[BUILD] 1176LN Rev D DIY

[vcorvino]

Have not found an answer on this one yet.  I know it has to be a simple issue, but i can't find the problem.  1176 works perfect in 20/1 ratio.  All other ratios do nothing (no compression).  I can't find any bad connections.  Any advice?

 

[dbonin]

Have not found an answer on this one yet.  I know it has to be a simple issue, but i can't find the problem.  1176 works perfect in 20/1 ratio.  All other ratios do nothing (no compression).  I can't find any bad connections.  Any advice?

So you are measuring the drop using your DMM on the output?  Ignore the vu meter for now  until you get the output reading correctly.

Put your DMM to measure AC volts on pin 2 and 3  of the output - apply your test signal, set attack to fastest setting, turn up the input to push the unit to trigger compression then cycle through the four ratios...  you should see  the voltage  DECREASE as you go from 20:1 (least compression) to 4:1 (most compression, lowest volts).
[/quote]

Also see this thread for a more precise test of output voltage after calibration:
http://groupdiy.com/index.php?topic=20058.msg708603#msg708603

 

[vcorvino]

Correct,  I'm ignoring the VU and measuring for voltage drop.  There is no voltage drop at all when I engage the other ratios.  When its set 20/1 I consistently get a 10db drop on the VU and 10db drop in voltage when switching the attack on and off.  When I do that to any other ratio, there is no change.

 

[fidelitybassist]

Hello everyone, first time on this forum. I recently assembled the hairball 1176 rev D, which I've been working on for a few days straight now, with lots of reading into the forum topic on the subject (not all of it, 'cause there's a LOT there), but I haven't been able to track down why I may be getting the issues that I am having.

The build itself went smoothly, and I tested all the resistors and capacitors when loading all of the boards, and they were all fine. The problems started when I got to the calibration part of the build.

Qbias calibration:
When sending the 1khz @ 0.775vac signal into the input, I only get about 0.147vac or so at the output xlr when the input and output controls are around the middle. So I used the mnats method for calibrating the Qbias.

Discrete Meter circuit/Null adjust:
At this step, I have R75 maxed out CW, and with R71 adjusted so that the meter shows 0, but the measurement across R74 will not drop below 0.407vdc.

Here is where I stopped, and started to re-test most of my passive components. I also used the schematics with normal voltages from the mnat troubleshoot page, and found that I am getting close to normal (+/-5%) everywhere except the GR meter driver section, where I am getting only 8.93vdc at Q11, and the other readings are also low about 20-30% on Q12 and Q13.

I also took some voltages asked by mike for for a different problem I saw on the forum and filled this out.

Post your AC voltage between pad 22 and ground for:
20:1 - 0.034
12:1 - 0.019
8:1 - 0.012
4:1 - 0.006

Post your DC voltage between pad 21 and ground for:
20:1 - -6.380
12:1 - -2.970
8:1 - -1.702
4:1 - -0.636

Post your DC voltage between pad 19 and ground for:
20:1 - -0.284
12:1 - -0.284
8:1 - -0.284
4:1 - -0.284

When trying to complete the gain reduction meter tracking adjustment, I get no movement from the needle when turning the GR on or off.

I have been trying to figure this out as much on my own, but if you could provide some insight I would be very grateful. Thank you so much for your time!

 

[Echo North]

First things first.  Lets start with your Qbias, something is wrong there, and everything starts there.  So let's ignore everything else for now.

Make sure you attack switch is in the off position and confirm pad 22 has continuity to ground.  Your side chain is now completely removed from the circuit.  You essentially have a 2 stage mic pre with about 45 dB of gain.  Sounds like yours has no gain so let's fix that first.  The FET is the first active component in that stage and it should be only slightly into conduction (see my trap door analogy in the build materials).  You do this by setting the Qbias.  If you can't complete this adjustment forget everything else till you can.  With the QBias set right the compressor is a brick.

Questions:

1.  Are your +30V DC and -10VDC test points reading correctly?
2. As you turn the Qbias adjustment, measure the the V DC at pad 18 (referencing ground).  Does the V DC change as you rotate the trimmer?
3. With the 0.775 V AC input, what is your AC (referencing ground) at 15 and then pad 17 (controls mid way and attack off)

Mike

 

[fidelitybassist]

Thanks for the reply!

1: yes the power supply voltages check outm and I do have continuity from 22 to ground. 
2: the vdc does change when i adjust the qbias at pad 18, in the range of -0.240vdc to -0.150vdc-ish.
3: with given settings:
at 15: 0.060vac
at 17: 0.006vac

 

[Echo North]

Thanks for the reply!

1: yes the power supply voltages check outm and I do have continuity from 22 to ground. 
2: the vdc does change when i adjust the qbias at pad 18, in the range of -0.240vdc to -0.150vdc-ish.
3: with given settings:
at 15: 0.060vac
at 17: 0.006vac

You have an issue in your first stage.  In the signal amp or maybe in the t-pad/input transformer.

Next...

0.775 V AC input, Attack off (pad 22 to ground), in and out at 12 o'clock.

What is your AC between these points and ground:

1. Pin 2 XLR
2. T-Pad in +
3. T-Pad out +
4. Transformer in +
5. Transformer out +
6. PCB input +

It will drop at each stage so don't worry, but what are you getting for AC?

