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MXL 990 Microphone Circuit and Capsule Upgrade, Pt. 2

Here's the completed PCB for the MXL 990B microphone. Easy to build, and as I mentioned in Part One of this project, the instructions are very well written and clear.

After the board is populated, it gets hosed down with a flux remover.

I did this outside. It's not on the surface of the moon. It's the green tarp I paint over. That black/grey color is paint, not moon rock!

The backside (hee hee) of the PCB.

Here's one part of the build I wanted to highlight.

On the right in the picture is the switch to choose the cardioid (front diaphragm of the capsule) or omnidirectional (both diaphragms) pickup patterns.

Since the inputs from the capsule are high impedance, the connections are made up off the board, so that they are air insulated. The red arrow points to the joint between one switch terminal, the JFET, and a resistor. The front diaphragm lead will be soldered here in the final assembly.

The other side of the switch is for the omni pattern, and the back diaphragm lead will go there.

Now we need to disassemble the donor MXL 990 microphone and put the new capsule and circuit board in.

The headbasket (screen at the top over the capsule) just comes off with 2 screws.

Unsolder the two capsule leads and the three leads going to the output connector.

You can see the old PCB near the body of the mic. I'm saving it, but I'm not sure I'll ever use any parts from it.

There's a pedestal that the capsule mounts on. Two screws hold it to the chassis.

We'll reuse the pedestal with a new mount that comes in the kit.

The old capsule just slides out of the ring that holds it in place. I handled it carefully, and stashed it in the box the new capsule came in. Again, who knows if I'll reuse it, but I'm not throwing it away.

Unscrew that ring mount from the pedestal.

Screw the new "supersaddle" on to the pedestal. It's recommended to use some thread locker on the screw, so I followed that advice.

The new RK-47 capsule next to the old one. The new one is larger, and as I mentioned, will support 2 different polar patterns.

It also looks really cool.

Handling the capsule carefully, we mount it on the new saddle.

This tiny, 2mm screw has to go on the backplate (side) of the capsule. I magnetized it so it would stay on the screwdriver.

Here I'm putting that backplate lead on the capsule.

This picture is a little out of order in the assembly sequence. I wanted to show how the new capsule goes on the chassis just as the old one was mounted. The capsule leads go down through the pedestal just as the old one did.

Then slide the PCB on where the old one was, and solder the capsule leads to the PCB, as well as the output leads. Clean those connections with the flux remover - on a q-tip instead of spraying.

To protect and insulate the connections to the switch I highlighted above, I painted the switch joints with conformal coating. First time I've ever used it. It's like paint (smells a bit like nitro lacquer) and insulates that sensitive, high-impedance joint from becoming corroded.

With the PCB installed, the voltage for the capsule has to be set. This determines the microphone's sensitivity and self-noise.

Plug the mic into phantom power, connect one DMM lead to the chassis (ground), and one to a specific diode on the board (cathode end of D5 if you're building one...). I did this on my recorder since I don't have phantom power in The Dungeon!

The voltage is adjusted by a trim pot on the PCB. It gets set between 60 and 63 vdc. I set mine at 63 volts on the button.

There is a diode in the circuit that can be replaced with a different value to increase headroom and make the mic more able to handle high SPLs. I won't be putting this in front of any really loud sources, so I'm sticking with the stock configuration maxed out for best sensitivity and lowest noise.

Here's the assembled microphone after testing and the conformal coating applied. It sounds good in a very brief test, and worked perfectly. If you're careful and follow the instructions, this is a straightforward project. The main thing is to make clean solder joints, get components right down on the board, and deflux it.

Microphone Parts offers a custom headbasket for this project, which I sprung for. It has only one layer of mesh on it and looks nicer than the stock basket.

On the left is my finished mic. The old headbasket is on the right. It's painted the same sickly champagne green the body was. You may recall I resprayed the body a hammertone gray; I wanted a classic vintage German mic look and I think I captured it.

Now to go record something!


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