Car Amplifier Reverse Polarity Repair

I wrote this up a long time ago, but never finished it until now.  I took pictures, but unfortunately it looks like I lost them during a hard drive failure.  But, here it goes anyway…

I recently fixed a broken car amplifier (Pyle brand) for a family member, and I wrote this up to document the results.

Background

He was powering up the car amplifier after doing some re-wiring, and smoke came out of the amplifier.  The amplifier no longer worked after this.  He had earlier blown the fuses (there are two 30 amp fuses in parallel) and had bypassed them by wrapping a wire around the blades of one of the fuses.

Diagnosis

After opening up the case, I saw that a diode had overheated and cracked open.  A fairly wide circuit board trace leading away from the fuse block had also overheated and had broken.  The broken diode was part of a reverse polarity protection circuit.  The circuit is shown here:

When reverse polarity is applied to the amplifier power terminals, the diode protects the amplifier in two ways:

1. The diode clamps the voltage on the positive bus so that it does not swing too far negative (I would guess that it would not get below -1 or -2 V).
2. When the battery tries to lower the voltage further, a large current starts flowing through the diode.  This quickly causes the fuse to blow.  The blown fuse prevents damage to the diode or to the rest of the amplifier.

The family member said that he hooked up the amplifier correctly, but the blown diode tells a different story!  To be kind, there is a very small chance that the diode just happened to fail on its own at that particular moment…

I assume the sequence of events went like this:

1. Since the fuse had been bypassed, the diode continued to conduct longer than designed.  It overheated and failed closed (i.e. it shorted and became like a wire with very low resistance in both directions).  The diode is a 1N5401, which is only rated for 3 amps.
2. With the diode shorted, a high current continued to flow through the positive terminal of the amplifier, through the fuse block, through the diode, and out through the amplifier’s ground terminal.
3. The circuit board trace overheated and broke.

In a round-about way, the protection circuit worked.  The diode clamped the voltage and cut off power, and the circuit board trace acted as the fuse.  If the diode had failed open, the amplifier probably would have had more extensive damage.

On a side note: I am impressed with the circuit board design – there is a black mask on the top of the circuit board which mirrors the copper on the bottom.  This makes it easy to trace the circuit just by looking at the top component side.

Repair

The repair was straight-forward.

1. I soldered in a replacement diode from Radio Shack.
2. To repair the circuit board, I first cut off the loose trace (which had come off the board after burning) with an exacto knife.  This was probably unnecessary, but I was nervous that the loose copper foil might eventually weaken or break.
3. I cut one of the blades off one of the blown fuses.  It was about the right width and length to replace the missing trace.
4. I soldered the blade across the circuit board gap.  I used a high wattage soldering gun for this.
5. I replaced the fuses.

After re-assembling the amplifier, it tested out OK and is working fine.

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