[R-390] OT: Scratchy speaker

Mon, 18 Aug 2003 15:35:32 -0500

#1,

Thanks for the advice.  The problem is definitely coming from within the
voice coil.  I have the speaker out and can hear the "crinkle" sound when
gently depressing the speaker cone.  The "buzz" I hear is when running the
speaker directly from an audio generator so the mounts, etc., don't figure
in here.  That's how I knew the frequency range that produced the most
noise.

I'm not sure, but I think there are one or two small pin-hole punctures in
the dust cover.  I'm guessing someone stuck a wire or something into the
voice coil.  There is no grill cloth so the speaker is rather exposed to
whatever comes at it through the holes in the plastic grill.

I'm figuring a "new" speaker will eventually be needed, but I might just get
lucky and fix this one myself.  I'm guessing it isn't going to be that easy
and I'll probably destroy the thing in the process, but it's really not much
good in its present condition.

Sorry for the OT thread.  There doesn't seem to be a T.O. list with much
activity on it so you guys are some of my primary sources when it comes to
anything BA related.

Thanks again,

Barry(III) - N4BUQ

-----Original Message-----
From: Barry Hauser [mailto:[email protected]]
Sent: Monday, August 18, 2003 3:24 PM
To: Scott, Barry (Clyde B); R390 (E-mail)
Subject: Re: [R-390] OT: Scratchy speaker

Hi Barry III

Are you sure that buzz is from the speaker?  One or two of the caps to the
"tone organ" -- an early form of graphic equalizer ;-) -- are often bad.

So, try these in this approx. order:

1.  Try switching the tone switches around.  Also, try running the speaker
off an audio generator and see if you can replicate the buzz.

2.  As I recall, you don't have the correct bracket and bushings.  The
original bracket mounts the speaker to the chassis and has rubber isolating
mounts.  After 50 years, they're almost all still rubbery.  This mount
design is critical to avoiding rattles and buzzes.  Another related
possibility:  The 1 volt tubes and maybe even the 3V4 are of the "don't tap
during the shorts test" type.  That means the elements are delicate and
might also be microphonic-prone.  See if the buzz goes away when you apply
some pressure to the speaker frame or run the speaker removed from the
makeshift bracket you have there.

3.  Check the braided flex wires that go from the speaker terminals (or
transformer-to speaker terminals) to the cone.  They're in the back of the
basket.  Make sure they aren't touching the cone or the frame.  They should
have smooth bends, not be kinked.  (A kink can form a sort of audio
filter/tuning fork whatsits.) Make sure nobody loaded 'em with solder, like
a solder wick -- which is what they'll do.

3.  Assuming it's the speaker, if the spider (accordion pleated circular
dingus) is intact, and the voice coil dust cap is also intact with the glue
seal all around, it's highly unlikely that anything got into the cylindrical
voice coil gap.  What can happen is if the very edge of the voice coil
became brittle and the speaker were driven hard, some fiber particles may
have formed in there, but I haven't come across that yet with these.

4.  With more modern long throw, high compliance drivers, the cone surround
and spider can sag due to gravity especially in humid environments.  The
cure for that is generally to turn the speaker cabinet upside-down or flip
the woofer 180 degrees on its mounts.  However, the suspension on these
Zenith speakers is much stiffer and they're fairly well moisture resistant.
Could have happened.  Try running it upside down. You could try gently
pushing on the voice coil cap to listen for rubbing, but if you're only
getting this at 200 Hz, I doubt you'll hear anything.

5.  Check for any debris on the speaker, especially tiny metal filings.
Look closely in the frame side.  A small iron filing will dance on the cone,
held in place by the speaker magnet.

Just sort of reconnoiter as much as possible and sneak up on it downwind.  I
don't recommend running to remove that dust cap.  You can use some solvent
to soften the glue, but you will make a bit of a mess.  Even so, short of
removing the whole cone/voice coil assembly, you won't be able to get at the
voice coil slot fully.  Murphy's Law sez the grit will be on the wrong side.
Usually, cone removal is only done if there is a reconing kit in hand -- new
cone/voice coil.  Not possible with these.  Try everything else first.
Failing that, get a junker with a good speaker (and mount).

While this is a bit far afield of R-390 componentry, the incisive,
logical-deductive, hypocratic "do no harm" exercise is central to
three-ninety-ism credo and doctrine.

Hope this helps.

Barry (I)

----- Original Message -----
From: "Scott, Barry (Clyde B)" <[email protected]>
To: "R390 (E-mail)" <[email protected]>
Sent: Monday, August 18, 2003 12:45 PM
Subject: [R-390] OT: Scratchy speaker

> More investigation into this Zenith T.O.  The speaker appears to have
> something in the voice coil that is rubbing its magnetic housing.  It
causes
> a buzz at frequencies around 200 cps.  The cone itself is in good shape.
>
> If I remove the felt center cover, is there anything in there I might
could
> fix myself or is this a case of a blown coil and best to find a
replacement?
> I haven't done much other than fix tears in cones so I figure this is
> something more like meter repair where I can do much more damage than good
> in a very small amount of time.
>
> Thanks,
>
> Barry(III) - N4BUQ
>
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