[BUILD] 1176LN Rev D DIY

[audiophreak]

OK , here's what I got after performing the "Q" Bias Adjustment and Discrete Meter Circuit Adjustment

-1.83vdc @ Q11 gate with no signal

applied input signal +1.513vac @ input XLR pins 2+3  , adjusted input and output controls to get a -10db GR

measured:
PCB input @ +0.164vdc
pad 22 +0.441vdc
Q11 gate -1.70vdc

  please educate me on what this means  :-\

  Thank you

 

[mnats]

  please educate me on what this means  :-\

Not possible to evaluate.

We need to know the magnitude of the audio signal you are applying at the various points mentioned. Therefore, DC measurements at those points aren't going to help us.

We do need to know the DC voltage at the FET gate of the device that reduces the gain of the signal. It looks like you measure the FET that controls the gain reduction meter circuit which is a clue, but not what was asked.

 

[audiophreak]

ok ... I miss labeled some of my measurements and measured the wrong FET  ...  :-[

-2.08vdc @ Q1 gate with no signal

applied input signal +1.513vac @ input XLR pins 2+3  , adjusted input and output controls to get a -10db GR in VU mode

measured:
PCB input = +0.162vac
pad 22 = +0.442vac
Q1 gate = -2.08vdc

 

[Echo North]

That seems like a REALLY high bias voltage to me.  I'm not sure you're doing your qbias right.

 

[mnats]

ok ... I miss labeled some of my measurements and measured the wrong FET  ...  :-[

-2.08vdc @ Q1 gate with no signal

applied input signal +1.513vac @ input XLR pins 2+3  , adjusted input and output controls to get a -10db GR in VU mode

measured:
PCB input = +0.162vac
pad 22 = +0.442vac
Q1 gate = -2.08vdc

You cannot have gain reduction if the gate voltage remains the same. So which condition isn't true?

 

[audiophreak]

that seemed not correct to me too , but didnt want to make an assumption and further my embarrassment.
  I checked a few times and the gate on Q1 is -2.08vdc with or without the +1.513vac input signal at input XLR .
  I am home now , but will go back to the shop in a couple hours. unfortunately I am going back out tomorrow at noon and wont be back till next Monday. 
I will redo the Qbias and null adjustment procedures - I suppose I could be doing them wrong , but I'm playing the calibration videos and doing the procedures along  with them and they correspond accordingly.


Thank you both for helping me along with this, I'm sure its user error  :-[

EDIT:  but I am hearing and seeing gain reduction

 

[mnats]

EDIT:  but I am hearing and seeing gain reduction

Avoid confirmation bias and measure gain reduction by switching it in and out with the GR off switch.

Oh yeah, it was a lucky mistake that you measured your GR metering FET as it shows that you will of course have deflection on the meter in GR mode. That should provide a clue to you.

I'm deep into my own projects myself so probably won't be checking in for a while...

 

[audiophreak]

Got it !!    would that be with meter switch in VU mode and shorting pad 22  in and out and not only measuring at Q1 gate but also at output XLR ??

heading out to the shop in few minutes

 

[audiophreak]

OK ...
    after re-calibrating the Qbias and Null adj. - with no input signal the Q1 gate measures -2.08vdc
applying a 1k signal @ +1.18vac at input XLR Q1 gate also measures -2.08vdc
Input pad on PCB measures +0.133vac
Pad 22 measures +0.436vac

Q11 measures -1.83vdc with no signal and -1.73vdc with same signal applied as above

In Bypass mode measured +1.18vac on input XLR and +1.02vac on output XLR
when put in GR mode with same input signal - output XLR measures+0.40vac

Oh yeah, it was a lucky mistake that you measured your GR metering FET as it shows that you will of course have deflection on the meter in GR mode. That should provide a clue to you.

 

I suppose it should ... but still lost , I'm guessing from your previous statements that the gate of Q1 should be changing with input signal so problem with signal preamp  or GR control amp as that splits off the ratio switch and has the Qbias ?

 

[Echo North]

OK ...
    after re-calibrating the Qbias and Null adj. - with no input signal the Q1 gate measures -2.08vdc
applying a 1k signal @ +1.18vac at input XLR Q1 gate also measures -2.08vdc
Input pad on PCB measures +0.133vac
Pad 22 measures +0.436vac

Q11 measures -1.83vdc with no signal and -1.73vdc with same signal applied as above

In Bypass mode measured +1.18vac on input XLR and +1.02vac on output XLR
when put in GR mode with same input signal - output XLR measures+0.40vac

Oh yeah, it was a lucky mistake that you measured your GR metering FET as it shows that you will of course have deflection on the meter in GR mode. That should provide a clue to you.

