To raise the high frequency output, you can try increasing the blind hole size by one drill bit size (I went from #58 to #57 and the high end comes up a bit). Also, what is the depth of the blind holes? If you make them a bit deeper, that can increase the HF response as well. I find 4mm a good depth for any of my capsules. One of my first capsules I tried had a 2.5mm blind hole and that didn't work at all - it sounded like a telephone. It seems as though one wrong move with a capsule and you get a telephone frequency response.
Yes, I use weight to tension my capsules, but I also will check the capacitance with and without polarization to check for tension. The weight works well if your initial tension is accurate. Mine isn't so I add the capacitance check. I used to check for diaphragm suck-in voltage except that if there are any fingerprints on the backplate, you can have a problem with the diaphragm sticking to the backplate and not wanting to release due to the finger oil. I quit doing that now, although I now added a few cleaning steps to the capsule making process in addition to not doing the diaphragm collapse test. Neumann glass-beads their newer capsules (K67/K103 - maybe more) to avoid this problem.
Most of this is from observation and experimentation, so I don't know how much is 'fact' and how much is just coincidence, but....
Highly damped capsules like the M7 and C37 seem to be less fussy as to diaphragm tension. The K67 and CK12 capsules are quite fussy from what I've seen. In a damped capsule, mostly you set the diaphragm to be at a decent tension, but the exact frequency is not quite as critical. There is a bit of a peak there so you can 'voice' the mic differently. But I find it subtle, there seems to be far more effect on a slightly inaccurate hole drilling.
The K67 requires identical tuning for both sides, or the sound isn't right in any pattern, except possibly cardioid on one side. Cardioid on the other side won't sound right though. It usually takes me two or three tries on each side to get a K87 to sound right.
The CK12 capsules have the diaphragm resonance as part of the acoustic circuit so they need to be tuned not only to a specific tension, but a specific resonant frequency. I've only tried re-skinning a nylon CK12. I got it to sound about right but it's hard to deal with. I also tried making a CK12 clone. I might try it again some day, but I've been quite busy making M7-like capsules. I think Tim has quite a bit of experience with the chambered capsules and I don't really want to duplicate all of his efforts.
That is a very impressive amount of progress you've made in relatively little time!
-Dale
