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

[King Sound Studio]

Hey all,

First time builder here. Forgive the noobness.
I'm trying to calibrate my q-bias.  Everything is going great until I'm supposed to adjust the qbias control (r59).

Turning the q-bias in either direction doesn't adjust anything.

I'm so close, yet so far. Also, our air conditioner died, so we're doing this in the heat..super bad idea. Any help troubleshooting would be immensely appreciated!

 

[King Sound Studio]

Hey all,

First time builder here. Forgive the noobness.
I'm trying to calibrate my q-bias.  Everything is going great until I'm supposed to adjust the qbias control (r59).

Turning the q-bias in either direction doesn't adjust anything.

I'm so close, yet so far. Also, our air conditioner died, so we're doing this in the heat..super bad idea. Any help troubleshooting would be immensely appreciated!

So far, I've tried tracing DC voltage. I'm getting -2.128 vdc at pad 18 (brown wire) and -2.079 vdc at pad 7.

 

[Echo North]

Hey all,

First time builder here. Forgive the noobness.
I'm trying to calibrate my q-bias.  Everything is going great until I'm supposed to adjust the qbias control (r59).

Turning the q-bias in either direction doesn't adjust anything.

I'm so close, yet so far. Also, our air conditioner died, so we're doing this in the heat..super bad idea. Any help troubleshooting would be immensely appreciated!

Hi,

So basically the QBias is a negative DC voltage that gets sent from the negative rail of the power supply, through the side chain (attack, release) and ends up at the Q1 gate.  If adjusting the bias does not change your output signal, the first thing to suspect is that the negative DC voltage is not getting to the FET.  So let's trace it.

It helps to look at the schematic.  Even if you don't fully get it, it helps to see the direction we're looking at:
http://mnats.net/files/1176REVA_125_DOCUMENTATION.pdf

Let's see where this voltage shits out. Set your DMM to read volts DC and put out common probe on the 0V/CT of the power supply on the main PCB (like when you measure the power supply points).

1. Look at R60 and R80.  They are a little voltage divider.  The negative rail is -10 VDC and the bias don't need all that.  Just 1-3 VDC so this dividers job is to knock that down.  On one side of R80 you'll should have -10VDC (+/- 10%) and on the other you'll have something like -2.8VDC.  Check?

2. Should have -2.8VDC at pad 18.  Check?

(Checking your new post).

Mike

 

[Echo North]

Hey all,

First time builder here. Forgive the noobness.
I'm trying to calibrate my q-bias.  Everything is going great until I'm supposed to adjust the qbias control (r59).

Turning the q-bias in either direction doesn't adjust anything.

I'm so close, yet so far. Also, our air conditioner died, so we're doing this in the heat..super bad idea. Any help troubleshooting would be immensely appreciated!

So far, I've tried tracing DC voltage. I'm getting -2.128 vdc at pad 18 (brown wire) and -2.079 vdc at pad 7.

Seems fine.

What do you have at the G of Q1?  It's the lead closest to Q11.

You're reading will be a little messed up because of the high impedance point, but it should give you something.  That voltage change when you twist your pot?

Mike

 

[King Sound Studio]

Hey all,

First time builder here. Forgive the noobness.
I'm trying to calibrate my q-bias.  Everything is going great until I'm supposed to adjust the qbias control (r59).

Turning the q-bias in either direction doesn't adjust anything.

I'm so close, yet so far. Also, our air conditioner died, so we're doing this in the heat..super bad idea. Any help troubleshooting would be immensely appreciated!

So far, I've tried tracing DC voltage. I'm getting -2.128 vdc at pad 18 (brown wire) and -2.079 vdc at pad 7.

Seems fine.

What do you have at the G of Q1?  It's the lead closest to Q11.

You're reading will be a little messed up because of the high impedance point, but it should give you something.  That voltage change when you twist your pot?

Mike

Mike, thank you for the help. The G of Q1 (lead closest to Q11) gives me a reading of 1.73.  Checking that and twisting the pot DOES yeild change in DC!

Step in the right direction?

 

[Echo North]

-1.73 or + 1.73 VDC.  Needs to be negative.

Are you measuring the output with a DMM when doing the qbias or are you just relying on the VU meter?

Mike

 

[King Sound Studio]

-1.73 or + 1.73 VDC.  Needs to be negative.

Are you measuring the output with a DMM when doing the qbias or are you just relying on the VU meter?

