MXL 990 While researching on the internet, I came across a lot of examples of people suggesting modifications to cheap condensor circuits (like the 990) to remove some of the limitations of the cheaper circuits and headbaskets. While a lot of people said they got good results from removing the internal two mesh grilles and changing some capacitors, I didn't find a whole lot in the way of instruction, so hopefully this might help assist someone considering these mods. First, removing the two internal mesh layers was pretty easy, but scary and time consuming. There are two halves inside, so I would recommend starting with one side, pulling the bottom edge up, then continuing to the rest. Once one half is off, It is easier to start at the top on the second half. Basically just get in there with some needlenose pliers and make sure you're just taking off the bottom two layers.
Next, I replaced capacitors C3 and C4 (both clearly labelled on the circuit board) with two .22uF poly film capacitors from RadioShack. If you're pretty familiar with soldering, this was a very easy task. Supposedly the circuit for the 990 and the mxl 603 pencil condensor are the same so information for the 603 (widely available, just search google) should also work for the MXL 990. After doing all this modification, I wanted to compare the first mic I worked on, with my other, still stock, 990. I recorded both through a DIGI 003 set on the same gain with no extra post-processing. I recorded both at the same time, without letting myself know which mic went to which input, sort of a blind test. To my suprise there was actually a pretty big difference. The Modified sound was a lot hotter and had more high end. In comparison, I thought the stock 990 sounded a little muffled. I liked it enough that I performed the same modification on my stock 990 also.
Headbasket with first half of internal mesh removed, second half started