Tech Question for my 1176-schmtic included

[college101]

OK...I have tried and tried over the last 2 weeks to get my 1176 to show 10 db reduction to the signal in GR mode with compression ON and in the circuit...

My problem started with me thinkin that R44 was bad, which I replaced...I have re measured and re-inserted into its correct placement, BUT with no amount of turning, it does nothing to the needle to zero it out.. R71 and R75 work just fine, as well as my q-bias trim.

I took some measurements and it seems in stead of 9.28v and 9.75v- I am actually getting around 13 volts...which seems high to me...everything else in circuit is within spec and +/- a tenth of the schematic voltages except above q12 and q13...

When I go from setting the front control's to Zero with compression off, and -10 with compression ON...I go from +4 to GR mode, the needle moves, and I have to re-zero the meter....(This is where I think I am messing up***) I re -zero the meter with compression off...I turn compression ON, and the needle drops to -7  ????

No amount of adjustments can move the needle from -7...

I have attached a schematic drawing with information to show what I am looking at for your reference

 

[gyraf]

..please use the dedicated help thread..

Jakob E.

 

[college101]

Jakob...I have been using the dedicated Mnats thread...

Except nobody else is! Nobody is helping anyone in that thread

 

[PRR]

> around 13 volts instead of 9.75v What do you think is wrong?

R71

 

[college101]

R71 =2K pot- which is the zero adjust pot=  I have mounted to the faceplate* instead of having a trimmer pot mounted to the PCB board...

After you calibrate frontpanel settings and then go back into GR mode...you re-zero the vu meter..I have to adjust it, because it drops from 0 to about -5...when I switch to GR

 

[gemini86]

Then q bias is wrong.

 

[college101]

I will try to recalibrate the q-bias again, but that would just be pulling the 2k trimmer, putting in a fresh one, re calibrating...

I put in a 1k tone at .775(measured that first with a DMM before I attach the xlr "In" cable)... then turn the input up

controls
input full off
output controll three quarters of the way
attack OFF-no compression
release-fully clockwise
20:1 ratio
meter to VU or +4

Place the wiper of the Q bias closest to ground(which is corrected at Mnats site as being farthest away from ground...or at the highest setting the trim can reach)

apply 1k
turn up input control to show needle at +1 on vu meter
turn the q-bias adjustment down to re-zero the meter which then places the FET into action

I will re-do the calibration again since I have not tried a new 2k trimmer in the qbias location, but I dont think that is problem.

 

[gemini86]

That calibration doesn't sound correct. Q bias should be performed without any signal. Gr mode. Set q bias so fet is off, no reduction, then adjust bias until you see 1db meter movement
rezero meter. Done.

 

[college101]

Have you seen the mnats calibration video for the q bias adjustment procedure? Its on YouTube

In order to have signal in +4 vu mode, you have to have signal

 

[gemini86]

You are correct, I remembered wrong...

[edit] I use this calibration method from the Gyraf site...(I'm sure it's just like mnats, but cannot view the video right now.) Works without fail.

----

On adjusting the unit...

- Once the unit is built and checked, proceed with the adjustment procedure. You could use the procedure from the original .pdf manual, but that could be a bit tricky because we changed the bypass to be on the meter switch in stead of on the attack switch. So when they say "turn attack fully counterclockwise to bypass", it could translate into something like "bypass the unit with the meter switch". But that is not easy to follow, so here's a simplified adjustment procedure:

There is four adjustments.

1: "Q Bias". This is set by bypassing the unit, putting through a signal, adjusting in- and outlevel so output is around +1dB, and adjusting the trimmer untill a level drop of 1dB is acheived. This sets the FET start point in the beginning of it's conducting range.

2: "Dist. trim". Bypass the unit, input a ~1kHz, ~0dB signal, set unit gain to unity (by input- and output controls), and set trimmer for minimum distortion. If you have no dist.meter, simply set at middle position. Somehow this is not an important adjustment soundwise.

3: "GR meter Zero". No input signal. Meter switch in GR mode. Adjust untill meter reads 0dBVU.

4: "GR meter Trck" (tracking). Set ratio to 1:20, set Meter to bypass. Input a 1kHz, 50mV signal. Set input level to ~12'oclock. Set output level so you get 50mV at the output also (now you have unity gain). Now set the meter switch to "GR". Slowly turn up the level of your input signal (at the signal source, not with the input level control) while you monitor both input- and output levels with a voltmeter. When you reach a point where output level is half the input level, you have 6dB of gain reduction in the unit. Now set the "GR meter Trck" trimmer so the VU-meter reads -6dBVU. Remove input signal and (re)set "GR meter Zero" for a reading of 0dBVU. This calibrates the gr indication.

 

[college101]

So the Gyraf calibration procedure sets it to -6  in the 20:1 ratio postition?  I bet that circuit is different from Mnats then

 

[college101]

I just realized after my last post, even if I could calibrate it that way, I get NO movement out of R44 for the final inside the compressor adjustment... so that would not work either