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

[damian]

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

Hi Mike,

When I send 0.775VAC to the input, with attack set off and input and output knobs set to
midway the needle is all the way to the right and the output voltage is 6.51VAC when the
qbias is set to max I can turn it down to 2.45VAC

But when I try Calibrating the Discrete Meter Circuit the voltage is fixed at 0.45VDC across R74 when turning R75, but turning R75 does make the VU meter needle move.. I cant get the voltage to move up or down

Thanks,

Damian

 

[AntonSwe]

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

Thanks, that´s really nice to know!

So now I have checked the input circuit again and (sadly) everything looks right... I did as you said and removed one leg and measured R4 again just to be sure but it measured 270,3 Ω. I then removed Q1 and did a new frequency measurement but with the same rolled-off results... This really is very strange. Where is my bass going!?

Here is a picture of the input circuit just in case I´m overlooking something really obvious. Just tell me if you want more pictures and I´ll mail or post them.

Thanks again for your efforts!
//Anton

 

[Curran]

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

ok still in "compressor not working hell"! My DMM does not read that fine of a VAC I think i can only read it a (.7) or (.8) I get NOTHING on the xlr output with the DMM on the same setting. BUT  it passsing signal through?!?!?!?!? sooooooooooooooooooooo what hell do I do now? I know I am frustrated but damn it I am haveing a bad shitty day. I just want to compress and smash some death metal guitars for no good reason with my compressor. please continue to humor me and help me

 

[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

ok still in "compressor not working hell"! My DMM does not read that fine of a VAC I think i can only read it a (.7) or (.8) I get NOTHING on the xlr output with the DMM on the same setting. BUT  it passsing signal through?!?!?!?!? sooooooooooooooooooooo what hell do I do now? I know I am frustrated but damn it I am haveing a bad sh*tty day. I just want to compress and smash some death metal guitars for no good reason with my compressor. please continue to humor me and help me

Test your DC at the Q1 gate and turn the qbias pot to the the bias as negative as possible.  What is that DC? Can you pass signal then?

Mike

 

[Echo North]

I'm stumped Anton.  Please email me "info at hairballaudio dot com" and we'll talk about trying some stuff through the mail.

Mike

 

[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

Hi Mike,

When I send 0.775VAC to the input, with attack set off and input and output knobs set to
midway the needle is all the way to the right and the output voltage is 6.51VAC when the
qbias is set to max I can turn it down to 2.45VAC

But when I try Calibrating the Discrete Meter Circuit the voltage is fixed at 0.45VDC across R74 when turning R75, but turning R75 does make the VU meter needle move.. I cant get the voltage to move up or down

Thanks,

Damian

You want to adjust your output put so you have 2.75 between pin 2 and 3 on the output.  You'll need to turn the out pot down to get the output XLR to 2.75.  Then do your adjustment.

 

[damian]

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

Hi Mike,

When I send 0.775VAC to the input, with attack set off and input and output knobs set to
midway the needle is all the way to the right and the output voltage is 6.51VAC when the
qbias is set to max I can turn it down to 2.45VAC

But when I try Calibrating the Discrete Meter Circuit the voltage is fixed at 0.45VDC across R74 when turning R75, but turning R75 does make the VU meter needle move.. I cant get the voltage to move up or down

Thanks,

Damian

You want to adjust your output put so you have 2.75 between pin 2 and 3 on the output.  You'll need to turn the out pot down to get the output XLR to 2.75.  Then do your adjustment.

Hi,

Yes that's how I set Qbias  I didn't explain properly in my message. Its Calibrating the Discrete Meter Circuit is my problem, probably why I couldn't set qbias using the VU meter as in the video. But it set fine using mike's instructions

I have a constant 0.45VDC across R74 which seems to be my problem.

Thanks,

Damian

 

[Echo North]

Actually with the Rev A you don't measure across R74.

You take R44 out of circuit by moving the jumper closes to the boards edge and measure across the TP10 and TP11.

Mike

 

[The Beatsmith]

Hi,

Just got my Rev. A mnats with Hairball kit built by a buddy and it's great!

Only issues: I opted for the LED meter.

It was wired directly from the power supply and so has a bit of a 50Hz flicker which bugs me a bit.

What's a better way to do this and power the LED meter?

Thanks!

Ed

 

[Echo North]

Hi,

Just got my Rev. A mnats with Hairball kit built by a buddy and it's great!

Only issues: I opted for the LED meter.

It was wired directly from the power supply and so has a bit of a 50Hz flicker which bugs me a bit.

What's a better way to do this and power the LED meter?

Thanks!

Ed

You could build a little bridge rectifier with 4 1N400* diodes.  Put it on a little perf board or float it.  Or just power it off your 30VDC test point with a dropping R.

 

[damian]

Actually with the Rev A you don't measure across R74.

You take R44 out of circuit by moving the jumper closes to the boards edge and measure across the TP10 and TP11.

