Theory Question: Speakers don't work unless I poke soldered points with screwdriver

Question

In case you couldn't tell by the title, I don't really have any idea what I'm doing with electronics. Something happened though, and I am really curious about it. Don't know how to ask it specifically, so here's the backstory.

I brought out some old speakers from storage. I plugged them in, and they didn't work. However, if I turned, twisted, and pulled the volume turner just right, the speakers would start working. Usually only one worked, and the other was staticy, but which one was working would switch back and forth. After using them like that for about a month, I decided I had enough and took it apart to see if there was a loose part with the volume knob. Everything was soldered down tight, and nothing was loose with the knob. With everything on, I poked around at the soldered points with my screwdriver and heard some loud pops from the speakers. Just like that, they started working again (not perfectly, but 100x better).

This leads to a few questions. Why does poking at soldered points make them work again? Does static build up overtime or something? Also, is this dangerous to do again? The screwdriver is insulated, and I'm not touching any metal with either hand, so why is there any effect at all?

The speakers started acting up again recently, and the same solution worked once more.

Answer 1

Most volume controls have the terminals rivetted to the resistive track. Turning the volume control up and down a few hundred times (especially if it has the in-off switch in it) or pushing and pulling it loosens the rivets and the resistive track disconnects until something (your screwdriver) moves it a little.

My Sony Walkmans and clock radios had that problem.

Answer 2

There's a hairline crack somewhere really close to the part that you poke to make things work. The crack is either at the solder joint itself, or it's on the circuit trace close by -- probably at the point where the solder ends, because that would be a high-stress point.

Very close examination of the affected part, with the best magnifying instrument you have, may show you the crack. Poking around with a continuity checker may tell you where the thing opens up -- or not.

If it's a crack where the wire goes through the solder or the solder goes onto the board, then de-soldering that joint, cleaning it, and resoldering it should make the thing work solidly, probably for at least as long is it worked before.

If it's a crack where the pad meets the trace, then you need to bridge it with a bit of conductive something. A bit of component lead or a bit of fine solder-wick is usually what folks would use in this case.

If you can't figure out where the problem is or don't care to, and if the connection to the part doesn't have traces going off on both sides of the board, then you can kill all possible birds with one stone by desoldering, then soldering it back together with a bridge (i.e., that piece of component lead or solder wick).

If the speakers use transistors and ICs, then the highest voltage in there is almost certainly whatever the wall-wart that supplies the thing is rated for. If that's 12V or less you're fine. You're probably OK up to 48V, but that's getting kind of sporting, and it'd have to be a pretty hefty set of speakers to need that much voltage.