G7 PSU Help Needed

[Guest]

Don't get discouraged. You are making progress and learning as you go. Good news on the transformers.

I am not sure what the writing on the rectifiers was but as long as you have good ones now we can go to the next step. Bridge rectifiers are usually marked to show which pins are AC and which pins are DC in different ways. I think maybe the rectifier is in wrong and is blocking the signsl. To be sure we need to measure the rectifier. If your meter has a diode setting use this, if not, use the resistance setting With black lead (common) on an AC pin, touck the red lead to the DC pins. It will have a 0.5 or so reading on diode check mode (1M resistance) to negative terminal and block the positive terminal. If you are not sure of the treminal. touch the black lead to a treminal and the red to the terminals on either side. When you find the one that passes on either side, that is the positive one.

Measure the bridge and make sure it is good. You checked caps already so after the bridge, look at the regulator. Are the pins correct? We will measure this when you get voltage going.

 

[reggie]

thank you again for your patience!

now it seems that my psu is working! I did not get the DC due to bad connections on the pcb from de-soldering too many times. I fixed that now and I could trace both correct DC voltages (6V and 124V) to the psu outputs.
I also checked the rectifiers as you describe, I really learn a lot from this )

Then I hooked the microphone up, but for now all I get is a loud hum. So maybe I did not connect ground right somewhere? Should the ground from the psu pcb be connected to chassis ground? (though I did that and it changed nothing).
When touching the mic I get an even louder hum.
I remember having read something about connecting the 0V/gnd and Out- and F- from the mic pcb to the mic chassis?
Would it matter which way the capsule front/back are hooked up, is there any difference?

thanks,
reggie
(seems like I´m getting closer now to hearing my mic for the first time ;-)

 

[Guest]

Good job on the power supply.

The hum sounds like a ground is not right someplace or a capicitor is not filtering. Looking at the schematic, 0v/GND should be to pin 1. Connect this to the mike chassis. Heater 0v is on pin 7 and should remain seperate for now. Do not connect the signal to anything else.

Check to make sure your pins are all correct. Do this with an OHM meter from one end of the cable to the other. You want ground reference between the power supply and the mike. The mike body is the ground/shield for all the circuitry there so it should be on pin 1 as well.

 

[gyraf]

..and make sure that you have 0V/Ground connected to chassis (mic housing) at the microphone end!

 

[AudioJunkie]

In another discussion, someone had a problem with a broken trace.

I asked if he was using Gustav's boards because noticed on mine that the trace from 0V/Gnd to F-/gnd was incomplete on the tube end by the F- pad. And I asked if that was the problem on his board, but he never got an answer.

Here's the thread: http://www.groupdiy.com/index.php?topic=566&highlight=g7+hum

Daniel

 

[gyraf]

Daniel,

Could you post a pic of the pcb? or somehow show us what you mean?

The G7-pcb's I got from Gustav looked right?

Jakob E.

 

[AudioJunkie]

Here's a pic. My camera doesn't do very good close up.
http://pg.photos.yahoo.com/ph/klarinet_18/detail?.dir=9ef2&.dnm=c735.jpg

Daniel

 

[zebra50]

I'm not sure this matters, does it? It gives you the choice of grounding the heater supply somewhere else - for example at the power supply (or even not at all).

 

[gyraf]

Thanks, Daniel!

That must be an error..

You could either just solder on a wire bridge, or - as Zebra suggests - experiment with different heater grounding..

Jakob E.

 

[reggie]

0V/Gnd is definetely connected to pin 1, but for sure not connected to the mike chassis (I didn´t think about that)! So I will do that tomorrow morning right away.
I will also connect the mike end metalpiece to 0V/Gnd then (this is separated from the rest of the mike body in my construction by a rubber piece).

I do not quite understand what ground reference between the power supply and the mike means. Should the mike body be the ground for all the DC circuit, and the Psu Chassis be the ground for the whole AC circuit?

thanks,
reggie

by the way, today I hooked up successfully an old Siemens 24V psu and a V276 that I just bought ...no problems there ...makes me feel already a bit better ))

 

[Guest]

For this circuit there should be common grounding. The mike metal body shields the circuit inside when it is referenced to ground and gives a nice big hunk of metal to be grounded to. You should ground one point in the mike body, close to the connector. The same ground is in the cable and in the power supply box. Inside the power supply box you should star ground at one place.

 

[AudioJunkie]

[quote author="gyraf"]Thanks, Daniel!

That must be an error..

You could either just solder on a wire bridge, or - as Zebra suggests - experiment with different heater grounding..

Jakob E.[/quote]

Thanks Jakob,

It's the same on all 4 G7 boards. Maybe these were a faulty batch?
Anyway, I figured i would just use a wire jumper to complete the connection. But glad to know it may not be critical.

Daniel

 

[gyraf]

[quote author="AudioJunkie"]glad to know it may not be critical[/quote]

Don't worry - it's not!

Jakob E.

 

[reggie]

I just grounded everything as you said and it removed most of the humming. But I still do not get any sound ;-(

So I have everything lying open here while I did some measuring. I can measure about 12V DC between the front/back capsule where the wires from the capsule go into the pcb. And I get around 0.5 V between front/center and back/center. Would it matter, which way I hook up the capsule? I assumed that the two red wires are front/back and the white one is center (which is also physically in the center).

The two DC voltages (6V and 98V) are going into the mike pcb. But I get nothing between Out+ / Out- .

Now I measured the pins of the tube. I get 58V DC between Shield and Anode. Would that be enough? The tube get´s hot though.
There´s 6V between F+/F-

Since I did not have a 33M resistor I used 3 x 10M and 1 x 3M and connected them in series. I hope I did that right?

thanks for more help,
reggie

 

[zebra50]

You won't get sensible measurements for the capsule polarisation voltages using normal test gear - the impdences are too high.
You should not get a DV voltage at output +/- so that sounds OK.

Can you post photographs of your work? It might be easier to spot wiring errors?

 

[reggie]

thanks, so that seems to be ok.

I also measured 48V DC between pattern and 0V.

Too bad, I don´t have a digital camera, I might have to check if I could borrow one.

 

[reggie]

so would 58 V for the tube be enough to at least hear a faint signal or would it just produce nothing? If so, then I might have a clue as to what´s wrong ...

thanks,
reggie

 

[zebra50]

check out..

http://www.omnipressor.com/Other/G7mics/Comments.html

for some voltage measurements on my working G7s

58Vat the plate should be near enough. Something else is wrong.

 

[reggie]

thanks! I had already looked at the voltage measurements. But since it says there that 136V is already on the low side, I figured that 58V is not nearly enough.
So I´ll keep looking for the problem ...

 

[reggie]

now I spent the last 2 days just checking and checking why I´m getting no sound from the mike ;-((
I looked very closely at the pcb for bad soldering or bad contacts(I resoldered here and there), measured all the wires, checked parts again and again ....but cannot discover anything unusual anymore. I´m getting the right voltages from the psu (6V for the heater and 130V, but only 68V going into the anode after the 100k resistor) and no more loud ground hum.

How could I check if the Lundahl LL1538 is working ok ?
Could the tube be bad, even though it gets hot and how could I check for that?

I also switched the wire hookup to the capsule every which way possible, makes no difference. When I touch the center wire to the capsule I get a very loud hum that fades out then.

Maybe someone has an idea, what else I could check for, this is really getting soooo frustrating ....please help.

thanks,
reggie