[R-390] OT: Scratchy speaker
Bill Smith
[email protected]
Mon, 18 Aug 2003 18:01:49 -0700
Tips on repairing speakers:
Open windings: First test of any speaker. Connect an ohmmeter to the
speaker terminals and listen for a pop and an indication on the meter. Of
course if you are testing an electrodynamic speaker, you won't hear a pop
since the field magnetism is only developed when current is flowing through
the field coil.
Look carefully at the speaker terminals and also where the flexable wire
meets the paper. A small solid wire (the voice coil winding) is attached
there. Trace the wiring back as well as you can with meter probes. You may
need to use a safety razor to carefully scratch away insulation to make such
tests. Use a magnifying glass to visualy inspect the wire. Hopefully you
will find a small break. Care and patience yeald great rewards here.
Torn cones: Even cones that someone or something put a fist or a foot
through can generally be patched back together with airplane glue (clear
Corona dope). Get the "non-shrinking" variety from a hobby store. The
glue is sold to afix fabric to the superstructure of a model airplane.
Don't use clear cement such as commonly available in a tube. Most brands
continue to shrink over time and will introduce serious warps in the speaker
cone.
Use a toothpick to apply. Wet the torn edges of the speaker paper with only
very small amounts of glue on the tip of a toothpick and gently reposition
the paper. Use another toothpick if really torn. Keep the top on the
bottle of glue as the solvent evaporates quickly. Also, use in a well
ventilated area. Use small amounts of toilet tissue to fill holes. Use a
minimum of glue, and inspect the cone carefully for small inciedental tears
and holes. Even the smallest tear will make a loud buzz at some frequency.
Use a minimum of glue, it is readily adsorbed by the paper. Don't be too
worried about excess glue, either. It will shrink quite a bit as it dries.
Don't worry about how ugly the speaker looks, either. It will sound as
good as new if it is not warped.
New applications of glue will remelt old glue, so you can try again if you
aren't successful. Avoid warps and try to retenion the paper as it was when
new. Usually the paper will tend to fall back into place without much
effort.
Scratchy voice coils: There are three causes of scratchy voice coils. (1)
dirt (2) warpage (3) loosened windings. Dirt can often be removed by
running 60hz ac through the speaker to shake it loose with the center felt
removed. Best is the hum from an old radio that needs replacement filter
electrolytics. It often helps if you gently excercise the voice-coil with
your fingers during this process, and hold the speaker upside down. This is
especially true of speakers that are mounted face-up, such as in a
Hallicafter's S-38. As an aside, S-38 electrolytic filter
capacitors have an amazing ability to reform. Let them run a while before
you decide you need to replace them. You will think the radio is going to
blow up, but eventually the receivers quiet down again, and are servicable
with the original electrolytics still in place.
If the speaker is still scratchy after an attempt at cleaning, it could be
the cone has warped and the voice coil is rubbing against the magnet. The
solution is to insert small wedges, perhaps from sheet-plastic or heavy
composition paper that has been cut into strips, between the voice coil and
the magnet. Again, the center felt has been removed to expose the voice
coil and magnet assembly. The strips should be snug, and will become more
snug as you install them around the voice coil, but don't force them. If
you can't install them symmetrically around the magnet, you may need to use
thinner material.
With shims in place, wet the cone with a damp cloth or sponge (don't
saturate) and let it dry thoroughly. If you are successful, the paper
mosture will relax the stress that caused the warp. Make sure the cone is
dry before removing the shims.
Unfortunately, there isn't a ready solution for coil windings that have
separated from the voice coil. Typically, speakers that sound highly
distorted with weak sound will also feel very scratchy when the center of
the speaker is pushed gently by hand. If this is the case, or if the
speaker winding is open somewhere inside the voice coil, the speaker will
have to be replaced or reconed.
73 de Bill, AB6MT
[email protected]
----- Original Message -----
From: Scott, Barry (Clyde B)
To: R390 (E-mail)
Sent: Monday, August 18, 2003 1:35 PM
Subject: RE: [R-390] OT: Scratchy speaker
#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
>
> _______________________________________________
> R-390 mailing list
> [email protected]
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