 

I suppose it should ... but still lost , I'm guessing from your previous statements that the gate of Q1 should be changing with input signal so problem with signal preamp  or GR control amp as that splits off the ratio switch and has the Qbias ?

If you look at the schematic you signal is sent through the GR amp where it is converted to a DC voltage.  It leaves the GR amp @ pad 19 after CR2.  This DC voltage travels through the attack pot to enter the PCB @ pad 7 and is fed to the Q1 gate.

The bias at the Q1 gate is negative something.  Generally between -0.75 to -2VDC this DC fed from the side chain is positive and moves it positive.  The more positive your DC bias gets the more GR you get.  A 0VDC bias being max GR.

 

[audiophreak]

ok , trying to understand ... If my Q1 gate voltage ( Bias voltage ?  )  is not changing with input signal, how am I getting measured GR at the output XLR ?

 

[Echo North]

ok , trying to understand ... If my Q1 gate voltage ( Bias voltage ?  )

Correct.

is not changing with input signal, how am I getting measured GR at the output XLR ?

That's the million dollar question.  My guess is incorrect wiring. 

Try your best to look at the schematic and understand the basic flow.  The signal AC signal is leaving the output pot, traveling through 1/2 the rtio switch and getting converted to DC at the output of the GR amps where it's sent back to the Q1 gate.  I think something is wrong there with your pad 15, 22, 19 or 7.

Mike

 

[Sillen]

Okey
I took my 1176 to an electrical engineer i got in contact with to try to finish it up. We looked over it for some hours and got stuck at the Gain reduction meter tracking adjustment at the point where you put pad 22 back in circuit and there is supposed to be a -10 db drop. However no drop at all occurs when we do so. What we found out were that the attack trimpot were bad, so we swapped that one out for a good one, thought this would solve the problem but NO. All solder joints seemed fine and the typical voltages from mnats schematic matched. The wiring seemed like it was fine too. Do you guys have any clue what could be wrong here? Where should we look especially? Does the transistors effect the VU Meter somehow?

Kind regards
Isak

 

[mnats]

ok , trying to understand ... If my Q1 gate voltage ( Bias voltage ?  )  is not changing with input signal, how am I getting measured GR at the output XLR ?

You cannot have gain reduction if the gate voltage remains the same. So which condition isn't true?

 

[studiostuff]

Anyone have an extra WIMA Film Capacitor, 100V, .15µF, 10% laying around that I can buy from you? Mouser is out of that part for 6 weeks and I was hoping to get finished before then? I'm in the Dallas area...

Thanks

 

[fluxofpinkindians]

Can I use stranded wire to lengthen the Secondary wiring or must it be solid ?

The leads seem real short, especially when wound. I can't find an ideal place to mount the tranny and reach both the PCB and the output XLR.

with thanks for any help

pace

 

[Echo North]

Can I use stranded wire to lengthen the Secondary wiring or must it be solid ?

The leads seem real short, especially when wound. I can't find an ideal place to mount the tranny and reach both the PCB and the output XLR.

with thanks for any help

pace

Stranded is best IMO.

 

[Echo North]

Anyone have an extra WIMA Film Capacitor, 100V, .15µF, 10% laying around that I can buy from you? Mouser is out of that part for 6 weeks and I was hoping to get finished before then? I'm in the Dallas area...

Thanks

It doesn't need to be 100V.  ANything above 50V will be fine.

 

[studiostuff]

Great. Thanks for the info...!!

 

[beatnik]

hey guys,

i have built one revD and i'm stuck with calibration!

i can set the qbias and the null adjustments (first and second video) but then i can't do the tracking adjustment.

when i switch the attack pot from compression on/off (i used the classic hairball components), there is not any drop on the vu meter.

i checked all the transistors voltages compared to the mnats schematic with voltages, and everything seems ok.
the preamp and output sections are perfect, as it's the gr control amp. the meter section is basically right, only lower about -10%.

i measured all the resistors before placement and checked capacitors value and every wire for continuity or shorts.

the fets are matched and i even tried with another pair.

it probably is something simple but i can't figure it out. i really need some help!

let me know if you need all the transistor voltages and i'll post them.

thanks in advance