Re-foaming Beyer M380

I’m lucky enough to own two Beyer M380, and since they’re both probably over 20 years old, their foam needed to be replaced. In one microphone (gold M380) the foam was almost completely gone, but for the other it was more of a precautionary measure – the foam had holes in it, but wasn’t exposing the whole capsule. Anyway, these guys can go bad if even a small hair gets in the capsule, so it’s a good idea to replace the foam as soon as it starts showing signs of breakdown.

Greg from Electrical Audio tipped me off to McMaster Carr and said to search their website for “the thinnest Reusable Polyurethane Foam Air Filter”. It’s this one, and in case the link goes dead in the future, the catalog number is 9803K301. The thinnest one is 1/8″ thick, and that’s the same thickness of the foam that’s already in the microphones.

The next question is the porosity, 30 or 60 pores per inch? I got both and compared them to the existing foam in the microphone. The answer is 60 PPI. See for yourself!

The difference between 30 and 60 pores per inch is very obvious:

One sheet was enough to re-foam two microphones with some extra material left over.

There’s really not a lot to it once the microphone is open. One side the body comes off completely and the other is holding the capsule in place with a couple of screws.

I started on the gold microphone because its foam was in worse shape. However, it was deteriorating so badly that I couldn’t use the old foam as a template. So in retrospect, I should have started with the black M380 whose foam was in a better shape. Anyway, it took a bit of experimenting to figure out how to cut the foam. Like I said, it’s better to use the old foam as a template, but if it’s in rough shape, start with a square that is bigger than the grill. Fit the square of foam in the “tub”, and cut the four lines for the corners. Now the foam is overlapping at the corners and need to be trimmed. This is kind of tricky and here’s what I learned: the body of the mic has these vertical slats, right? They go all the way around to the rim of the tub, so if you cut to much foam along the length of the tub, you might leave holes in the foam that overlap with a slat. That’s bad. So the trick is to cut the excess at the top and bottom of the tub. Also, make sure to first cut the lines for the posts where the screws go in. Anyway, that’s basically the only tip I have for how to actually cut the foam. Here are a couple of shots of how my foam pieces turned out.

And the two M380s fully re-foamed.

 

Old foam (from the black M380):

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Important: Don’t try to glue the foam to the grill, that’s not how it’s supposed to be and the glue will probably eat the foam. When I got my black M380 I was dumb and decided to glue it to the grill, and when I re-foamed it I had to clean out a bunch of old gunk. It sucked.

(This was done in September, but I only now remembered to post about it)

EV RE20/PL20 Repair, Part II

I took the top grill off and of course there was disintegrated foam everywhere. I cleaned the top grill and fixed the actual grill a little (it was pushed in).

The capsule has this little black plastic basket that protects it. It comes off by undoing the two little screws on the perimeter of the capsule. Be careful to not drop the screw driver into the capsule, though. I didn’t take a picture with the basket on, and I also started cleaning the capsule before taking pictures, but here’s the capsule with quite a bit of dirt still, and the basket before cleaning it.

A guy I talked to through the TapeOp messageboard said that he didn’t have to clean the capsule too hard to get the bass reponse back. I tested it in the condition shown above, but I still got no bass. So I kept cleaning.

At first I was cleaning with a q-tip dipped in 99% isopropyl alcohol. It worked, but I was told that the q-tip is a little too rough for the delicate membrane. So I switched to a fine artist’s brush dipped in 99% IPA. That worked well and it was better for when cleaning around the tiny coil wires or getting into the crevices. Also, the IPA is only necessary for getting the gunk loose. It can be brush off with a dry brush, and remember to brush towards the outside, away from the center.

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Clean membrane, but notice the off-center pole piece.

There are still some specks of dirt under the membrane, but now the bass response was back. Check it out. I don’t have a very boomy voice, but for comparison, you can check my friend trying out the microphone on his voice prior to handing it to me.

At this point I was ready to move ahead and order new foam, clean the whole thing and work on the wiring (low cut switch is not working). However, if you’ve spent any time reading about these microphones and the ways in which they become dysfunctional, you probably saw mentions of “baby rattle”. There are examples of it online, but basically the rattle is there when the microphone is used on low end sources with fast transients. So I decide to test it on bass, and sure enough, with some low notes that were loud enough, there was this rattle that almost sounded like static. Here’s what it sounded like.

Honestly, I don’t know if that qualifies as baby rattle, because baby rattle is the result of the pole piece being loose, and this one is not loose. I also noticed that the buzzing is still there, although to a lesser extent, when the microphone is facing away from the speaker cabinet.

I asked a friend who worked on a couple of RE20s about it, and he said that he had one with a similar problem and he believed that it was dirt dirt in the motor and that some notes would make it resonate against the coil. Whether this is the problem, or it is a variant of the baby rattle or another pole piece related problem, this is the end of the road for me. I do not know how to remove the membrane, I do not have the tools to do it, and I don’t think the owner of the microphone will want to pay to learn how to do all this on his microphone. So I contacted Ben at MicDaddy.com. He’s universally recommended for fixing EV microphones and his rates are supposed to be lower than EV’s. I still haven’t heard from him, so I don’t know what are his rates yet. If he’s too expensive, the owner of the microphone said that he’ll have me replace the foam, and then the microphone at least will be usable on voice.

EV RE20/PL20 Repair, Part I

A friend of a friend brought to me a very beat up PL20 to repair. At the moment it has no bass response and the capsule moves around. The foam had obviously deteriorated and needs to be replaced. I should say that I never worked on an RE20 or a PL20 before.

The first issue was trying to open the thing. YouTube has videos and people show how to do it, but basically there’s a small hex screw at the bottom of the top grill.

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Location of the screw

It supposed to use uses a 0.035″ key (confirmed via the Ampex Mailing List). The screw was stripped and neither my 0.035″ or 0.9mm keys would grab onto it. I thought that maybe the screw was seized, so sprayed some WD-40 on it. When you do that, cover the grill with a paper towel or a rag so no WD-40 gets inside. That didn’t help and the key kept slipping.

Removing a stripped set screw that is sunken in its socket is tough. Some people say to use a very small flat head screw driver. I tried with one but I didn’t have a good grip on it. I went to the makers space I’m a member of and found a Pittsburgh Precision tool that had a 0.9mm bit (which I was sure was square, but looking online it says it’s hex). I put the bit into the socket and tapped it lightly so it grabbed into the screw. Then I used a handle to turn it slowly while applying some pressure. The screw came out.

I should really get that Pittsburgh tool thing, it’s cheap and it came in handy. It seems sturdy, too.