[BUILD] 1176 Rev A - Back to the beginning...

[rainton]

Ok probably I'm just leading a monolog here, but I guess this can also help



regarding the -10V rail I traced it down:

- after the diodes (CR9 CR10) the DMM reads -15.56V
- after the first 1K resistor (R81) the DMM 10.00V or 10.01V
- after the second 1K resistor (R82) the DMM reads 4.5V (checked both resistors before installing them both measured correctly)
-> at the testpoint it also reads 4.5V

regarding the +30V rail:

- I get 31.5V DC all the way to IN pin of VR1
- on the OUT pin of VR1 I get 23.4V

- on the + pin of C23 I get the same 23.4V

BUT on the - pin of C23 I get -30.6V

So what does that tell you?

As mentioned above I already changed VR1 for another one since the last one also showed me 23.4V at the +30V testpoint so that can't be it, right?

Any help would be highly appreciated!
Thanks!

 

[Echo North]

Habe you been through my new power supply guide (which incorporates the MNATS guide)?

http://hairballaudio.com/docs/FETGUIDE/Section%20D%20-%20Power%20Section.pdf

Read that and if you're still stuck after post back here.

Mike

 

[rainton]

Habe you been through my new power supply guide (which incorporates the MNATS guide)?

http://hairballaudio.com/docs/FETGUIDE/Section%20D%20-%20Power%20Section.pdf

Thanks Mike but that's exactly what I used.

My power supply looks exactly like the one in the guide and since I get 25V at both of the AC points at the terminal block that part should be fine, right?
I even get about the right +30V DC and -10V DC at some point, but not at the given test points - see my measurements in my post above for more details...

Also:
- I only stuffed the power supply so far
- I double checked all parts except VR1 and the diodes before soldering them into place
- I already changed VR1 for another one to see if it was the problem (it wasn't - problem persists)
- I desoldered C23 - checked it again - it measured 7.01uF - but I guess that's within tolerance right? I then went through all the 6.8 uF electrolytic caps I have and the all measure the same...

So again please see my measurements above to see what that tells you - because I'm really stuck here...
Thank you so much

 

[Echo North]

Habe you been through my new power supply guide (which incorporates the MNATS guide)?

http://hairballaudio.com/docs/FETGUIDE/Section%20D%20-%20Power%20Section.pdf

Thanks Mike but that's exactly what I used.

My power supply looks exactly like the one in the guide and since I get 25V at both of the AC points at the terminal block that part should be fine, right?
I even get about the right +30V DC and -10V DC at some point, but not at the given test points - see my measurements in my post above for more details...

Also:
- I only stuffed the power supply so far
- I double checked all parts except VR1 and the diodes before soldering them into place
- I already changed VR1 for another one to see if it was the problem (it wasn't - problem persists)
- I desoldered C23 - checked it again - it measured 7.01uF - but I guess that's within tolerance right? I then went through all the 6.8 uF electrolytic caps I have and the all measure the same...

So again please see my measurements above to see what that tells you - because I'm really stuck here...
Thank you so much

Check the value of R87 and R89 VERY closely.

 

[damian]

Hi,

I have calibrated my 1176 but my VU is way off, with a around 0db signal going in, compression turned off and adjusting the input and output knobs to get 0db back into the mixer the 1176 is only showing aound -15db

I tried calibrating again and still this happens, do you have an idea what is wrong or what I have done wrong?

Thanks,

Damian

 

[Echo North]

Hi,

I have calibrated my 1176 but my VU is way off, with a around 0db signal going in, compression turned off and adjusting the input and output knobs to get 0db back into the mixer the 1176 is only showing aound -15db

I tried calibrating again and still this happens, do you have an idea what is wrong or what I have done wrong?

Thanks,

Damian

Is it a digital mixer?  Are you looking at -15dBFS?  That is pretty close to 0dBu.

 

[damian]

Hi,

I have calibrated my 1176 but my VU is way off, with a around 0db signal going in, compression turned off and adjusting the input and output knobs to get 0db back into the mixer the 1176 is only showing aound -15db

I tried calibrating again and still this happens, do you have an idea what is wrong or what I have done wrong?

Thanks,

Damian

Is it a digital mixer?  Are you looking at -15dBFS?  That is pretty close to 0dBu.

Hi,

Its an analog mixer, It has a meterbridge for each channel so I can see exactly whats going in.

