my new G7 is working but not working right - help please

[plumsolly]

I finished putting together my g7 in a mxl v69 body with cinemag 2461-NiCo output and dale m7 capsule. I'v got sound, but the polar patterns don't seem to work. polarising the back capsule at 0v gives a figure-8-ish pattern with no bass response, 80v gives an omni-ish pattern with more bass, and 160v gives a cardiod-ish pattern, but towards THE REAR ??? diaphragm and with lots of noise and intermittent pops. All the polarization voltages seem to be correct when measured between the 33m and 10k resistors referenced to ground. (i don't think i can measure any closer because of the high impedance, no?)  I've got 61.7v at the junction of the 470k resistors measured to ground - (low but it should work, right?) and Iv'e got 0v at ground on the mic referenced to case ground at the power supply. So something is going wrong in the high-z area right? I switched capsules and put in a one-side peluso cek-89. that didn't work at all ??? so i put the dale m7 back in and changed which was the front diaphragm, no help either. Thanks so much in advance, Ben

 

[gyraf]

..double-check what wires goes where between capsule and pcb..

Jakob E.

 

[plumsolly]

thanks for responding Jakob - yeah i've septuple checked - they are definitely right....
-Ben

 

[gyraf]

Could well be either the front membrane not grounded properly, or that the centre (backplate) dosn't get it's +80V. Check that you have +80 at the junction of the two 470K's and the 1G - and that this 1G is properly connected.

Jakob E.

 

[zebra50]

If you can manage it, some photos of the capsule mount and Hi-Z area might help readers spot something.

 

[ioaudio]

maybe the 1n cap?

-max

 

[rodabod]

Is your back-plate well insulated from ground?

 

[zebra50]

Is your back-plate well insulated from ground?

I think this is something to look at very carefully. I had a capsule recently in which the backplate was leaking to ground (via dirt in this case - it was an old capsule). Symptoms were weird pattern behaviour, lack of bass in some positions, low sensitivity etc.

As max points out, the 1n cap could be suspect or dirty - might provide a leakage path.

 

[plumsolly]

Thanks so much for the suggestions! It really is appreciated. I got it working now, but with the cek-89 capsule, so no switchable polar patterns until I can get the m7 in there. I think I had a combination of problems. first I pulled up the 10nf polystyrene cap. I had mounted it sort of sideways to get the body to slide on properly and it was sitting on the pad at one side of one of the 1G's and had melted a little. I had thought that the case of a cap would be non-conductive enough to sit there, but I guess this part of the circuit is super sensitive. I replaced it with a wima to get it to fit and be out of the way and I thought that would solve it, but i turned it on and had the same problems. I then explored rodabod's suggestion about the back-plate being isolated completely. I tried a few things and eventually put the cek-89 capsule AND MOUNT from the another mic in there and it worked. I think there was leakage through the metal post I had  my homemade capsule-holder mounted on (even though it has a plastic base). either that or my capsule-holder somehow made contact with the grill. Anyhow I'm going to get a new capsule mount either from budij or lewilson when they become available. even if mine worked electronically I'm afraid it was acoustically unseemly. Thanks again for all your help. I'll post pictures when i get it painted. and I'll report on the sound when i get everything going properly.
-Ben