Speaker Amplifer - Repair and Capacitor Replacement
A french NSX owner, active on NSX prime sent me his Bose speaker amplifiers (and the radio- but we'll come to that in a separate thread) for repair. The car has been left in a garage for a considerable amount of time and the audio system was non-functional as not even the head unit would turn on. The PCBs were from 1992 and in a bad state:
The right door amplifier hat a blown MOS-FET transistor which charred the PCB. The Bose PCBs are rather robust and withstand a lot of abuse but there are limits, of course. The reason for this failure is (of course) capacitor leakage but also due to the design of the amplifiers. Their main ouptut stage is a MOS-FET transistor bridge that connects each speaker pin to either GND or VBAT - that's why there are four transistors.
The downside of this design is, that it fails catastrophically if a pin is connected to GND and VBAT at the same time - which happens if both transistor pairs are conducting at the same point in time. Such situations are usually avoided by switching the transistors correctly but in this particular failure case the acid from the capacitors carries voltage to one of the transistors, making it conduct permanently - if the second transistor is now activated it becomes a direct connection between GND and VBAT. Since the acid-affected MOS-FET typically conducts only a little at first, it keeps a remaining resistance which causes it to literally heat up and burn.
Both left side and centre speaker amplifiers appeared normal from a first look. Due to the contamination with capacitor acid, all of the hot glue, all capacitors and the coils had to be removed. The PCB of the right door amplifier was washed with water and a brush, then rinsed with natural alcohol. Contaminated (dark coloured) traces were scraped of and polished with a fibre pen, then covered with solder. New capacitors installed and powered-up for a test.
.. to be continued
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