 

[rainton]

Habe you been through my new power supply guide (which incorporates the MNATS guide)?

http://hairballaudio.com/docs/FETGUIDE/Section%20D%20-%20Power%20Section.pdf

Thanks Mike but that's exactly what I used.

My power supply looks exactly like the one in the guide and since I get 25V at both of the AC points at the terminal block that part should be fine, right?
I even get about the right +30V DC and -10V DC at some point, but not at the given test points - see my measurements in my post above for more details...

Also:
- I only stuffed the power supply so far
- I double checked all parts except VR1 and the diodes before soldering them into place
- I already changed VR1 for another one to see if it was the problem (it wasn't - problem persists)
- I desoldered C23 - checked it again - it measured 7.01uF - but I guess that's within tolerance right? I then went through all the 6.8 uF electrolytic caps I have and the all measure the same...

So again please see my measurements above to see what that tells you - because I'm really stuck here...
Thank you so much

Check the value of R87 and R89 VERY closely.

Ok I checked R87 and R89 again: R87 measures 1.07Kohms and R89 measures 220Ohms

I also checked R81 and R82 again both measured 0.994Kohms...

Everything as it should be , right?

I'll send some pictures...

 

[Echo North]

Hi,

I have calibrated my 1176 but my VU is way off, with a around 0db signal going in, compression turned off and adjusting the input and output knobs to get 0db back into the mixer the 1176 is only showing aound -15db

I tried calibrating again and still this happens, do you have an idea what is wrong or what I have done wrong?

Thanks,

Damian

Is it a digital mixer?  Are you looking at -15dBFS?  That is pretty close to 0dBu.

Hi,

Its an analog mixer, It has a meterbridge for each channel so I can see exactly whats going in.

What is your mixers metering calibrated to?  0dBu = how many V? 

If it's -10 metering and you 1176 is in +4 mode....

Measure the ACV across pin 2 and 3 of your output XLR and adjust your in/out so it reads 1.25VAC.  Your meter should then be at 0 in +4 mode.

 

[rainton]

here's a picture of how it looks like in my power section:

 

[Harpo]

regarding the -10V rail I traced it down:
- after the diodes (CR9 CR10) the DMM reads -15.56V
- after the first 1K resistor (R81) the DMM 10.00V or 10.01V
- after the second 1K resistor (R82) the DMM reads 4.5V (checked both resistors before installing them both measured correctly)
-> at the testpoint it also reads 4.5V

regarding the +30V rail:
- I get 31.5V DC all the way to IN pin of VR1
- on the OUT pin of VR1 I get 23.4V
- on the + pin of C23 I get the same 23.4V
BUT on the - pin of C23 I get -30.6V
So what does that tell you?

Your DMM reads -15.56V in respect to what? (should be about -34VDC in respect to 0V reference)
At least your "- pin of C23 I get -30.6V" tells, your black multimeter probe, plugged into the meters COM port is not connecting to 0V reference voltage (the transformers center tap connection that you hopefully by now have corrected for the red+orange transformer secondary wire junction) when you probe DC voltages with your red wire probe.
Maybe don't insist to know it better than the transformer manufacturer or the build guide, how to connect you 2x25VAC dual secondary windings in series in order to make the transformer behave like a single winding center tapped 50VAC secondary.
To get your +30V fixed from your pic, just replace your 22 ohm resistor with a 220 ohm resistor for R89.

 

[rainton]

Sorry Harpo,
I didn't insist to know it better - I was just confused that's why I posted here before firing up my unit...

R89  is a 220 Ohms resistor - I just checked it again.

Ok I checked R87 and R89 again: R87 measures 1.07Kohms and R89 measures 220Ohms

I also checked R81 and R82 again both measured 0.994Kohms...

I measured all DC voltage with black probe at CT of the terminal block.

And at the moment I have black & orange of my power transformer secondaries connected to CT since that is what are both of the 0V wires according to the
schematic of my power transformer:

 

[rainton]

Ok - I got it...

...I know it sounds stupid but I just got another DMM from a friend next door - since mine showed low battery.
And his is measuring +29.85V DC at the +30V TP
and -9.84 at the -10V TP

So I guess this is tolerable right.

My friend also said I have to be aware that a DMM can give wrong measurements when running low on battery...

so thanks for everyone trying to help!
I think I can now go on stuffing the board...

