G7 problem solved !!

[seavote]

my past threads about my g7 are long and have solved some problems so im starting fresh with this one. this problem has me stumped for a while. i'm using gustavs pcb's. my psu is going to be a dual psu one side for my G7 the other for a royer mod. here is a photo:

The problem presents itself with or without the mic plugged in. with the mic attached im getting ther correct voltages out of the psu and in the mic circuit itself, everything seems fine. when the 3 pin xlr (audio out) is connected to either of my 2 preamps voltages read fine. when i plug the preamp out to my pcs' break out box the B+ voltage drops from 170V to about 110V.??? since i had fried my psu circuit due to an ac outlet that was wired incorrectly and much of it was wired point to point i decided to populate a new psu pcb (and a new mic pcb while i was at it, i purchased enough of everything to make 2 when i first ordered)to see if the problem was located on the pcb boards. both the mic and psu are neatly and cleanly put together.
my break out box is an M-Audio delta series break out box with 1/4 balanced inputs. the only components that are the same are the ac inlet,power switch, the indicator light(which i connected in parallel across the + and - ac wired as i had seen in some guitar amp schematics) and the transformers. where should i be testing next. i'll be getting a new xlr to 1/4 balanced cable and checking that although the cable tested ok with my dmm. please help im real close to getting this up and running. i've got parts for other projects waiting patiently but i wont start them till this is up and running. thanks

 

[stinky]

Is there a DC voltage on the signal pins of your XLR jacks? Pins 2 & 3 of the output XLR, or the pins that carry signal on the XLR jack that feeds power to the mic? If there is a voltage drop when you connect the output to the rest of the world then there may be some current floating there that shouldn't be.

 

[seavote]

i'm getting between 6.5 - 7 V ac on those 2 pins. when i measured for dc the meter moves quickly up and down, up and down, from 0V slowly raising to close to -400 mv, then droping back down to 0V again and repeats the cycle. is this Voltage negligible? or is that the problem?

 

[stinky]

You get 6.5 - 7VAC on the output without the microphone connected? Or is this with the microphone connected? With the microphone disconnected and the PS not plugged in to anything else, I would expect to see no voltage on the outputs. Neither AC or DC. I guess I would start by checking that I have a solid ground connection. Make sure every point that goes to ground really does go to ground. Double check the 7pin XLR to make sure there aren't any errors. And check continuity of your cable that goes between the mic and the PS. Be sure that electrons are going where they are supposed to go. The G7 has a transformer on it's output so unless there is a wiring error, there shouldn't be a DC voltage on it's output. And even if the microphone is connected and it is in front of a live rock band, I don't think you'd be getting 6.5-7VAC signal at it's output. I had a frustrating dilemma on one of my royer modded mxl 2001s that was the result of a poor ground connection. When in doubt, reflow!

 

[Gus]

http://www.groupdiy.com/index.php?topic=24011&highlight=microphone

is this the same microphone?

 

[stinky]

Have you checked that your chassis is truly earthed and doesn't have a voltage floating on it? If you've got a good chassis ground connection to earth double check that all points that go to ground or 0 V truly make it back to the ground plug on your power cord with little to no resistance.

 

[seavote]

stinky: ac voltage readings were without mic attached. is that voltage just a big ground hum? i haven't bothered to listen to the mic.

gus i didn't seem to be getting to far with that mic so i built a new one to narrow the possible sight of the error down. same capsule and tube but the rest is new.voltage divider resistors are matched.i had everything else ready to go for a second G7. for the psu everything befor the pcb is the same and everything after is new.

earth to chasis connection is fine. the only place i have the circuit 0 volt ground connected to chassis is on the xlr ground lug in the microphone.
now if i understand correctly current to this lug travels through the shielding in the cable and on the other end of the cable(psu side) connects to chassis ground through the 7 pin xlr. am i wrong about this? when i tested my 7 pin xlr in the past i checked all 7 pins for continuity. i never tested the sheilding. tonight i relized i had no continuity from the ground lug of the mic xlr to the 7 pin lug, or the chassis in the power supply. shouldn't i get continuity here?
since the b+ voltage drop occurs without the mic connected to the circuit i should be looking elsewhere for the answer to that problem???

 

[stinky]

If I understand what "ground lug" you are talking about, no that does not make contact with the shielding of the mic cable unless you solder a connection to it, and I have never done anything with those "ground lugs" on a connector. If I am thinking of the same "lug" then it sounds like your 0V point never gets to ground, which could cause problems. To double check; open the mic body up and find one of the 0V points. That should make it back to chassis ground (ground prong on your power cord). If that isn't grounded it could definitely cause problems like this.

 

[seavote]

after posting i tested some other xlr cables and relized that the ground "lugs" on the xlr connectors are not connected. i've reread the g7 post in the meta and from what i can tell i have grounded the mic correctly. 0V does not always have to go back to chassis ground does it?
at this point i'm going to hook up 2 alligator clips and test different ways of getting 0V back to chassis. any other possibilities? could my transformers being out of phase have any thing to do with this? i believe they are wired correctly as the present hook up was the only variation that gave me the proper voltages.if this is worth exploring i can give tranny specs and other info.could the heating of the second transformer have caused some damage that could cause this symptom? thanks for any help.
edit: just checked back at the old thread. i noticed gus suggested a possible connection from pin two or 3 of the xlr to the phantom resistor in the pre amp. i never checked this. i'll check this now .

 

[stinky]

How did your tests go? I hope that getting your 0V to ground helped your microphone. I read some posts from gyraf and he spells out that 0V goes to ground. I'm curious, thanks!

 

[seavote]

my G7 is up and running!!! my power supply is a dual psu. after the second(reversed) transformer i split the signal using a 2 row ,2 sided barrier strip and eyelet connectors on my wiring. from there it goes to the two psu pcbs. a screw on the 115V side of the strip was grounding out on the psu chassis. the mic sounds great. thanks to gyraf,gus,ioaudio,stinky,babyhead,steppenwolf,cj and any others who put up with my inexperienced sometimes ignorant ramblings. ive learned plenty

 

[gyraf]

:thumb: Time to change thread title, then.. :wink:

 

[seavote]

done! :grin:

 

[ioaudio]

:sam:
congrats!