Mike

Thanks Mike that worked, which calibration instructions should I be following because there was no mention of TP10 and TP11 in the video I was following, just have the last step to do now

Thanks,

Damian

 

[Curran]

negative bias at the Q1 Gate is (-.082DC) I still pass signal, when I turn it the qbias control the other way the negative DC drops and the signal stops passing though

 

[Curran]

sorry I am in idiot (-.82) is the reading

 

[Echo North]

sorry I am in idiot (-.82) is the reading

That all sounds normal.  As is moves to 0V most of your signal gets dumped to ground.

If you move it as neg as it will go (about -2 to -3 V I would guess) can you then complete the qbias adjustment?

Keep in mind the qbias is at high impedance so your meter is loading it a little.  The reading is a little less that is actually is.

Mike

 

[rainton]

Hi Mike,

I just started stuffing my pcb and as suggested began with the power section.
Now I triple checked everything but on my +30V DC test point I only get 23.35V DC
and on my -10V DC test point I only get -4.5V DC?

That's too much off, right

 

[Curran]

I turn the Qbias and the lowest it goes is (-.82vdc) so I need to adjust it to -2 or -3? it wont go there as far as I can tell. if I need it to go to -.2 or -.3 then ok.

 

[rainton]

Hi Mike,

I just started stuffing my pcb and as suggested began with the power section.
Now I triple checked everything but on my +30V DC test point I only get 23.35V DC
and on my -10V DC test point I only get -4.5V DC?

That's too much off, right

BTW: I have the pwr xfmr secondaries like this: red & orange tied together and black & yellow each to AC
Measuring between CT and AC at the terminal block I get 24.6 V AC - so that seems to be correct, right?

I was actually a little confused with the secondaries because in the written manual it said "tie the 0V leads together" but then again it said "red & orange go together" - I use the supplied Avel Lindberg tranny and there it says black & orange = 0V...
...I also found other people in this thread being confused by it but I haven't found an answer to this yet...
...so for now I went with red & orange together

Anyway regarding my DC voltage being a little to low on both the +30V and the -10V rail where should I look first?
(FYI I tried different 7284 VRs already - no change)     

 

[Harpo]

BTW: I have the pwr xfmr secondaries like this: red & orange tied together and black & yellow each to AC
Measuring between CT and AC at the terminal block I get 24.6 V AC - so that seems to be correct, right?

If you measured between CT (red+orange) and the 1st.AC (black) wire at the terminal block, getting 24.6 V AC AND measured between CT (red+orange) and 2nd.AC (yellow) wire at the terminal block, getting maybe another 24.6 V AC (this would measure 49.2V AC between black and yellow), this would be a little lower than expected but enough to work correct, assuming your transformer is a 2x25VAC secondary from the avel lindberg Y23 range.
From your 'I only get 23.35V DC and on my -10V DC test point I only get -4.5V DC' you might have this transformer center tap junction (red+orange) not connected to the middle 'CT' connection at the AC-CT-AC connector on pcb with the black transformer wire going to one AC connector and the yellow wire to the other AC connector.
Another possibility is, you have wired your transformers primary windings in series (needed for 230V mains) and connect to your -Absurdistan or whereever you might be located, please update your profile- local maybe 115V mains.

From the manual you have to read the complete sentence "Just make absolutely sure you have tied the two "0V (centre tap)" wires together as indicated in Mr. Burnley's OUTPUT CONFIGURATIONS/SPLIT SECONDARY FOR +/- POWER SUPPLY illustration."

 

[rainton]

I'm from Germany Harpo  ;D

So the primaries are correct. Regarding the secondaries: I already had black&orange connected and red & yellow to AC because that is what the schematic on the side of the tranny said - like: Black 0V - Red 25V - Org 0V - Yel 25V

But Mnats guide for exactly this avel lindberg xfmr showed red & orange together.

So I was searching the thread and found other people that stumbled over this, but I never found anybody confirming that in this case black & orange have to go together - actually everybody kept writing "tie red & orange together"...

So I decided to go the red&orange route...

Actually this is the complete statement from Mnats guide:  "Just make absolutely sure you have tied the two "0V (centre tap)" red and orange wires together"

According to the schematic on my xfmr I can compare the above statement to:
If I'm asking for the way and the answer would be "make absolutely sure you make a left" while the person is pointing to the right. 

Anyway so I guess I try both of the real "0V" wires...

 

[rainton]

Ok now I tried black & orange together - same result

at the AC-CT-AC I get around 24.7 between CT and AC and 23.4DC at the +30V testpoint and -4.5V DC at the -10V testpoint...

It has to be something in the power supply.

Like I said I double checked all resistors with the DMM before soldering them into place - the same with the capacitors...
I even changed the 7824VR for another one to see if there's a problem...



I mean the AC seems to be alright at the terminal block, right? Only like .3 off
And also I checked the connections from the power xfmr to the terminal block - it's all there.
But the DC at the testpoints is way off...