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

[Echo North]

Do you have any suggestions?

Tripple check your load resistor R4 270Ω and R5 (27K), C1/C2 (0.15uF).

Mike

I have now checked the input circuit again. Just to be sure nothing was broken I desoldered R4, R5, C1 and C2 and measured them with my DMM and they all worked and were the right values.

I also disconnected the T-pad from the input transformer and sent a 50Hz and 1kHz signal through the T-pad and measured on its output and of course it worked as it should.

The change in level between treble and bass occurs after the input transformer, on the input of the main PCB. I also checked the transformer PCB and even resoldered the input transformer to it (in case of bad solder points) but nothing changed. None of the pins on the transformer are shorting to the chassi except pin 6 as it should.

All of the grounding is in order (audio ground on the PCB, input transformer shield to pin 6 and then compressor chassi, input and output XLR, chassi to IEC inlet) with very low resistans, <1 ohm.

Is there a way to measure only the input transformer with the proper load on its input and output? I have a bunch of resistors laying around that I can use.

I know it seems very unlikely that both of my input transformers would have the same problem but I am all out of ideas now...

Thanks again!
/Anton

I checked an input transformer with nothing attached to it.  Attached to PCB is fine.  With a 50Ω sweep to the input I get a flat response 20-20K, with a 600Ω output there is a roll off of about 1dB below 50.  Normal.

Increase in source load will cause rolloff.  That is why the T-Pad is there. to maintain the 500(ish) Ω load.

Try your input and tell me what you see.  I should help to know the source load of the signal generator.

Also measure the + and - of your t-pad input, then output with your ohmmeter.  Not attached to anything other than it's own PCB, no signal.

Mike

 

[Echo North]

Anton,

Also, are you sure your polarities are right on the input wiring?  Not the polarity markings on either side of the input transformer PCB are swapped.  (+ - on one side and - + on the other).

Also that pin 1 (ground), 2 (hot), 3 (cold) are correct on both XLRs.

Mike

 

[Curran]

ok the issue now is that during the calibration for the Q bias, the needle never moves. it is sitting at the far left of the meter. NOW when I probe through the circuit and get to other sections of the the needle will slam over to the other side SO I think that there is a simple little problem here. If you are telling me my Qbias is looking like it works, and the super filtered thin sound is ok then my next move should be what? I do have the attack set to "OFF" position for the qbias mode, the meter set to +4, and the ratio at 20:1. I am pretty sure I can here the compressor working like a champ when I probe through the circuit. I am running the audio from a dvd TV show though it, so there are alot of dynamics going on!

 

[damian]

Hi,

I have a problem with my 1176 I just built, audio does pass through but it wont calibrate or compress, When I checked for errors in my build I noticed that I had C9 (6.8uf  electrolytic the wrong way around) I put it the right way around but its still the same,

What damage could I of done by this?

Thanks,

Damian

We'll it won't compress UNTIL it's calibrated.  So really you have one issue right now, setting your qbias.  What is you DCV at the gate of Q1?  What happens to that voltage when you rotate the Qbias.

The level is correct, however it should be a negative value.  Is it negative?

I doubt C9 is your issue.

at the gate of Q1 I get a range from 0.003 up to 2.360 when turning Q bias

Damian

The level is correct but it should be negative.  Is it a negative value?

Yes its a negative value, when I try to calibrate the q bias, the meter stays at +1 when I turn the trimmer I can't get it down to 0, but it is changing the voltage at Q1

 

[Echo North]

Damian and Curran,

Sounds like you both need to adjust your Qbias but your VU isn't working right in +4 mode?  Start by adjusting the Qbias using your DMM as described here:


If you have a version with the "off" switch on the attack pot, you can just switch that.  That will ground pad 22.  Feed a 1khz signal into the input at about 0dBu.  You can put your DMM across input xlr pin 2 and 3 and look for 0.775VAC which is about 0dbu.  Your DAW might say it's sending out a -16dBu FS signal or something.  Just do what ever you need to do to get 0.775 VAC across 2 and 3 at the input.

