MXL’s 991 is also known as the 603. The model 551 appears very similar, as do other MXL pencil small condenser mics.

That completes the mods so far. The mics are flatter, more open and natural sounding. They are suited for string instruments, room mics, and vocals with a windscreen / pop screen. No, they don’t sound just like KM84s, but they are “84-ish”, excellent mics in their own right. See this post for frequency sweeps, but the main result of the electronic and acoustic mods is to give a more natural voice to these inexpensive mics. Sound sample here.

Next I opened the “gill slots”. Grinder & files.

On reassembly, the screen is simply dropped in in front of the diaphragm. It’s not firmly glued in place, so you need to be careful not to poke at the mic in the future. While I had the mics apart, I stripped off the paint and polished the bodies with steel wool and paste wax. On reassembly my capsules look like this. 991 capsule (brass) on left, 990 (silver) on right.

Don’t lose those clear plastic spacer rings!

They fly away if you breathe hard.