Analysis of damaged/defective parts (ISO9001)

[harbethpr]

Thanks to our excellent QC control during design and manufacture, a Harbeth speaker system is capable of giving many years of service; many of our speakers sold in the early 1980s are still in daily use! Unfortunately, accidents do happen to speakers in the User's home and using our new video analysis facility we can not share with you some of the (few) problems we see. The most significant issue concerns the (rare) failure of the driving power amplifier - 'going DC'. If you do purchase an old amp please consider having it serviced. Amplifiers, just like all of us, do age!

We have made some experiments to find the smallest video file size that offers the best picture quality, voice quality for the spoken commentary and compatibility. We've decided to use MP4 as this produces a very small streaming file size that should open on PC, MAC
and Linux.

On a PC, if you do not have a player that can handle MP4 video (or indeed any other video files) we highly recommend and use the free open-source GNU Public Licence VLC Video player which seemingly can play any and all video/audio file format.

[harbethpr]

Download the video dissection of this drive unit here. Commentary and dissection by Alan. May take a few seconds to commence playing on your PC.

Conclusion: Voice coil shows no sign of excessive movement so it is unlikely that the speakers were being played too loud.

Our verdict: Destruction of these drivers was caused by suspected amplifier fault whereby the amplifiers output stage short-circuited and the PSU was directly connected to the P3s woofers. Both bass units in this pair of speakers were equally effected.

Timescale: from time of amp failing to irreversible destruction of drivers: perhaps 30 seconds.

[A.S.]

This drive unit was returned as "Warranty Claim". Download the video dissection of this drive unit here - an MP4 streaming file. Commentary and dissection by me. Will take a few seconds to commence playing on your computer. (On my PC it plays inside my free QuickTime player).

Conclusion: Drive unit destroyed by excessive power applied by amplifier (probably about 200W). Not a Warranty issue.

Our verdict: Destruction of this driver was most probably caused by an amplifier fault. which connected the amp's PSU directly to the woofer's voice coil. The failing amp produced a loud pulse or 'pop' which initially caused the voicecoil to gump outside the magnetic 'gap'; it then rapidly became so hot (200 deg. C or more) that it cut its way through the cone like a cheese wire through cheese.Once the voice coil was displaced from the gap, there was no cooling effect and thermal runaway occurred. The user could do nothing to save this drive unit - by the time he had heard the pop, it would have been too late.

Timescale: from time of coil jumping from jap to irreversible destruction of driver: about 15 seconds.

Suggestion: Don't use such a powerful amplifier as the more potential energy in the amp (i.e. the higher its watt rating) the more potential for damage to the speakers if/when the amp fails.

Postscript: Amplifier faults are nothing new. In the 1970's when QUAD introduced the revolutionary 100W current-dumping 405 amp, they included a crowbar protection circuit which continuously monitors the output terminals. If excessive voltage is detected indicating a failure of the amp, the protection circuit leaps into action and short circuits the amp's outputs, which blows the amplifiers HT fuses. The speakers are saved.

[A.S.]

We've had this (pair) of drive units returned because the customer's amplifier failed. This has destroyed the voice coil which has become so hot that the varnish on the enamelled copper wire has boiled and bubbled

Beware of using an old unserviced amplifier bought second hand at a flea market - it can be a very expensive mistake. Amplifiers, just like cars and -shock, horror, people - do age. My advice is, if the urge to buy is irresistible do at least test the amplifier with some old speakers. But that will not insulate you from instantaneous failure which does happen.

I urge you to ask yourself this essential question before buying second hand "who can I turn to who has the specialist knowledge and source of parts that can restore it to 'as new' and put it right when it fails at a reasonable price in a reasonable timescale?". If you can not satisfy yourself about this do not buy.