On Jul 28, 6:29*am, Ron wrote:
On 28/07/2010 14:11, Arfa Daily wrote:





Owner of local high-end hifi store that I do some service work for,
bought these in with the surrounds rotted away on the bass drivers.
Apparently, these things were 8 grand apiece 12 years ago when new. He
managed to get some new cones / surrounds from the States somewhere for
a reasonable price, and fitted them. One of the amps also needed a new
level pot, so he put one of those in, too. When he repowered them, one
worked perfectly, but the one that he had put the pot in, buzzed so
loudly that it nearly destroyed his new cone work. He couldn't see
anything that he had done wrong, so he sent them both to me. One double
hernia later, I had them both up on the bench. They are signal-sensing
for main power turn on, so first I had to work out which was the relay
for this, and bridge it such that the amp could be powered via a variac..
At about 40v input, the amps started working (separate amps for bass and
mid / top drive) but the bass amp started motorboating violently. I
spent a very long time checking caps in the power supply and bass amp.
Interestingly, I found one with very poor ESR on the driver board, but
replacement made no improvement, and then a 470uF that read very good
ESR - in fact so good, that it rang alarm bells, so I whipped it out and
put a good old analogue ohm meter across it. It was short circuit (near
enough). Remember we were talking about just this a couple of days back
in the thread about SMPS cap testing, where someone asked if an ESR
meter showed a cap as good, could it still be bad, and someone else then
commented that a straight ohms test should be carried out "for sanity" I
think was the phrase. Well here we are with a good real life example.
Anyway, that turned out to be a red herring too. I don't know what that
cap did as I have no schematics, but a replacement had no effect on the
instability problem.

At this point, having already spent too long on the job, I phoned the
store owner to tell him where we were at with the problem. He then went
through what he had done again, and then casually tossed in that the
voice coils on the bass units that he had re-coned, were unusual in that
there were two of them per driver ...

Now some real alarm bells started to sound in my head. I asked him if he
meant that the drivers had four wires going to each of them, and he said
yes. I hadn't actually seen the backs of the drivers, as they come out
from the fronts of the cabs, and the backs are packed with damping
material. I rang off, and pulled one out, and indeed, there were four
terminals, two of which had a fairly heavy piece of twin connected to
them, and the other two, had a piece of thin screened wire connected to
them. Looking at the soldering, he had definitely had these wires off,
so I removed the screened wire, and reversed it. That cured all the
problems, so I have to conclude that the second 'voice coil' is in fact
some kind of feedback winding, and when the cone went say forward, this
winding, being reverse connected, was telling the amp that it had moved
backwards, playing havoc with the stability.

In many many years of servicing all sorts of hifi and audio
amplification equipment, I don't ever recall having come across a bass
driver with a feedback coil like this.

Didn't Phillips do something like this back in the 70's? Motion feedback
or some such daft name?

I think they called it 'David', it was a motion feedback woofer in a
powered-speaker
form factor. This kind of system can control the low frequency
behavior
of the cone very accurately. Mossbauer apparatus uses the same
hardware for motion control with .01% distortion, but that's a
linear-motion
measurement, no idea what the air movement effectiveness is.