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Speaker - Repair

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Failure of the NSX active speakers is a common fault, especially for the early build years. The modules are class-D switching amplifiers with a MOS-FET driver bridge for low impedance (2 Ω) speakers. The amplifier can be divided into these main sections:

  • Output drivers (MOS-FET transistors) and Filter (coils)
    One pair of transistors connects one pin of the speaker to either GND or VBAT. The IC U2 ensures that two opposing transistors are never conducting at the same time. If it happens, a direct connection between GND and VBAT would be created, with fatal consequences.
  • PWM Control (U2)
    The Bose proprietary IC U2 controls the high frequency switching of the MOS-FET transistors. It's a fixed frequency that's always active even if no signal is present on the input.
  • Sound Processing (U1)
    The Bose proprietary IC U1, together with most of the other components on the PCB, control and shape the sound of the amplifier. The differences in sound between door and foot well speaker are realized there, as well as the typical "Bose sound".

Typical failure is due to capacitor leakage. The fluid travels underneath the hot glue and depending on the area covered by glue, different failure modes can be observed.

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a puddle of liquid after removal of a piece of hot glue

If the fluid drains without reaching critical parts (like the transistors or the Bose ICs) the amp will fail due to issues with the capacitors. Functionality can be restored by replacing the electrolytic capacitors.

When fluid reaches the transistors it can transmit voltage and cause them to conduct independent of the PWM control. This creates a short between GND and VBAT across both transistors of a speaker pin. Due to the nature of MOS-FET transistors, such a small voltage makes them act like a resistor and heats them up to the point of destruction.

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A destroyed MOS-FET

The MOS-FET transistors can be checked for acceptable resistance values (the transistors can be measured in circuit and don't need to be removed).

  • Drain to source resistance: ~1 MΩ
  • Gate to source and Source to gate resistance: >100 kΩ

Failure can be assumed if very low values (like 70 Ω) or strong differences between the transistors on the board are measured.

If the amp looks OK after visual inspection, capacitor replacement is the first step. If in doubt about the MOS-FETs, replace all four with IRFIZ24N which are pin compatible replacements. Transistor failure can kill one of the two Bose ICs (U1 or U2) which are non-standard parts. Replacements are expensive and it's difficult to de-solder these 20 pin chips.

Similar amplifiers are used in the Corvette C4 and corresponding troubleshoot information is helpful, too.

Updated 24-03-2021 at 06:02 AM by Heineken

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