In the late 1970s, electret condenser microphones slowly started to push dynamic microphones out of the amateur recording market, and for a good reason: Electret condenser mics have a flatter frequency response, are easy and very cheap to manufacture, have a higher sensitivity than dynamic mics in the same price range and are ideal for your tape recording amateur who wants to pick up the acoustic wonders of mother nature or her grandson’s first sousaphone solo. If you look at HiFi catalogues of the late 70s, you’ll find that almost all audio gear manufacturers who were selling OEM microphones by other brands started switching to electret models, and by the beginning of the 1980s, there were virtually no dynamics left but the really, really cheap karaoke grade models.
Some of the more expensive models are actually very good small diaphragm condensers with sonic characteristics you’d never expect in a microphone by Teac, JVC, Technics, Kenwood or Universum. But the cheap and middle price range electret condensers by Monacor, Prefer and numerous cryptic brands like Yu Brother, H&H, Hanumpa or Chinon abound and can be found everywhere almost for free – whether or not you like their thin, harsh and peak-heavy sound and their tendency to distort even at fairly moderate SPL, or whether you even find it usable at all nowadays is entirely up to you, your phone probably has a similar sound quality like some of the lower-grade electret condensers sold in the 1970s.
Anyway, whether top shelf OEM product or run-of-the-mill cookie cutter electret, this new-gained quality came at a very low cost – a single AA battery, inserted into the microphone, to supply power to the electret. A very low cost indeed – only that the late 1970s were obviously the time in which many of these microphones were last used, and then put back into their neat cases. Now, in the year 2015, you find one of those on a local flea market, open it up, and BOOM: a horrible acidic mess. Not really acid, more like harmless electrolytic fluid, but still, the horror…
Don’t ask the guy „Good lord geez, why didn’t you remove the battery before putting the microphone back into the cupboard unter the stairs?“ Here’s the answer: Forgetfulness, lack of interest or even anarchy. Instead, buy it for much less than you would have paid if you hadn’t opened it up and don’t be afraid if your cool electret condenser microphone doesn’t work right from the start, the mess in the battery compartment most likely hasn’t affected the electret capsule at all.
The first thing you need to do is take out the battery. If you can, of course. Sometimes a lot of force is necessary to open the microphone, if the battery acid has spilled all over the metal parts inside the mic and they have oxidised. This tends to happen a lot with closed battery compartments, where you insert the battery into the mic and then screw on the capsule or the connector. One possible way to free it is the brute force approach: Use your electric hand drill to drive a long screw into the dead battery and then rip it out, seething with rage. Or remove it carefully, so you don’t rip the battery contacts and parts of the ciruitry out with it, if this is an option.
Once the battery has been safely removed, get busy cleaning the contacts. Usually there is one metal plate for the plus side and a spring or plate for the minus side of the battery. Now let’s check if the wiring on the metal plates is still intact. Due to corrosion, rust and battery acid, the solder points could have gone dead, or have lost the attached wire. Clean them thoroughly with a solution of baking soda and water, or whatever cleaning solution works best for you – to combat battery acid, anything goes, it is water-soluble and non-hazardous. For additional effect you can apply some contact spray once most of the metal is clean. If the plate is uncleanable or completely eaten up by the acid, simply find a fitting piece of metal, anything conductive that can be soldered – battery contacts from some old useless remote control are usually a safe bet.
If there’s a spring to maintain battery contact, chances are it has been damaged really badly by the acid and can’t be cleaned anymore. This can be fixed in the same way and with the same resources (grab one from the battery compartment of some dead electronic device), but requires a bit more handyman skill. Once you have clean battery contacts, you can resolder the cables.
If there is a splotch of dried battery acid on the inside of the housing, you can try using steel wool or sand paper to get it clean. Again, use anything that works best for you. Now it’s time to test: If you can hear your voice clouded by scratching noises or drowning in a mean howling ocean, the contacts might not be clean enough just yet.
Another way to upgrade your electret condenser mic and get it to work again, would be to simply bypass the battery and use phantom voltage. Most electret mics can handle the +48 phantom voltage without any problem, although a 47k resistor, or, if you want a balanced connection, a small circuit might be necessary. And while you’re at it, maybe you’d like to tamper with your electret condenser capsule a bit more and build a new preamp for it?