Mike

Measuring the output with the DMM. The VU meter isn't moving. The DMM however moved as I adjusted the qbias.
That is, of course, measuring from the G of Q11.  At the XLR Out 2 and 3, moving the qbias gets me no movement on the VU meter or the DMM.

 

[Echo North]

-1.73 or + 1.73 VDC.  Needs to be negative.

Are you measuring the output with a DMM when doing the qbias or are you just relying on the VU meter?

Mike

Measuring the output with the DMM. The VU meter isn't moving. The DMM however moved as I adjusted the qbias.
That is, of course, measuring from the G of Q11.  At the XLR Out 2 and 3, moving the qbias gets me no movement on the VU meter or the DMM.

Is that 1.73 voltage - or +?

 

[King Sound Studio]

-1.73 or + 1.73 VDC.  Needs to be negative.

Are you measuring the output with a DMM when doing the qbias or are you just relying on the VU meter?

Mike

Measuring the output with the DMM. The VU meter isn't moving. The DMM however moved as I adjusted the qbias.
That is, of course, measuring from the G of Q11.  At the XLR Out 2 and 3, moving the qbias gets me no movement on the VU meter or the DMM.

Is that 1.73 voltage - or +?

Whoops. Sorry about that! It's -1.73

 

[Echo North]

We'll that's weird.  Possible you have too much voltage there.

Two questions:

1. With the input and output at 12 o'clock, GR OFF, 20:1, what is the ACV you see between pin 2 and 3 of the output.  The is the AC V that is not changing when you rotate the Qbias.  What is the voltage?

2. When you rotate measure the DCV between ground and pad 18 and rotate the pot.  What is the smallest you can get that DCV.  Smallest meaning closest to 0VDC.  So -0.25 is smaller than -0.85.

Mike

PS Make sure you carefully switch between reading AC and DC since your doing both to make sure you get the correct data.

 

[King Sound Studio]

We'll that's weird.  Possible you have too much voltage there.

Two questions:

1. With the input and output at 12 o'clock, GR OFF, 20:1, what is the ACV you see between pin 2 and 3 of the output.  The is the AC V that is not changing when you rotate the Qbias.  What is the voltage?

2. When you rotate measure the DCV between ground and pad 18 and rotate the pot.  What is the smallest you can get that DCV.  Smallest meaning closest to 0VDC.  So -0.25 is smaller than -0.85.

Mike

PS Make sure you carefully switch between reading AC and DC since your doing both to make sure you get the correct data.

Checking now.

1.  Checking AC with GR off, once everything settles, I get a reading of .4. And correct, the ACV of .4 is not moving when I rotate the qbias.

2.  When measuring the DCV with the GR switch in the off position, I get a reading of 0dvc with no change rotating in any direction. If I push the GR switch in, turning the unit on, I get a reading right now of 2.02DCV and this number DOES change as I rotate the qbias.

 

[Echo North]

We'll that's weird.  Possible you have too much voltage there.

Two questions:

1. With the input and output at 12 o'clock, GR OFF, 20:1, what is the ACV you see between pin 2 and 3 of the output.  The is the AC V that is not changing when you rotate the Qbias.  What is the voltage?

2. When you rotate measure the DCV between ground and pad 18 and rotate the pot.  What is the smallest you can get that DCV.  Smallest meaning closest to 0VDC.  So -0.25 is smaller than -0.85.

Mike

PS Make sure you carefully switch between reading AC and DC since your doing both to make sure you get the correct data.

Checking now.

1.  Checking AC with GR off, once everything settles, I get a reading of .4. And correct, the ACV of .4 is not moving when I rotate the qbias.

2.  When measuring the DCV with the GR switch in the off position, I get a reading of 0dvc with no change rotating in any direction. If I push the GR switch in, turning the unit on, I get a reading right now of 2.02DCV and this number DOES change as I rotate the qbias.

Oh ya you have an entirely different issue.  You're not really passing signal.  Give me a sec to search for something.

 

[Echo North]

You should be seeing way more ACV on your output. 

Start with the input and output at 12 o'clock, attack in OFF position, 20:1.  Set you Qbias so you get pad 18 closest to 0VDC. Should be fully CCW. Inject you 0.775 VAC 1K signal into the input and again adjust it till you 0.775 VAC across input 2 and 3.

Again follow the schematic.

Set your DMM to AC and reference the 0V/CT on the main PCB power supply.

1. What is your AC @ the input transformer + on the in side? (should be smaller)
2. What is your AC @ the input transformer + on the out side?  (should be smaller)
3. What is your AC @ the main PCB input marked "+"  where the transformer comes in?  (should be very small like 0.085 VAC)
4. What is your AC @ + lead of C7 (should be WAY higher)
5. What is your AC @ -  lead of C7 (should be same)
6. What is your AC @ either lead of C8

Please post results.