Now put your DMM across pin 2 and 3 on the output xlr and turn the qbias (in and out knob mid way).  You should see the output moving up or down.  Turn the bias until it gets to the max and doesn't get any higher. 

Then adjust the output knob so you see +11dbu (2.75VAC) at the output.  Now pull the qbias back until you get a 1dbu drop 10dbu (2.45VAC).  That will set your bias.

Mike

 

[Arthurelletson]

Hi all, I am just finishing my first Rev A, I already have two rev F that work perfectly

but with this one, I am only getting 10VDC at the test point for 30VDC  I have double checked the orientation of the diodes, Caps, and the regulator and they are all correct, I have also tried replacing the regulator with another one. no change. all Resistors are of the correct value as i have taken them out of circuit and measured them.

Now i have tried to raise the voltage by force by raising the value of R89, I have changed it to 500ohms and I am now 14. VDC. I am slightly reluctant to keep bringing this up as it would seem that there is something else wrong and this is not the correct way to fix it, but i am out of ideas and would really appreciate some input from you guys.

Many Thanks

Arthur


P.S this is the equation for the regulator that i used to help me select a new resistor for R89 but i am not sure what value IQ is, is it total current draw for the entire circuit? or Bias Currant for the regulator.
VO = VXX(1+R2/R1)+IQR2


Edit:
enter your own joke about having a low IQ

 

[Echo North]

Hi all, I am just finishing my first Rev A, I already have two rev F that work perfectly

but with this one, I am only getting 10VDC at the test point for 30VDC  I have double checked the orientation of the diodes, Caps, and the regulator and they are all correct, I have also tried replacing the regulator with another one. no change. all Resistors are of the correct value as i have taken them out of circuit and measured them.

Now i have tried to raise the voltage by force by raising the value of R89, I have changed it to 500ohms and I am now 14. VDC. I am slightly reluctant to keep bringing this up as it would seem that there is something else wrong and this is not the correct way to fix it, but i am out of ideas and would really appreciate some input from you guys.

Many Thanks

Arthur


P.S this is the equation for the regulator that i used to help me select a new resistor for R89 but i am not sure what value IQ is, is it total current draw for the entire circuit? or Bias Currant for the regulator.
VO = VXX(1+R2/R1)+IQR2


Edit:
enter your own joke about having a low IQ

Have you checked the AC at the AC pads (using CT as the reference)?  See if your getting the correct AC from the transformer?

 

[Harpo]

P.S this is the equation for the regulator that i used to help me select a new resistor for R89 but i am not sure what value IQ is, is it total current draw for the entire circuit? or Bias Currant for the regulator.
VO = VXX(1+R2/R1)+IQR2

Yes for the 78xx fixed type voltage regulators. IE Vout = 24V(1+R89/R87)+R89*0.0046A, giving 29.812V for a typical Vregs quiescent current IB 4.6mA. (with IBmax. 8mA Vout would rise to 30.56V).
With R87 1K1 your R89 500R would rise Vout to typical 37.21V up to 38.91V if there would be enough raw DC voltage in front of the regulator, so probably not the best descision.

 

[Arthurelletson]

Hi all, I am just finishing my first Rev A, I already have two rev F that work perfectly

but with this one, I am only getting 10VDC at the test point for 30VDC  I have double checked the orientation of the diodes, Caps, and the regulator and they are all correct, I have also tried replacing the regulator with another one. no change. all Resistors are of the correct value as i have taken them out of circuit and measured them.

Now i have tried to raise the voltage by force by raising the value of R89, I have changed it to 500ohms and I am now 14. VDC. I am slightly reluctant to keep bringing this up as it would seem that there is something else wrong and this is not the correct way to fix it, but i am out of ideas and would really appreciate some input from you guys.

Many Thanks

Arthur


P.S this is the equation for the regulator that i used to help me select a new resistor for R89 but i am not sure what value IQ is, is it total current draw for the entire circuit? or Bias Currant for the regulator.
VO = VXX(1+R2/R1)+IQR2


Edit:
enter your own joke about having a low IQ

Have you checked the AC at the AC pads (using CT as the reference)?  See if your getting the correct AC from the transformer?

