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Loudspeaker fire: a close escape

Seeing the speaker on fire on the October 2011 cover made me remember an incident that occurred a few years ago now, which could have had dire consequences. It is worth telling for several reasons.

I was working late in the home workshop shed and at around 11:00pm heard a rather loud noise disturbing the neighbourhood and I immediately wondered about the sanity of whoever was running a compressor or mower at that time of night. After about 10 seconds of this I realised that the noise was in fact coming from my house.

I went inside just as the noise abated somewhat and was still led to the home theatre room where there was now a bad smell, smoke and finally a lick of flame coming from the active sub-woofer port. This unit was a US Audio-branded device and was in standby mode at the time it cremated itself. I unplugged the unit, extinguished it and put it outside for investigation next day.

When I got it apart I noted that the fuse was still intact but that both the speaker driver and the amplifier had been alight. The PCB for the amplifier was very crispy and not much was recognisable in the way of components. The speaker cone had suffered burning at the centre but the voice coil was still continuous.

This was not the first time I have been directly involved with consumer electronics in standby mode catching fire, with PC monitors and TVs being quite common. This is the reason I now have a real switch to remove all power to most devices around the home.

The subwoofer in question was rated at 60W RMS and this low power is why I believe it burnt so well, as there was not quite enough current to open the voice coil or fuse but still enough to produce sufficient heating to make flame and to do so within about 40 seconds of the fault occurring.

I was lucky; I was home at the time! How many people leave these things on standby even when away on holiday? With a fire just waiting to happen? Since that time I have seen one other subwoofer with very charred innards and I wonder about the general safety of these units.

David Woodbridge,
Camillo, WA.

Tamper-proof screw solution with an angle grinder

I read the Serviceman story in the September 2011 issue that Leo Simpson had trouble with tamper-proof screws.

Well it does not matter how tamper proof they make things, we will find a way to get them apart. To cater for such circumstances, I have converted my tamper-proof bits into adapters by cutting a slot with a 1mm thick cutting disk mounted in an angle grinder. I always have these 125mm diameter disks on hand as I use them for all my metal cutting (up to 12mm plate would you believe?).

Anyhow all you do is drop the modified bit down into the recess and then undo the screw with a flat bladed screw driver using the slot in the top of the bit.

Ron Groves,
Cooloola Cove, Qld.

Comment: another reader has commented that long-shaft tamper-proof screwdriver bits are available on eBay.

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