Still troubleshooting G7 ...WORKS NOW

I´m still trying to find out desperately what could be so wrong with my G7 mike. I just cannot get any sound.
In the last 2 weeks I checked for about anything I could think of with no results

I measured all voltages on the mic pcb according to the omnipressor site dealing with low voltages. I get exactly the same results.

Now my question:
with the capsule NOT mounted (but the mic connected to the psu) I get
1.4 VAC between the +/- output pins (the same on my output xlr). Now when I hookup the capsule there is 0.2 VAC. Would that be the normal values you´d expect to measure there?

How about some pcb traces being so narrow (I use Gustav´s pcb) and maybe solder getting right to the edge, that something could pull everything to ground? There are NO connections, I triple-checked, even with magnifying glasses!).

thanks for any clues ...this is really becoming a nightmare for me (expensive one too) ...

reggie

To be able to measure anything, you need to assemble the microphone completely AND mount it in it's housing with chassis grounded - including the wire mesh around the capsule. If you try it out-of-the-box, all you will hear is hum.

And hum it must be, your mentioned 1.4 and 0.2Vac. Connecting the capsule reduces hum a little, you say - this indicates that your amplifier is working.

Have you checked the microphone with a mic preamp? Does it hum, or is it just dead-quiet? Is there more noise compared to a shorted mic input at the same gain? If you tap the microphone at very high gain, can you hear a bit of "ringing" (that is microphonics from the tube)? Check that you have around 60VDC at the centre electrode of the capsule.

That's it for now..

Jakob E.

ok, I assembled the mic now as you said.

I have connected it to the xlr input of my Mackie Mixer, now it´s almost dead quiet. But when I turn up the input gain a bit more I suddenly get a loud "clicking" sound (that disappears as soon as I turn down the gain on the mixer).
I cannot hear any "ringing" from a microphonic tube when I tap the housing.
When I take off the wire screen to measure DC voltage at the center electrode I only get 0.7 V !?! And shortly after touching the center wire with the MMprobe I can hear a very loud hum.

You will NOT get a sensible reading at the capsule with standard test gear - impedence is way too high (in the 1 gig region).

thanks, that´s good to know, at least

Check for DC at the output of the microphone (at the XLR).

Double-check that your mic pre is working

Double-check your 7-pole cable between mic and psu.

Check that the tube heats up.

Check continuity from the output transformer in the mic to the XLR output

Check if it's the right tube you have. If possible, check it out-of-circuit to make sure it actually works.

Check DC current draw (when heated up) at both the 6.3V and the 160V lines. This will give an idea about what is wrong.

Jakob E.

Can someone tell me roughly what the HT current draw is for the G7 ?

The transformers I`m using are ones I happen to have in my workshop & have a higher HT voltage than the specified ones. I need to change the 10K dropper resistor in the PSU to get the 160v. Since I don`t know the current draw this is a bit tricky !

Rob,

It is some 1.5-2mA.

But you'll need to experiment with supply series resistors to get them right.

Also, use a 160V protection zener at the powersupply's HT output, so that you won't blow the 160V capacitors before tube is heated up and HT current draw starts..

Jakob E.

the microphone is WORKING NOW !!!

This morning I checked again my Mackie Mixer, just to be 100 % sure (thanks for your great checklist, Jakob!). I had checked it with 2 other microphones just 2 days ago and it was working perfectly. Now since yesterday I got this weird clicking noise and guess what, today I get the same clicking noise with my other microphones, no mic sound at all anymore!

So I just hooked up the G7 to my VTB-1 and went straight into the computer from there and it works perfectly :grin:

Now I´m just a bit worried as to what could have happened to my Mackie. I did not really put any extreme loud levels into it. I guess I need to have it fixed now

So I just want to say a BIG BIG thank you to everyone that helped me out with advice! I think I learned a lot from this, if it had worked right from the start, I never would have gone into such detail with this circuit ...I´m sure I benefit a lot for my next project.

btw, the mic sounds fantastic ... I will record with it right away :grin:

thanks,
reggie

Well done, Reggie!

Now record some music, before you get stuck here!

:grin:

Jakob E.

well, I guess I´m already stuck here :grin:

there´s just too many interesting things to read!
I know, sometimes I should make more music ...

reggie

Great work. Any pictures?

Gustav

Thanks for the info Jakob

what capsule did you use??

@jeroddumas
I used the CEK12 capsule from John Peluso. After what I heard from just today it sounds amazing to me. It´s definetely the best sounding mic I have ever owned (but I don´t have any Neumanns, can´t afford those, I just have a Studio Projects TB1 and an AKG).

I will post pictures soon. I can borrow a Digicam in a week or so, then I will upload them.

reggie

pics? :grin: