Heineken's Garage

Introduction
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)
  Transistors used are from either STMicroelectronics (ST) or International Rectifiers (I▽R) with the BOSE-internal part number 137110
  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".

Diagnostic
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.

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.

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 MOSFETs are know to be good and no floating input is observed, check for the high frequency switching signal arriving at their Gate (pin 1) with an oscilloscope. They are always present when the amplifiers are powered up, even if no input signal is provided. They should be detectable on all four transistor input pins - if not, assume U2 to be broken.

General Approach
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.

If there is still no sound, note that transistor failure and spilled capacitor acid can kill the Bose ICs U1 or U2 (most often it's U2) which are non-standard parts. Replacements are expensive and it's difficult to de-solder these 20 pin chips without damaging the PCB. If one of them is broken, consider disconnecting their legs with a pair of side cutters, then remove each individual pin separately from the PCB and installing an IC socket.

U2 (Bose 134499) shows up on the usual parts scraper sources, from time to time. U1 (Bose 141661) isn't available anywhere but can be replaced with Bose 133524.

Special Observations
These are specific failures cases, observed and confirmed, including how to fix them.

Distorted Sound and overheating of a single MOS-FET
If the sound is distorted when reaching medium volume after replacement of the MOS-FETs, one transistor heats up quickly and not all transistors are switched even though U2 is know to be good: Check if the connections from U2 to the gate of each MOS-FET (realized via a 10 Ω resistor) are OK. Broken VIAs have been observed resulting in the gate of Q3 floating and causing these issues.

a repair wire from U2 to Q3

Too low Volume
If the amplifier works but volume is too low (not very much too low but clearly noticeable), check the voltage of pin 17 on U1 - if it's below 5 V it causes the compressor circuit to shut down, reducing output volume. If the low voltage is limited to pin 17 only, it can be restored by soldering a 1 kΩ resistor across C203.

Too low Volume II
If the same behaviour as above is observed but the 5 V are missing at several places (no only pin 17 of U1) check this resistor for damage. It should read 27 Ω in circuit. If it's much higher (like 100 kΩ), it will make the 5 V drop to ~3.7 V causing identical issues at U1.

Q4 Heat-up
If Q4 heats up even thought everything else seems OK, check the resistor closest to the "Etch-Triangle" near the connector. It's the pull-down resistor for Q4. If broken it will cause Q4 to float and heat-up.

Way too low Volume
If the amplifier works but the volume is way too low and Pin 17 of U1 shows the correct voltage (5 V), it's most likely a broken U1.


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