I'm closing up for the day and will most likely be off line till the AM.  Good luck! We'll get it working.

Mike

 

[King Sound Studio]

You should be seeing way more ACV on your output. 

Start with the input and output at 12 o'clock, attack in OFF position, 20:1.  Set you Qbias so you get pad 18 closest to 0VDC. Should be fully CCW. Inject you 0.775 VAC 1K signal into the input and again adjust it till you 0.775 VAC across input 2 and 3.

Again follow the schematic.

Set your DMM to AC and reference the 0V/CT on the main PCB power supply.

1. What is your AC @ the input transformer + on the in side? (should be smaller)
2. What is your AC @ the input transformer + on the out side?  (should be smaller)
3. What is your AC @ the main PCB input marked "+"  where the transformer comes in?  (should be very small like 0.085 VAC)
4. What is your AC @ + lead of C7 (should be WAY higher)
5. What is your AC @ -  lead of C7 (should be same)
6. What is your AC @ either lead of C8

Please post results.

I'm closing up for the day and will most likely be off line till the AM.  Good luck! We'll get it working.

Mike

Hey Mike,

I wish that I could report back and say that I was having a lot of luck, but to be honest, I'm feeling a little in over my head. Electronics are a little Greek to me, and as much as I want to be able to knock this sucker out myself, I don't know if I'm going to be capable.

But if you won't give up on me, I'll keep trying. Here's what I got.

Input is set to 24 (12 o'clock)
Output is set to 24 (12 o'clock)
Attack is set to "off"
Release is set to 7 (fully clock wise)
Ratio 20:1
Meter is set to GR

In order to get a 0VDC on pad 18 with my red lead touching pad 18 and my black lead against the ground bolt on the back of the chassis, I had to turn the Qbias CLOCKwise, not counter-clockwise.  The further I turned it clockwise, the lower the number would go below zero, into negative numbers.

A couple observations at this point.
1. My VU meter is all the way to the left in GR mode with the Qbias turned to 0 (completely clockwise). Turning the qbias CCW will eventually move the needle on the vu meter back to the right towards "0". Not sure if this even matters at this point. Just an observation.

2. Looking at my trim pots, they are arranged precisely as they appear in FIG 4-13 in the guide. Didn't know if maybe I had one reversed after having to turn the qbias clockwise to get to 0 VDC  instead of CCW. Comparing with Fig 4-13, everything seems to be organized appropriately.

I bought, and am using a Fluke 177.
Using the mA (Hz) mode on the 177, I was able to determine that my 1khz signal in my DAW needed to be at about -19.2db to get a reading of .77/.76 (bouncing back and forth) on the Fluke in this mode, with my leads touching lugs 2 and 3 on my XLR input.

Maybe this is the incorrect mode to be using to measure this? I don't know. I'm looking for three decimal places (.775). The mA setting only gives me two decimal places. Am I incorrect in using the mA setting?

Now, going back to your questions on your last post.

With my .775 VAC 1k signal going in to the input with my q-bias all the way right, I'm getting basically no reading on my XLR input lugs 2 and 3.

I'll get a reading around 2 VAC upon first contact, but it will quickly go disintegrate to near 0, usually hovering around 0.3.

The same thing happens when AC @ the input transformer + in and out.

Does this have something to do with the Qbias being set all the way right/CW to get 0? Something tells me it does since I have a feeling that with it all the way right, and it killing the input signal, of course I'm not going to be getting anything going through.

I guess I'm just afraid that when I turn it CCW/Left that the number is going down further away from zero. Should I just be brave and keep cranking it to the left?

Your thoughts?

tl;dr

I'm getting 0 VDC with my qbias all the way to the RIGHT...when I turn to the left, I'm getting negatively further away from 0...

Something tells me this is the crux of all my problems.

 

[Echo North]

Hey Mike,

I wish that I could report back and say that I was having a lot of luck, but to be honest, I'm feeling a little in over my head. Electronics are a little Greek to me, and as much as I want to be able to knock this sucker out myself, I don't know if I'm going to be capable.

But if you won't give up on me, I'll keep trying. Here's what I got.

No worries dude.  I won't give up!  I enjoy helping.  We'll get it working.  It's probably one little thing.

