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

[Chrome Heart]

Is it possible to check the voltages on the bench with the PCB out of the box or will the readings be wrong? I have the main board out and connected only to the secondaries and ground. There are no other wires attached to the board. BTW is that wiper voltage (-1.171) acceptable?

 

[Echo North]

Is it possible to check the voltages on the bench with the PCB out of the box or will the readings be wrong? I have the main board out and connected only to the secondaries and ground. There are no other wires attached to the board. BTW is that wiper voltage (-1.171) acceptable?

That's in the ballpark.

You need to hook it back up because the qbias travels through the release.

 

[Chrome Heart]

Im also using 2n3707's but the BOM says 2n3708. Is that a problem?

 

[Echo North]

Im also using 2n3707's but the BOM says 2n3708. Is that a problem?

No.

You have an issue somewhere between your - rail and the gate bias most likely.  Probably with your C22 or your release pot.

 

[Chrome Heart]

C22 is a .22 wima that measure correctly on the meter. I also noticed the -v dropping on the minus sides of C7 and C10

 

[Echo North]

C22 is a .22 wima that measure correctly on the meter. I also noticed the -v dropping on the minus sides of C7 and C10

There shouldn't be DC at the - of C7 (I don't think).  That's just sitting in the audio chain before the output pot.  It's job is to block DC.  So it's probably a good thing what you see there.

You got me stumped.  I'm pretty sure the issue is in that qbias chain.  How about the release pot resistor.  Is it a 270K?  Make sure it's not 270R.

 

[Echo North]

I have a second PCB, Im just gonna stuff it and see WTF happens.

Unfortunately sometimes this is the best route for your time and sanity.  Let us know how it goes.

 

[Chrome Heart]

Different PCB. Different problem. Meter will not respond in GR mode. It wont zero and it wont move. But it appears to be compressing according to audio I/O. Meter output drops as I  increase comp ratio and it "sounds" like its working. Voltage at gate of Q1 moves between near 0 and -2 volts as I turn the bias pot, but no GR meter. Meter works in output mode. Ugh.

 

[Echo North]

Different PCB. Different problem. Meter will not respond in GR mode. It wont zero and it wont move. But it appears to be compressing according to audio I/O. Meter output drops as I  increase comp ratio and it "sounds" like its working. Voltage at gate of Q1 moves between near 0 and -2 volts as I turn the bias pot, but no GR meter. Meter works in output mode. Ugh.

Your meter driver isn't work.  Small circuit, let's go through it.

You've set your QBias?  You attempted the meter null and tracking adj with no luck?

Everything in this section is DC.  So you have a - bias at the meter FET gate correct? You have the jumper pin in the right spot correct?

Check you meter voltages agains the MNATS schematic with voltages:
http://mnats.net/files/1176REVA_125_VOLTAGES.pdf

What's off?

Mike

 

[Chrome Heart]

I cant set the Q Bias because the meter wont show GR. No luck wit the null adjust or tracking adjust. Ive spun them both ways
Jumper is inside the white outline closest to trim pot

Q11           
G -.81             
                 
S -1.7
D -11.3

Q12
-5.83
10.09
-1.17

Q13
-1.70
11.28
-1.9

TP10 10.1
TP11 11.2

Q10
14.7
29.7
14.1

Q7
5.25
16.9
4.7

Q8
16.9
29.7
16.2

Q9
3.7
14.7
3.7

Q1
G -1.04
S 0
D 0

 

[Chrome Heart]

From what I can tell these voltage readings are pretty close. The only one in question is Q1 No reading on S or D.

 

[Echo North]

From what I can tell these voltage readings are pretty close. The only one in question is Q1 No reading on S or D.

You have to get your Qbias set before you do anything.  If your Qbias is off, your meter will not work properly if at all.  The MNATs video does the Qbias +4 metering which work right.  I use a DMM:

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

 

[Chrome Heart]

I was able to set bias by way of the DMM method above. I tried the null adjust as described on mnats video for the rev D ( I assume its the same ) and the DC voltage at R74 remains at .87 no matter which way I turn the pot.

 

[Echo North]

I was able to set bias by way of the DMM method above. I tried the null adjust as described on mnats video for the rev D ( I assume its the same ) and the DC voltage at R74 remains at .87 no matter which way I turn the pot.

Pull your trimmer and place your ohm meter on the center lug and an outside lug and see if it works.  If it's a 2K you should be able to rotate from 0-2KΩ.  If it's working set it for 1K as a start point and put it back in.  Look at your schematic and confirm the value of the pots that feed it are correct.

Mike

 

[Chrome Heart]

It works. Are TP 10 and 11 for the null adjust? Mnats uses R74 in the vid.

 

[Echo North]

It works. Are TP 10 and 11 for the null adjust? Mnats uses R74 in the vid.

Ya in the Rev D (video) R74 is where TP10/11 is.  Use TP10/11 in the rev A with the jumper on the pins near the PCB edge (or no jumper).

Sounds like everything is working but your GR circuit.  It's super simple and only about 14 components w/ no caps.  Have a look at your R values.  Your 0 set pot wiring.  Make sure your trimmers are set right (ignore the PCB silkscreen), and check your 29/28 and X/Y pad wiring.

 

[Echo North]

Double check those voltages too.  Some seem way off to me.

If you're setting you're Qbias fine, passing audio, and it sounds good and compressing...then just focus on what is below.

Mike

 

[Chrome Heart]

Got it. I was able to trim the null down to 0v no problem on the TP's. All the other adjustments fell right into place and went textbook. Thanks for sticking with me Mike. Your a champ. I just finished running some bass through it and it sounds great. I learned more on this one build then the last 3 Rev J builds put together. Now Im gonna go back and troubleshoot the problem board and finish the next one.

 

[Echo North]

Awesome!  8)

Mike

 

[Chrome Heart]

In the rack.