 

[Curran]

still need an answer on my Qbias adjust. I have it reading (-.81) at the gate of Q1, do I need it to go to (-2, or -3) or to (-.2, or -.3) the last post abotu this I was not clear on it. If it was a typo or not. If it is supposed to go to (-2) and NOT (-.2) I am stuck

 

[Echo North]

still need an answer on my Qbias adjust. I have it reading (-.81) at the gate of Q1, do I need it to go to (-2, or -3) or to (-.2, or -.3) the last post abotu this I was not clear on it. If it was a typo or not. If it is supposed to go to (-2) and NOT (-.2) I am stuck

Sorry, you just want to get it as neg as possible.  That should be somewhere around -2 or -3 VDC.  Measuring at the gate is confusing since it's at a very high impedance.  Your meter loads it and drags it down.

Start by measuring at pad 18.  That is the wiper of your QBias.  Get that as negative as it will go.  What is that voltage?  Are you passing signal?

Mike

 

[Curran]

OHHHHHH I measured at the Q1 Gate!!! hahahaha I will check that today after work!!!

 

[Curran]

OK!!! off of the middle pad of the q bias, I got the DC to read (-2.0) I am still passing signal. what is the next move!!! If my VU is not working right then what should be my next steps so I can calibrate the beast and move on to my CLX-VU's and Vox AC30 I am building?! woo hoo getting closer!! thank you for all of the help!!! AND PATIENCE!

ok, I re-read the info from Mike about my issues here. I have re-done the qbias off of the wiper pad on the boards and have it as negative as it will go (-2.8vdc) I am still passing signal at (.775) in the input, nothing on the output XLR though which is obviously weird

 

[Echo North]

OK!!! off of the middle pad of the q bias, I got the DC to read (-2.0) I am still passing signal. what is the next move!!! If my VU is not working right then what should be my next steps so I can calibrate the beast and move on to my CLX-VU's and Vox AC30 I am building?! woo hoo getting closer!! thank you for all of the help!!! AND PATIENCE!

ok, I re-read the info from Mike about my issues here. I have re-done the qbias off of the wiper pad on the boards and have it as negative as it will go (-2.8vdc) I am still passing signal at (.775) in the input, nothing on the output XLR though which is obviously weird

If you're not passing signal through the compressor with GR set to OFF, obviously it's dying somewhere.  Start by comparing the voltages of your transistor against the MNATS schematic with typical voltages.  The is downloadable on the rev A page on MNATS.net.  Focus on the upper 1/2 of the schematic, the signal and preamps.  See if any of your voltages are off.

If they all seem fine then trace the signal.  Input a 0dB signal at the input (measure 0.775V AC across input 2+3) and set your in/out knobs 1/2 way.  GR OFF.  Let's follow it through the amp stages.

Your AC V should be 0.775 at the input 2+3. Now measure the rest of these AC voltages using the CT as your reference (black lead).
1) Check it at the T-PAD input +.  It'll probably be about 1/2 the V.
2) Check it at T-PAD output +.  It'll be even less.
3) Check it the input transformer primary +.  Should be the same as the T-pad out.
4) Check it at the input transformer secondary +.  Should be even less.  Maybe 0.1 V AC(ish).
5) Check it at the + of the PCB input.  Same as last measurement.
6) Check it at pad 15 on the main PCB.  Should be a lot higher than before.
7) Check it at pad 17 on the main PCB.  A little less now.
8.) Check it at BRN of the output transformer.  Higher.
9) Check it at pin 2 of the output XLR.

What did you get?

Mike

 

[Curran]

well I am passing signal through the entire circuit!!!! I have adjusted the Qbias to (-2.85vdc) the meter is working at this step of the game!!. I have plugged an sm57 into the front end of the compressor and it is passing signal all the way!!! I need to get some alligator klips for my DMM leads so I can get the meter calibrated. I found some way sh*tty wiring and solder blob jumper on the input transformer PCB.

I do not think it is compressing yet BUT I am still thrilled so far. I will buzz you all again with more stress and misery later this holiday weekend!!

 

[Echo North]

well I am passing signal through the entire circuit!!!! I have adjusted the Qbias to (-2.85vdc) the meter is working at this step of the game!!. I have plugged an sm57 into the front end of the compressor and it is passing signal all the way!!! I need to get some alligator klips for my DMM leads so I can get the meter calibrated. I found some way sh*tty wiring and solder blob jumper on the input transformer PCB.

I do not think it is compressing yet BUT I am still thrilled so far. I will buzz you all again with more stress and misery later this holiday weekend!!

Before you do anything else you need to set the qbias now.  Right now you basically have a 40dB mic pre.  Adjusting the qbias will turn it into a compressor.

Mike