Microphone diaphragms aren't so fragile(1). The big problem with this
issue is that most people don't know what's inside the microphone,
past the diaphragm, are terrified that they'll damage something, and
as a result get all spooky and do more damage than they otherwise
would. So they get shaky hands and oops, there goes the q-tip through
the diaphragm.
(1) Note, some early Neumann and AKG diaphragms were made of PVC rather
than PET and are subject to embrittlement due to loss of plasticizer
compounds. These diaphragms *are* fragile.
Microphones are cleaned all the time (when, and only when needed) _by
people who know what they are doing_ and can afford to make mistakes.
You have to allow yourself the mindset that if you break it, you can
afford to fix or replace it. Otherwise, leave it alone. The gap between
the diaphragm and the backplate varies from 10 microns to about 50
microns (that's 2/1000 of an inch maximum). Any pressure you put on
the diaphragm with any device like cotton, or a swab, will crash it into
the backplate. Get over it. So will a door slam, or a sharp snare drum
hit. If this causes damage, the diaphragm is already deteriorated to
the point where it can't be depended on for studio service. Make no
assumptions about the diaphragm unless you're able to check them yourself.
There is a lot of junk out there, particularly now that old studio mics
that were once tossed in a pile are worth $5000 and up. I have seen a
lot of mics that sellers claimed were "factory original" that were in
fact rather recently re-made (and the whole gamut from fraudulent junk
to factory-equal-or-better).
The accumulation of spittle and smoke on a large-diaphragm vocal mic
is often at least as heavy as the diaphragm itself. This causes readily
audible changes in response, and never (in my experience) for the better.
That it doesn't make it sound like an entirely different microphone is
proof that diaphragm mass isn't everything (in fact, the main controlling
component is the air cushion behind the diaphragm, but that's a topic
for another rant.) You need to remove this if the mic is going to sound
right. You also need to be sure that all external surfaces that may have
accumulated conductive deposits are clean so that you don't get fizzy
noises whenever the weather is damp. Particularly troublesome in this
regard are the Neumann and other mics that have a conductive spot in
the middle of the diaphragm, with an un-metallized band between it and the
diaphragm support ring. A thin film of spit, atmospheric crud, smoke, etc.
condenses on this ring-shaped band of clear plastic, and it doesn't
take much to bridge the gap. This deposit is often hydrophilic -- when
dry the mic works fine, but when breathed on, the moisture from your
breath combines with the salts in the deposit to form a conductive bridge.
Before you begin, scrub your hands absolutely clean with some harsh,
oil-removing soap. The object of the exercise is to avoid leaving
conductive or hydrophilic (water-attracting) deposits on the insulators.
My solvents of choice are isopropyl alcohol and distilled water. For really
awful cases, xylene is the backup solvent. I use a wooden-handled cotton
swab with an extra layer, about 1/2" thick, of long fiber cotton (rolled
cotton from the drugstore) wound on over the tip. Use a #2 or so sable
artists' watercolor brush, yes, the $20 kind, to remove the dust you can
brush off. Getting the brush a little damp by breathing on it (or better,
putting it in the steam from a tea-kettle so you don't make the deposits
worse) will make more dust stick to it. Almost all remaining deposits can
be removed with just water. Take your time, and keep the cotton damp but
not dripping so you have some control. When you think you have most of
the junk off, STOP. All solvents including the distilled water should be
kept in tightly closed bottles; pour out what you need into a dish and
throw it out when you're done. If the water isn't getting all the crud,
add a little alcohol (this is 91 or 99% isopropyl alcohol, with no other
chemicals in it, preferably lab technical grade at least but USP is good
enough if at least 91%) to the water, about 50% is usually enough. You
use long fiber cotton so you can see all the fibers you left behind, and
remove them before you reassemble the mic. On Neumann and other large
diaphragm mics with the clear band of plastic diaphragm between the
metallized center spot and the support ring, be particularly careful that
this band gets clearn.
Often, you'll see some particle caught between the diaphragm and the
backplate making a sort of "tent." You have to make a decision -- are
you going to tear the whole capsule apart, or send it to a good mic lab,
or buy a new one. Most of the screwed-together mic capsules can be taken
apart and put back together with no special tools except screwdrivers
that really fit the screws, and pin wrenches that fit the threaded rings.
Make a drawing of how it came apart, it has to go back together the same
way. Note however that some of these have the diaphragm tension held
constant only by the pressure of the ring -- no glue (these cannot be
disassembled safely unless you have a means to re-tension the diaphragm).
Measure the capacitances between each terminal and every other
terminal (for instance, a CK12 capsule has two diaphragms and two
backplates, that's four capacitances to measure) and confirm when you
put it back together than you're within a few percent of where you
started.
Modern diaphragms are made of mylar (PET, polyethylene terephthalate)
or PC (polycarbonate) and don't deteriorate much. However the metallization
may or may not be adhered well to the plastic. If you see pieces of it
coming off, or getting thin, STOP. A little missing gold won't hurt,
but if you lose some more it's not a microphone anymore. Metal diaphragm
mics use aluminum, nickel, cobalt alloys, stainless steel or titanium, all
of which can react with junk in the air to form corrosion products, which
are often conductive. If you see pinholes or cracks in the diaphragm, STOP,
use it as is if you can, otherwise get a new one.
Disclaimer and notice: Obviously these are delicate devices easily damaged
by people who don't know what they are doing. I'm not responsible if you
use this information and damage your (or someone else's) microphone. Only
you can make the determination whether you're qualified to attempt such
procedures as I have described here. Also, I don't do microphone repairs or
"upgrades" -- there are several people in the US and Europe who do, you're
on your own in selecting one.
Be patient, pay attention, take your time...
Josephson Engineering
|
Author
Topic: U87 Capsule cleaning video (Read 28804 times)