MXL 990 mic after suffering a drop - Page 1
Forum section: electronics / repair

author: Yamin
author_rank: Frequent Contributor
timestamp: January 03, 2019, 09:33:34 pm
content: Hi I just got a MXL 990 (to have a look at) which has undergone a drop. It works but the high frequency response is messed up. Could the only explanation be a bad capsule.My question is how can I check whether its the capsule?I opened up the mic, the pcb looks really dirty as well with some kind of white residue all over. I haven't checked any components yet. What else could be a culprit for this issue?I'm really intrigued to know more about this circuit. I have included a schematic from: MXL 990 Zener which is almost the same as MXL 990. What is the function of the npn transistor Q2? What are the functions of the two pnp transistors? Other pointers will be greatly appreciated tooThanks in advance

author: floobydust
author_rank: Super Contributor
timestamp: January 03, 2019, 10:02:32 pm
content: Q2 oscillator and voltage multiplier generate the capsule bias voltage, guessing around 60-80VDC.Loss of high freq. response, the capsule tension is soggy. It's like a drum head, tensioning rings might be there or a screw. Unless it ripped.Repairing these, I check the input JFET to make sure it's biased OK. Measuring source/drain voltage.Usually spit or dust/metal chip shorts the capsule, if low output.

author: Yamin
author_rank: Frequent Contributor
timestamp: January 04, 2019, 06:14:04 pm
content: Hello floobydust, thanks for the heads up. I have attached some close up pictures of the capsule. How could I check the tension and which screw did you mean? I noticed that "inside of the capsule" is slightly not centered - is this normal?

author: DaJMasta
author_rank: Super Contributor
timestamp: January 04, 2019, 06:45:42 pm
content: I don't know what it normally looks like, but is that a crack in the clear plastic on the backside of the capsule?Basically any physical damage can distort the frequency response, and if it ends up being that, it's unlikely that gluing the crack or something would be a viable solution.It probably should be roughly centered, the screws probably are for adjusting the position. Also looks like from the top it's not quite sitting evenly in the outer ring, one side is slightly forward from the other. Any sort of physical imperfection can effect response, whether it will or not is a more complex thing than I can comment on.

author: floobydust
author_rank: Super Contributor
timestamp: January 04, 2019, 09:54:52 pm
content: The capsule chamber seems to have stress cracks but is intact.I would look for diaphragm cracks or tears, through the screen (end) with a microscope or magnifier.They are very fragile, 6 microns thick. Don't even think of touching it. The tension is fixed on the ring.Either it has lost tension or small chance the electronics are might be damaged, here are a few voltages to look at. You could measure the capsule capacitance, this schematic mentioned 41pF.More information on the MXL603s capsule, and teardown pics:http://recordinghacks.com/2013/01/10/dissecting-the-mxl-603s-capsule/"Be aware that this disassembly will probably render the capsule unusable." It must be dust getting in.This low cost mic gets lots of mods, from better capsule to better capacitors. The through hole PCB is worth keeping.I did not see much for original spare parts for sale.