
    – Marcello Prete
    Jan 7, 2022 at 15:04
  • \$\begingroup\$ Excellent, much easier to understand what you're talking about now. Would you by any chance have an oscilloscope that you might be able to see what's happening when the segments light up? It would be good to rule out a broken VFD as the cause, because if that's the case, you probably won't be repairing this. \$\endgroup\$
    – Hearth
    Jan 7, 2022 at 15:56

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Pin 16 of an HD38800 microprocessor is a "body bias" or "back bias" pin (Vbb). The HD38800 microprocessor is a PMOS device, (a family of devices that is now largely obsolete, having been replaced by CMOS). Vbb biases the substrate upon which all the PMOS devices sit.

The BJT Q1 appears to be regulating the voltage of Vbb. It is peculiar, however, that its emitter is (indirectly) connected to the AC side of the bridge rectifier.

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