Mike

 

[fidelitybassist]

Okay here are the measurments:

1. Pin 2 XLR : 0.385
2. T-Pad in +: 0.040
3. T-Pad out +: 0.215
4. Transformer in +:  0.215
5. Transformer out +: 0.093
6. PCB input +: 0.093

 

[Echo North]

Okay here are the measurments:

1. Pin 2 XLR : 0.385
2. T-Pad in +: 0.040
3. T-Pad out +: 0.215
4. Transformer in +:  0.215
5. Transformer out +: 0.093
6. PCB input +: 0.093

1 + 2 should be the same.  Either way you PCB input + looks ok. 

Looks like something is bad in that first "signal amp" stage.  Check all of those parts, particularly that caps are not damaged or have trace issues.  Check all of your voltages.  Check everything in there.  Make sure pads that are linked by traces have continuity....

Mike

 

[Echo North]

If all else fails, try removing Q1 and see if that gives you your gain.

Mike

 

[fidelitybassist]

double checked everything, and components/traces looked good. Removed Q1, and now I'm getting lots of gain (5.70 VAC) at the output.

 

[Echo North]

double checked everything, and components/trances looked good. Removed Q1, and now I'm getting lots of gain (5.70 VAC) at the output.

Cool, your bias is messed up. I'll be away from the computer for most of the day. Hold tight.

 

[Echo North]

Either your bias circuit is messed up, or your FET is bad.

There is a little simple divider circuit by your R59 Bias trimmer.  If you look at the schematic, R35 and R60 make a simple two resistor voltage divider.  They sit between the -10V DC supply and ground. and their job is to knock that voltage down to something closer to the bias level for the R59 trimmer.  Something closer to 2.8 V DC.

What is your DC V at these positions:

1. Both sides of R35 (reference each against ground)
2. Both sides of R60 (reference each against ground)

Post those and TRIPLE check that your attack and release pots are wired correctly.  Confirm your resistor and caps on them are the right value and all of the wiring is 100% correct.  These are both part of the bias circuit.

Mike

 

[fidelitybassist]

checked all of the wiring, components, and traces. So with Q1 back in the circuit:

1. Both sides of R35 (reference each against ground) : -9.75/-0.204vdc
2. Both sides of R60 (reference each against ground) :  -0.204/0vdc.

Either your bias circuit is messed up, or your FET is bad.

There is a little simple divider circuit by your R59 Bias trimmer.  If you look at the schematic, R35 and R60 make a simple two resistor voltage divider.  They sit between the -10V DC supply and ground. and their job is to knock that voltage down to something closer to the bias level for the R59 trimmer.  Something closer to 2.8 V DC.

What is your DC V at these positions:

1. Both sides of R35 (reference each against ground)
2. Both sides of R60 (reference each against ground)

Post those and TRIPLE check that your attack and release pots are wired correctly.  Confirm your resistor and caps on them are the right value and all of the wiring is 100% correct.  These are both part of the bias circuit.

Mike

 

[Echo North]

checked all of the wiring, components, and traces. So with Q1 back in the circuit:

1. Both sides of R35 (reference each against ground) : -9.75/-0.204vdc
2. Both sides of R60 (reference each against ground) :  -0.204/0vdc.

Either your bias circuit is messed up, or your FET is bad.

There is a little simple divider circuit by your R59 Bias trimmer.  If you look at the schematic, R35 and R60 make a simple two resistor voltage divider.  They sit between the -10V DC supply and ground. and their job is to knock that voltage down to something closer to the bias level for the R59 trimmer.  Something closer to 2.8 V DC.

What is your DC V at these positions:

1. Both sides of R35 (reference each against ground)
2. Both sides of R60 (reference each against ground)

Post those and TRIPLE check that your attack and release pots are wired correctly.  Confirm your resistor and caps on them are the right value and all of the wiring is 100% correct.  These are both part of the bias circuit.

Mike

Kind of suspicious of your FET.  Are you in the USA? Maybe the easiest thing to do would be to send you a new pair of FETs to try.

Turning all trimmer all the way clockwise and counter clock wise doesn't improve the signal strength?  Remember it's like a 20 turn pot, you can here a light "clicking" when you get to the end of the turns.

Mike

 

[fidelitybassist]

I am in the US. I just tested whether the output improves with the qbias, and it doesnt seem to affect the output.

 

[Echo North]

I am in the US. I just tested whether the output improves with the qbias, and it doesnt seem to affect the output.

Does the DC V change at the Gate of the FET when you turn the Qbias?

 

[fidelitybassist]

Yes. -0.308vcd at full CCW and around 0v at full CW

 

[Echo North]

Yes. -0.308vcd at full CCW and around 0v at full CW

Ideally it goes to 2V DC or higher but that point has a very high impedance so getting a real reading is hard with a meter.

Email me your address to the "[email protected]" address.  I think we should try new FETs.

Mike

 

[patxiplg]

Hello everyone:

After finishing the Hairball build (which first of all I have to say it went flawlessly and the quality of the kit is better than expected, truly AMAZING) I'm having a problem in the 2nd step of the calibration process... I checked everything and I can't find the problem.

Step 1 went OK.
At step 2 (Discrete Meter Circuit - Null Adjust) I can't put the VU at 0 with R71, If I turn it max CCW I can only set it to +0,5 dB. If I turn it max CW then I slam the needle to the right!). Turning all the way CW the "Null Adj" pot I can only reach 0,755 VDC in R74. I can't lower that value by moving the "Null Adj" pot.

Do anyone know what this issue can be related to? Maybe a bad Q11 FET?

I really triple checked everything and measured every component when building the kit. And the soldering has been clean too!

Thanks a lot in advance!!