[R-390] OT: Scratchy speaker

[bw]

Barry,

My vote is that this is interesting. I'm learning a bit about speakers here.
A radio is a bit useless if you don't have some sort of speaker hooked up,
including headphone speakers.


Barry non-Hauser


> 
> 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

> 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)

> 
>> 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