Old Bird 21c microphone self-maintenance
(changing the protective covers)
The 21c mic evolved from the simple homemade "flowerpot" design, for which building instructions have been online at oldbird.org/mike_home.htm since 2002. While the electronic circuit design of the 21c is more sophisticated than the old "flowerpot" mic, the protective covers of both mics are essentially the same. The primary step in self-maintenance for both the old "flowerpot" mic and the 21c involves changing these covers -- the plastic wrap interior and the "cloth" exterior. Basic steps illustrating this process for the "flowerpot" design are inherent in its building instructions at this link. Instructions for changing the covers for the 21c are noted below:
i. remove cloth cover
ii. unscrew interior base from bucket
iii. remove plastic wrap and excess tape
iv. replace plastic wrap
v. insert interior base into bucket
vi. replace the cloth cover
i-ii video iii video iv video v video
i-ii
The material used for the cloth cover is thin polyester fabric commonly used for translucent drapes. This material can often be acquired at department stores by looking for white translucent drapery without any pattern. The specific material used in construction of the 21 mics is available from Fabric.com (item# 0380633). While polyester holds up relatively well against ultraviolet (UV) light, it still deteriorates over time -- especially in areas with high intensity UV (e.g., Mexico, Texas, Arizona). In such locations the polyester cover should be changed every 3 months. At northern latitudes (e.g., New York, Montana), this cover only needs to be changed about once a year. If you poke the cover with your finger and it goes through, it's time for a cover change. The thick dark rubberband that holds the cover in place holds up better over time (5+ years), though it loses elasticity. Other rubberbands or similar elastic bands can be used. The goal is to keep the polyester cover firmly in place.
Early models of the 21c mic (~2011-2012) used clear silicon sealant around the circumference of the cover to add extra durability. It yellowed and accured dirt and particles of debris over time. After 2012 this step was eliminated as the rubberband was found to be sufficient. If you have such an older model 21c, peal off as much as possible of the silicon in preparation for installing a new cloth cover.
There is a base inside the white bucket that holds the electronics and pyramid boundary. In order to change the plastic wrap cover that protects the microphone element, this base needs to be removed from the bucket. The base is mounted to the bucket with four screws visible on the bottom of the bucket. In the early design of the 21c mic, a dab of silicon sealant was applied to the end of the screws to prevent the nuts from unwinding off during shipping. In such cases, this makes unscrewing the four nuts to remove the base more difficult. For best results, pull off as much of this silicon as possible, then insert a long-necked Phillips head screwdriver inside the bucket to hold each screw steady while simultaneously unscrewing the nut with pliers. Beginning in 2013, tape was used to hold the nuts on during shipping and since 2015 a rubber band was used -- in these cases the nuts are easily removed. Nothing is needed to hold the nut on when it is not being shipped or transported long distances.
Once the nuts are off, the base inside the bucket can be jiggled and lifted to free the screws from their holes and then be pulled out of the bucket. You may need to ease the audio cable back into the hole on the side of the bucket to facilitate this process.
iii
The plastic wrap cover tends to get brittle over time from UV exposure. Generally it is a good idea to change this cover at least once a year, if not before each migration period. Removing it from the aperture of the pyramid is easy -- just puncture the plastic with your finger and peal it off. In construction of the 21c mic, 2 inch-wide translucent polyethylene tape is used to hold the plastic wrap cover in place. This too wears down over time due to UV exposure. Pulling off the old tape is relatively easy if the mic has been mounted outside for a year. More effort may be needed if the mic is relatively new or has been kept inside out of the sun (in which case the covers don't need to be changed as often). If your aim is simply to change the plastic wrap, there is no need to disconnect the pyramid from the circuit board and remove it from the base as shown in the video. Once you have removed the plastic and as much tape as easily comes off, you are ready for the next step - replacing the plastic wrap.
iv
The goal of this step is to waterproof the microphone element that resides inside the apex of the pyramid. If drops of water reach the mic element, it will cease functioning properly, soon corrode, and need to be replaced.
The plastic wrap cover material used in the 21c mic is called "Reynolds Foodservice Film 914M". It is polyvinyl chloride, likely around 50 gauge thickness (.0005 inch) -- a bit thicker than "Saran" wrap from a grocery store, which is typically 35-40 gauge. Any plastic wrap around this thickness will work. Typical plastic garbage bags are a hundred time thicker and block too much sound to be functional.
To replace the plastic wrap cover, cut a piece of new plastic wrap at least 12 inches x 12 inches. Lay it over the aperature of the pyramid and tape one side about 4 inches down the side of the base. Turn the base around and gently pull the plastic wrap over the opposite side. Cut off excess and pull (but don't stretch) the wrap until most of the wrinkles are gone over the aperture -- then tape that side about 4 inches down. Repeat this procedure for the other two sides, each time carefully removing remaining wrinkles without stretching the plastic wrap. The result should be a smooth or nearly smooth, taut but not stretched, membrane over the pyramid's aperture. In good light, inspect the membrane for perforations. If you find any, start over and repeat the process. Once you have a good looking layer, add extra tape so that any flaps of plastic wrap sticking out from the walls of the base are conformed to the walls. Also, consider adding a piece of tape over any exposed holes associated on the circuit board side. As noted in iii, polyethylene tape may be used, but other varieties may work as well.
v-vi
Carefully reinsert the base into the bucket, bolt it in, and add a fresh cloth cover. Bolts can be inserted in the holes of the base before lowering it into the bucket - aligning bolts with holes on the way down. Depending on the size of your hand/arm, you may be able to carefully reach in and push the bolt through the holes in the buckets, other times you may need a long-necked Phillips head screwdriver. It's important to be careful not to damage the plastic wrap cover during this stage. If you think you may have done so, remove the base and inspect the cover again. Once the base is securely bolted to the bucket, you are ready for adding the cloth cover. As with the plastic wrap, start with a larger piece than needed, secure with the rubberband, and then trim off the excess cover material. It is handy to keep around a 4-inches of extra cloth sticking out so you can pull it to remove wrinkles.