HI, I have 30.05V and 30.45 at the AC pads

 

[Echo North]

That's wacky.  You're measure the 30V dc referencing the CT?

I wonder if you have a bad cap or diode.  What is your DC V on the + side of C25?

Mike

 

[Arthurelletson]

That's wacky.  You're measure the 30V dc referencing the CT?

I wonder if you have a bad cap or diode.  What is your DC V on the + side of C25?

Mike

I have got 39VDC on c25, i just took it out to mesure and it is ok.  would C23 have any effect on the regulator output? im worried about constantly pulling components out the board.

I will pick up a new regulator tomorrow, i have a feeling that is the problem

 

[Echo North]

That's wacky.  You're measure the 30V dc referencing the CT?

I wonder if you have a bad cap or diode.  What is your DC V on the + side of C25?

Mike

I have got 39VDC on c25, i just took it out to mesure and it is ok.  would C23 have any effect on the regulator output? im worried about constantly pulling components out the board.

I will pick up a new regulator tomorrow, i have a feeling that is the problem

Ya seems like your regulator is messed up.  Get a new one in there with proper resistors.  Everything is fine with your rail until it hits the regulator.

 

[rainton]

Hi everybody,
I searched the entire thread but couldn't find an answer:

What happened to C27 on the BOM? I ordered all parts from Mouser according to the new Mouser shopping cart on hairballaudio.com (updated May 2nd) and now received the parts andjust went through all
of them and found the cap for C27 is missing(?)

The BOM says it is a 0.012uF cap. At some point in the thread I found Mike replaced it with a 0.01uF cap. But according to my order-history on mouser.com there was no such cap in my shopping cart.

Did I miss anything?

Thanks!

 

[Echo North]

Hi everybody,
I searched the entire thread but couldn't find an answer:

What happened to C27 on the BOM? I ordered all parts from Mouser according to the new Mouser shopping cart on hairballaudio.com (updated May 2nd) and now received the parts andjust went through all
of them and found the cap for C27 is missing(?)

The BOM says it is a 0.012uF cap. At some point in the thread I found Mike replaced it with a 0.01uF cap. But according to my order-history on mouser.com there was no such cap in my shopping cart.

Did I miss anything?

Thanks!

Do you have an extra 0.015?

 

[rainton]

No unfortunately I don't.

I'll have to check my local electronic parts store...
...I just wanted to know wether I missed something (like something has changed in the BOM in between) or the cap for C27 was just not
in the latest mouser cart I found on your website.

 

[AntonSwe]

Do you have any suggestions?

Tripple check your load resistor R4 270Ω and R5 (27K), C1/C2 (0.15uF).

Mike

I have now checked the input circuit again. Just to be sure nothing was broken I desoldered R4, R5, C1 and C2 and measured them with my DMM and they all worked and were the right values.

I also disconnected the T-pad from the input transformer and sent a 50Hz and 1kHz signal through the T-pad and measured on its output and of course it worked as it should.

The change in level between treble and bass occurs after the input transformer, on the input of the main PCB. I also checked the transformer PCB and even resoldered the input transformer to it (in case of bad solder points) but nothing changed. None of the pins on the transformer are shorting to the chassi except pin 6 as it should.

All of the grounding is in order (audio ground on the PCB, input transformer shield to pin 6 and then compressor chassi, input and output XLR, chassi to IEC inlet) with very low resistans, <1 ohm.

Is there a way to measure only the input transformer with the proper load on its input and output? I have a bunch of resistors laying around that I can use.

I know it seems very unlikely that both of my input transformers would have the same problem but I am all out of ideas now...

Thanks again!
/Anton

I checked an input transformer with nothing attached to it.  Attached to PCB is fine.  With a 50Ω sweep to the input I get a flat response 20-20K, with a 600Ω output there is a roll off of about 1dB below 50.  Normal.

Increase in source load will cause rolloff.  That is why the T-Pad is there. to maintain the 500(ish) Ω load.

Try your input and tell me what you see.  I should help to know the source load of the signal generator.