In order to get a 0VDC on pad 18 with my red lead touching pad 18 and my black lead against the ground bolt on the back of the chassis, I had to turn the Qbias CLOCKwise, not counter-clockwise.  The further I turned it clockwise, the lower the number would go below zero, into negative numbers.

Sorry I made a mistake.  You want CCW and you want a the biggest - VDC which will be like -2 something.  My mistake.

Mike

 

[Echo North]

Your bias looks fine.  You're signal is dying somewhere is seems.

Are you using mA?  That's amps.  You want to be using the V with the sine above it. Looks like it's directly to the right of "off" on your meter.

Mike

 

[King Sound Studio]

Your bias looks fine.  You're signal is dying somewhere is seems.

Are you using mA?  That's amps.  You want to be using the V with the sine above it. Looks like it's directly to the right of "off" on your meter.

Mike

Mike, you're the man.  Thank you!

I think we're talking about two different things.
I'm using the mA setting when trying to set my 1khz sign output DB in my daw to .775. I didn't know if that was correct or not for finding that .775.

That's the ONLY thing I'm using the mA setting for.

Everything else, when checking volts, I'm using the V with the sine above it.  BUT, I'm guessing with the Qbias set all the way to the right, like I've been doing, that's keeping me from getting anything on my XLR lugs.

It's been such a long day...I've made a choice. I'm going to put it down for the night and pick it up in the morning.

First thing I'm going to do is set my qbias to the right as far as it will go, and go from there and report!

Mike, thank you again for everything!!

 

[Echo North]

You should not be using the mA setting at all.  That is for something completely different and a completely different process.

Use VAC (V with sine) for EVERYTHING related to testing audio signal level.  You need to have it to this for setting the input and test the output level.

Start fresh in the AM, set your 0.775 with the VAC (V with sine) and check the output with the same.  If you have no output trace though like above. 

There might not even be anything wrong.  8)

Mike

 

[King Sound Studio]

You should not be using the mA setting at all.  That is for something completely different and a completely different process.

Use VAC (V with sine) for EVERYTHING related to testing audio signal level.  You need to have it to this for setting the input and test the output level.

Start fresh in the AM, set your 0.775 with the VAC (V with sine) and check the output with the same.  If you have no output trace though like above. 

There might not even be anything wrong.  8)

Mike

Okay, new day. Ready to start fresh. Here's where I am.

I turned my qbias all the way to the left (CCW) until I got a reading of -2.830 VDC
In Pro Tools, I ended up having to set my 1khz signal at -8.8dbfs to get my .775 VAC on the input. This seems a little out of range when reading the guide. Cause for concern? 

Moving on, I put my leads on the XLR output and adjusted the output knob until we got our reading of 2.73 VAC (couldn't quite get it to stop on 2.75). Cool, great. Went to adjust the q-bias CW/RIGHT to get to the 2.44 VAC, but moving the qbias had no affect on the output.

So, in short, the qbias is adjusting my input, but seems to have no affect on the output. Does that make sense?

 

[Echo North]

You should not be using the mA setting at all.  That is for something completely different and a completely different process.

Use VAC (V with sine) for EVERYTHING related to testing audio signal level.  You need to have it to this for setting the input and test the output level.

Start fresh in the AM, set your 0.775 with the VAC (V with sine) and check the output with the same.  If you have no output trace though like above. 

There might not even be anything wrong.  8)

Mike

Okay, new day. Ready to start fresh. Here's where I am.

I turned my qbias all the way to the left (CCW) until I got a reading of -2.830 VDC
In Pro Tools, I ended up having to set my 1khz signal at -8.8dbfs to get my .775 VAC on the input. This seems a little out of range when reading the guide. Cause for concern? 

Moving on, I put my leads on the XLR output and adjusted the output knob until we got our reading of 2.73 VAC (couldn't quite get it to stop on 2.75). Cool, great. Went to adjust the q-bias CW/RIGHT to get to the 2.44 VAC, but moving the qbias had no affect on the output.

So, in short, the qbias is adjusting my input, but seems to have no affect on the output. Does that make sense?

Hmmm not really.  That's a bit of a head scratcher.  Could be a bad FET but I'm still suspicious of the calibration set up.  Unfortunately I'm not sure where to look next.  Your Q1 gate has a negative voltage that changes w/ the bias rotation but you're not seeing an output drop?  If you turn the bias pot full CW what does the G show in DC? Still no drop at the output?

I'll just toss this out there so you know it's and option, we do repairs on these builds:
http://www.hairballaudio.com/shop/index.php?cPath=45

If it just need calibration and is built fine we'd probably just charge you $50.

Mike