Repairing a PCB
A PCB can become cracked if there is sudden pressure put on the PCB at certain points. Often this occurs when the equipment is dropped. The heavier the item, the more risk there is of cracking. The subwoofer is very heavy and so this does increase the risk of damage to the amplifier PCB if the subwoofer is dropped at any time, although the PCB does seem to be held in place OK.
When the PCB is cracked, the crack will usually be right through the board, although it may only go part way through. If the track is cracked, then this will break the circuit. Depending where the damage is on the amplifier PCB, this may have little effect, or it may affect one channel, both channels, the subwoofer channel, or the whole board.
A crack, especially a hairline crack, is not always easy to spot -
A crack can often appear at the point of shock. This can be where fixing screws hold a PCB in place, or heavy components are fixed or soldered to the PCB.
On the amplifier PCB, especially check the areas around where the heatsink and associated chips are connected.
Sometimes there can be poor connection where the component is soldered onto the track. This is usually caused by a poor joint or a dry joint and can result in an intermittent connection or no connection at all. Components are usually either surface mounted (the component is on the same side of the PCB as the track) or “through hole” mounted (the component is on one side of the PCB and the “legs” go through holes to the track on the other side of the PCB.
One problem sometimes seen is where the end of the leg has come away from the solder
Apart from poor joints, another problem can be where the track has lifted and cracked, normally near a component joint. Usually this occurs when pressure has been put on the component, either through the equipment being dropped or because the PCB / component has been mishandled. This can happen where, for example, a vertically mounted capacitor has been bent over, or near a socket that has had a bit of rough treatment.
Sometimes these are easy to see, sometimes not. Telltale signs could be a component that seems very loose on the board. Look at the track side of the PCB and gently wiggle the component. If you see the track move then, at the very least, the track has come away from the PCB. Usually the track cracks at the same time.
Mending a cracked track or joint
There are generally two ways of doing this
Don’t just clean the track and put a blob of solder across the crack! Whilst this may rectify the problem, it is more likely to fail again
1) Clean away the protective covering of the track either side of the crack to expose the shiny metal surface below. Solder a piece of solid core wire to the PCB either side of the crack. This is an ideal method if the track hasn’t lifted.
A simple way that I find of doing this is to use the “leg” of a component -
Bend the wire at a ninety degree angle and hold it in place it across the crack so that it is flush with the surface either side.
Solder the end to one side of the track, being careful not to pull on it or overheat the joint as this will lift the track from the PCB.
Solder the wire to the other side of the track. (Using an old resistor, and bending the wire at 90 degrees means that you can easily hold the wire in place and the resistor means that your end of the wire won’t heat up and be too hot to handle).
Cut off the excess wire
2) Use a wire two join an existing solder point either side of the crack.
If the connection is only going to be a short one and there’s no risk of the wire short circuiting against anything else, bare wire can be used. If not, use insulated wire. The wire can be stranded or single core as necessary. Depending on what you need to do, solid core may be better and it avoids the risk of spare strands shorting out against something else.
If you have a broken track at a joint, it’s likely that the track has also come away from the PCB, leaving the component leg loose with the solder joint and a bit of the track sticking to it. You will need to be able to secure the component at the same time. The easiest way to do this when there is another joint on the same track nearby is to clean the broken track and old solder from the component.
Using solid core wire, wrap one end round the end of the leg of the component so that it is flush with the PCB.
Solder the wire to the leg. The component should now be held solidly in place.
Solder the other end of the wire to the other joint.
Whatever method you use, check afterwards to make sure that there are no short circuits caused by stray bits of solder or wire.
Fig. FP1: Repaired PCB joint
The above photo shows a (rough!) repair to the relay PCB. Because of strain on the mains sockets, the PCB track had cracked at the joint and lifted from the board. This meant that the pin, solder joint and a scrap of PCB track were separated from the rest of the track and the socket was loose. There was no connection with the rest of the track. Method 2 was used to repair the joint.