Also measure the + and - of your t-pad input, then output with your ohmmeter.  Not attached to anything other than it's own PCB, no signal.

Mike

I have checked the input and output XLR wiring again and it was correct. I also have the transformer connected in the correct/swapped way so sorry to say that this was not the problem

I have now measured the frequency response of the input transformer alone. According to my sound card manual the output impedance of my RME fireface 800 is 75 Ohms.

I disconnected the T-pad and connected the transformer input wires to pin 2(+) and 3(-) of an XLR cable from my RME. I connected  the transformer output wires (which normally would go to the main PCB) to pin 2 and 3 on another XLR cable right back into the RME. The transformer was still attached to the chassis.

The frequency response was MUCH better! (See attached picture) At 20 Hz I have about -5 dB and at about 43 Hz is where I have -1 dB.  It´s not perfectly flat but good enough I think, or should it be really linear at this point?

I then measured my disconnected T-Pad as you suggested. I first measured between input + and -, then output + and -. The load varied when I turned the pot. The first half of the turn (starting from fully CCW) it decreased and the second half it increased. The lowest value was around midway. Both the input and output behaved the same way. This is what i found:

                T-Pad measurement
                  Input            Output
Fully CCW:  518,1 Ω        519,5 Ω
Midway:      370 Ω            365 Ω
Fully CW:   1 051 Ω        1 051 Ω

Is this as it should be?

Thank you again for your time!

 

[Echo North]

No it should be about -1.5dB(ish) at 20Hz.

Hang in there.  I'm thinking, testing, and bouncing this of some people.

Edit:  Yes your t-pad is fine.  It should generally be between 350-650, but does fire up to 1K just as it hits fully CW.

 

[Echo North]

Anton,

With your input tx disconnected from the main PCB and t-pad:

Measure the resistance between input + and - (should be about 51Ω)
Measure the resistance between output + and - (should be about 13Ω)

Now measure between each of the out/in +/- and the transformer can (should all be infinite Ω)

Mike

 

[AntonSwe]

Anton,

With your input tx disconnected from the main PCB and t-pad:

Measure the resistance between input + and - (should be about 51Ω)
Measure the resistance between output + and - (should be about 13Ω)

Now measure between each of the out/in +/- and the transformer can (should all be infinite Ω)

Mike

Ok I have now measured the resistance. Transformer input was 49.8 Ω and output was 13.4 Ω. And all of the tx in/out +/- had infinite resistance to the tx can. So it seems to be fine... I just cannot understand what could be causing this.

Just to check that it´s nothing wrong with my measuring gear or hookup wires I used the exact same cables and input/output on my sound card a did a measurement from the input XLR, through the shielded input cable (I´m using Belden 8451)  and then straight back from where the T-Pad is supposed to be.  That measurement was perfectly flat down to 8 Hz, so nothing wrong there.

I really appreciate that you are trying to help me figure this out.

 

[Echo North]

Anton,

I've been troubleshooting these builds for a long time and this is by far the weirdest one I have ever seen.  We WILL get this figured out, I'll stick with you on this until it's 100% working. 

I spoke with the transformer designer.  After looking at your readings he felt certain it was not a transformer issue.  I tend to agree with him.  Why was your transformer only curve less flat than mine?  Most likely do to discrepancies in loading.  Also, in 4 years I've never has a single bad input and it seems unlikely you would get the first two.  Generally when transformers are bad, either a winding is broken, or the polarity is reversed.  Neither of these seem to be your problem.

I believe (as does the transformer designer) that there is something at the input of the PCB causing this weirdness.  I know I've asked this before so I apologize, are you 100% sure the values in the image below are correct.  Notably that the load resistor is 270Ω and not 270KΩ?  Remove one leg from the circuit and test it with your ohm meter.

If all that looks good.  Try passing a signal with your FET (Q1) removed from your PCB and with GR off.  This will effectively remove GR completely from the circuit and turn your 1176 into a two stage pre.

Do you still have the FREQ issue?

If that fails, we may have to send some parts back and forth in the mail.

Mike