Sony Surround Sound System Intermittent Speaker Issue Due to Inductor Dry Joint

Had a bizarre dry joint problem in a 20 year old Sony surround sound system. Has had an intermittent sound on one of its 6 loudspeakers for years. Bought a failed old unit and after swapping everything I could I have found the problem. Initial thoughts were that is was a dry joint on the speaker connectors and indeed both units had this problem slightly on several connections, but resoldering these did not improve it. Eventually was about to give up when I found a small inductor between one of the 6 amplifiers and its speaker connector that has one bad dry joint. It's not that big compared with its lead size so it not overheavy for the joint size.

Bizarrely the other unit has exactly the same fault so swapping the PCBs over had made no difference. There are 12 of these inductors in each unit and only this one same lead on each had the problem. I suspect it’s because it’s the nearest joint on those inductors to one edge of the PCB and maybe the wave soldering machine was not set up well so did not properly solder that bit of the PCB. Visually though there is no sign of surrounding soldering problems.

These inductors filter out the 1Mhz left on the output signal from presumably its switching action (don’t understand Class D amplifiers). Does 1Mhz produce enough vibration in an inductor to vibrate a solder joint loose?

Any thoughts?

Were these through-hole components? (I assume so due to your reference to Wave Soldering).Wave soldering is fine on small components (resistors, capacitors etc) but on larger components like power sockets, other connectors, large heavier components, etc, it is usually woefully inadequate.  As I am fond of saying, if I had a dollar for every dodgy wave soldered joint I have fixed, I'd be able to shout a lot of beers.Causes... seems like fault was on for a long time so probably inadequate soldering from the start.  I'm not sure about component vibration due to the signal, but if it's a hifi amp it would get vibration from the sound (Pink Floyd played loud makes everything shake or just from being moved around occasionally.  But good work finding the problem.  Dry joints can be real sods to find.  Going over every other joint you can find is a great idea, where there's one there's probably more.

Thanks. Yes is through hole mostly, although the solder side has some SMD resistors etc. I assume these are put on first
and the wave soldering does not remove them again because of the glue used to
stick on the SMDs. The 12 ferrite inductors are only ½” diameter by 5/8” high
so are not large or heavy. It’s the left front channel so, apart from the left
and subwoofer channels, gets the most power through it. So I guess the varying
magnetic field from Pink Floyd etc. does not help. I see later models of this unit have a stronger mounting arrangement with 2 inductors in one plastic case so get 4 pads in a square arrangement to solder, giving more rigidity.

 I did resolder all the big joints whether
they looked good or not. Don’t think it’s lead free at that age so can’t blame
the brittleness of lead free. Has a big copper EMC shield over most of circuit
which was one reason it took a while to find. Speaker connectors were visible outside
the shield but not this inductor.

Solder joints near a board edge can be subject to a bit more flexing in the manufacturing operation (the board is usually held by the edges) so that may have something to do with it in this case. The other thing I've found is that solder joints on parts that are subject to intermittent heating can become problematic over time. I've been amazed how many older ham radios I've fixed by going over the solder joints in the power supply or transmitter sections.

I also struck a situation where the conformal coating on some components had made its way down the leads, and had resulted in poor wave solder joins,cheers,Richard

Edward...an inductor 1/2 in dia x 5/8 in high is not big, but compared to a transistor or resistor it is bigger and heavier.  And your remarks about the change to a different package inductor makes me think you were not the only one to have this problem.Having fixed this and hopefully all the other incipient dry joints, I hope you get another 20 years service from your system. Nice work

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