Fixing Jeremy’s Ford Mach 460 Bass Amplifier

Sometimes you get lucky.

Ford Mach 460 bass amplifier

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.

Ford Mach 460 bass amplifier circuit board

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.

Broken solder joints on Ford Mach 460 bass amplifier

However, it didn’t take long to discover the dramatically cracked solder joint on this lead,

Pi filter in Ford Mach 460 bass amplifier

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.

Glued pi filter in Ford Mach 460 bass amplifier

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.

Credits

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.

2 Responses to “Fixing Jeremy’s Ford Mach 460 Bass Amplifier”

  1. Baratine says:

    Really cool, I enjoyed reading :) . I am always impressed when I see an old device, there is always so much space between the components and no SMT. Anyway, cool !

  2. whackpak says:

    I like it when you post your repairs. I enjoy reading and always learn from it to fix my own stuff. Grtz from Belgium

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