Sometimes you get lucky.
My friend Jeremy has a 1995 Mustang that had the factory premium sound system in it when he bought the car used. The CD player was broken and he had the head unit replaced within a couple of weeks of owning the car. He later added a subwoofer.
I’ve always thought the stereo lacked clarity in the bass, and the head unit and EQ have had some quirks. Recently Jeremy pulled the head unit and found all sorts of interesting techniques used by the aftermarket installer that will be the subject of a later monologue … but one of the things we discovered is that the amplifier for the door woofers wasn’t working at all. Swapping it with the amp for the rear deck woofers caused them to go silent and the (shot) door woofers to work again (after resoldering their cut cables).
Turned out to be a delightfully easy fix.
The black connector on the bottom is input and the grey connector on top is power in and speaker out. Although both connectors are pinned for two channels, only one channel is actually connected — this is a mono amp. Interestingly, all of the audio interconnects in the factory sound system are balanced, which should help cut out alternator noise and clicks and pops from other electrical systems — a luxury neither of us has seen in aftermarket sound systems.
When first I opened the unit, I was disappointed not to find any smoked components. Since we got no output whatsoever, I really expected to find a melted power transistor or something else dramatic.
However, it didn’t take long to discover the dramatically cracked solder joint on this lead,
which turned out to be the inductor in the pi filter on the power input,
so named because of the schematic’s resemblance to the letter π (my favorite vitamin).
I wanted to be sure to get all oxidation and dirt out of the joint; so instead of merely reheating it and adding more (flux-core) solder, I scraped both the pad and the lead clean with a chisel tip. While doing so, I noticed that the whole inductor felt loose and discovered the less visually obvious second cracked solder joint.
My assessment is that the inductor was installed somewhat carelessly at the factory and left sitting slightly above the PCB. Lacking contact with the board to prevent downward motion, a decade (or perhaps less than a year) of vibration from the rear end of a sporty coupe with a tight suspension broke the solder joints, thereby breaking delivery of power and disabling the whole amp.
After cleaning both joints, I reinstalled the inductor, this time installing it flush to the board as well as bending the leads over for increased contact area before resoldering, in hopes of increasing the mechanical strength of the joints.
The capacitors to the left were already hot-glued in place to prevent damage from vibration, and before reassembling the amp, I glued down the whole pi filter.
I got in touch with Jeremy to let him know exactly how lucky he was, and last night we reinstalled the repaired amp and confirmed that it now works completely.
Thanks to flemworld.com for providing the name of the factory sound system, as well as wiring diagrams invaluable in the larger project but not directly relevant to this repair.