Testing Blown Speakers

From detritu–(at)–x.netcom.com Sun Nov 28 22:12:39 CST 1999
From: Lord Valve
Newsgroups: alt.guitar.amps
Subject: Re: Test for Blown Speaker
Date: Sun, 28 Nov 1999 15:47:40 -0700
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Xref: geraldo.cc.utexas.edu alt.guitar.amps:217146


>Any good quick tests for a blown speaker – like visual
>or DC resistance?

>Thanks in advance.

Lord Valve Speaketh:
DC resistance is sometimes a clue. If you don’t see
any, it’s a fairly safe bet that the voice-coil is
open. 😉 Sometimes the DCR can rise if the coil
is burned, or lower if the coil has shorted turns.
In order to make any useful measurements, you’ll
need a 4-1/2 digit DMM with a 200- or 20-ohm scale.
You’ll need to allow for testlead resistance, too,
since this resistance can fool you if not taken into
account. You can also *gently* flex the cone in and
out with your fingers; you’ll need to make sure you’re
not tilting the cone in any direction, because this
will cause the same rubbing/scraping sounds that indicate
a bad VC. The *best* way to check a speaker is to put
it on your bench amp and sweep it with a signal generator;
some rubs may not show up unless a specific frequency
is fed to the speaker. I do a preliminary test by
“shaking” the cone up and down at 6 Hz; this freq is
too low to hear (unless you’re a whale or an elephant)
so any noises you encounter while doing this will
be from the coil, suspension, etc. Be careful not to
clip the amp while doing this. A tube amp is best (I
use a Dynaco MK III) since many SS amps have low-freq
protection that either filters freqs this low before
they reach the input stage, or interrupts the speaker
output or shuts down the DC supply to the output
transistors. Speaker tests are best made with the
speaker *removed* from the amp, as cabinet rattles
can masquerade as speaker defects.

Lord Valve

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From rich–(at)–imeelect.com Mon Nov 29 12:53:10 CST 1999
From: Rich Koerner
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Subject: Re: Test for Blown Speaker
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Date: Sun, 28 Nov 1999 22:13:59 -0500
Xref: geraldo.cc.utexas.edu alt.guitar.amps:217208

> Any good quick tests for a blown speaker – like visual or DC resistance?
> Thanks in advance.

Blown would imply it’s dead. No sound.

Frozen in the gap, or the coil is open.

In addition to the suggestions given by LV, there is the THUMP Test.

You take the speaker and hold it up to your ear and Thump it close to
where the dust cap cover is located over the voice coil.

It should sound tight like a rack tom. Now if it sounds like a drum
snares making contact with the other head, you have a failure of the
cone/coil structure. Check the glue seems for separation. Re-glue
if found.

Otherwise, you have to rebuild it.


Rich Koerner,
Time Electronics.

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