Really hoping to get my 1176 working...

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Mbira

Well-known member
Joined
Jun 4, 2004
Messages
2,422
Location
Austin, TX
Sorry for the double post-The other post started different so I wanted to start a seperate topic. My meter is pegging when in GR mode. What could be causing that? At this point, I don't seem to be able to do any sort of "bypass". I can feed a signal in and the input and output post work (maybe even as they should). But I can't get any sort of bypass. I am assuming that bypass means that I will be getting unity gain on the output-like a "true bypass"?

I did stupidly wire up my input xlr incorrectly at first by switching pins one and two.

Thanks for any help...

Joel
 
no, the bypass shouldnt have anything to do with unity, it just switches the sidechain out so you still get the full gain swing of the line amplifier. Does the unit sound ok? Have you gone thorugh the calibration procedure? I wouldnt expect the unit to function properly at all until you've properly biased the fet and got the meter tracking right. I wouldnt worry about what the meter is doing until you go through the cal.

dave
 
This may be where my confusion is. Reading the gyraf site, I was thinking that calibrating and doing the meter are one and the same. Here are the steps listed on their site (sorry for the cut n' paste Jacob):

1: "Q Bias". This is set by bypassing the unit, putting through a signal, adjusting in- and out level 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.

OK, now reading back, I am wondering, when putting the signal thru and then "adjusting the in and out level", is theat the input and output pots on the unit itself?

I am using a signal generator sending 1Khz at .870Mv to the input (+1db). Is that correct?

I'll just start with number one for now.

Thanks for your help.
Joel
 
joel-

you want to feed the unit 1K and adjust the front panel input and output pots, usually around 10 on the input and 2 on the output in my experience. With the meter in "+4" mode, you should be able to dial in +1dB. Adjusting the drop is important as it turns the fet "on". Thats at least the way Ive looked at it to visualize it a little easier. Its kinda like setting a global threshold level so the box knows when to start compressing, sorta.

dave
 
OK,I got the Q bias done. I had put the little wire going to the Release pot in the wrong hole :roll: .

My meter still pegs in GR mode no matter if there is a signal or not=what could cause that?

What the hell does GR even stand for anyway?

Joel
 
did you do te rest of the calibration?

If so and you still have an off scale reading, check to make sure that there arent any shorts between your ratio selector switches. When you push all the buttons in, you'll generally get an off scale reading, you may have more than one ratio selected simultaneously.

dave
 
did you do te rest of the calibration?

Well the only calibration I see on Jacobs site is:
1) Adjust the Q bias (done)
2) Adjust for distortion (it says that's not important-and just set in the middle position) done
3) now do the GR meter calibration-That's where I'm at.

Is there more that I'm missing?

I'm in school now-Last day of finals-woohoo then off to the DIY land...

Joel
[/quote]
 
Meter calibration means push in the "GR" button on the meter and twist the little trim pot until it does something-right? I push the "GR" button and the meter instantly jumps all the way up and stays there. This no matter what I do-if there is a signal or not and however the input and output pots are turned.

Joel
 
well, thats what happens when you do the "all buttons" thing, you might search around the ratio switch wiring for shorts. Of course it could be something else, but that immediately comes to mind.

dave
 
OK, the voltage from the number 29 where the wire jumps from the board to the GR switch is putting out 7.2V DC. That is when the GR button is not pushed. When I push it, it drops to 4.36V. When I push any of the ratio buttons, the volts drop and then quickly (within 5 seconds) rises back up to the 4.36V.

Does this help shed any light on this problem?

Joel
 
I think I've got it...I'm gonna check something out and post when I know for sure. I don't want to look TOO stupid yet! :cool:

Joel
 
OK,
The trim pot associated with the TL071 (R55) had a short in it. I replaced it and now got the GR meter to calibrate. Now I'm on to the next part:
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.

I believe I am doing this correctly, but when I try and turn up the signal source, the output just follows it until it clips-It never does a gain reduction thing.

Also, my output wave seems that the ground is fluctuating... I measured it up till the output transformer and it's good til there. It seems to happen in the transformer?

Any thoughts?

Thanks for any and all help guys. My eyes are crosseyed now...

Joel
 
Hi,
I'm still having this problem...

I am assuming that 50mV is peak to peak?
The output wave is very ugly (even at 50mv output) There seems to be some interference making the wave ugly. It seems to only be happening after the output transformer.

Also, the output tracks the input as the input source is increased-there doesn't seem to be any gain reduction happening...

Thanks for any and all help.

Joel
 
Well, it seems like the impedence of the test cable I was using was the thing adding the noise to the output. I'm using a better cable and that noise is gone...

Still fighting the ratio board. I have discovered that the 1:12 setting DOES compress-I don't know if it is doing the right ratio, but it does work. None of the other ratio buttons compress though...
 
ok well, if one of the ratios work, I would bet the problem is in the wiring, the limiter obviously works. Did you etch a pcb for the switch bank or did you use DPDT's or what? Make sure your wiring is right, doing those switchbanks is a royal PITA. With one ratio working however, I woudl be suprised if the problem isnt in the wiring assembly for the switchbank, check for cold solders, right values, etc.

dave
 
I'm on it dave...I etched the pushbutton boards myself. They were from mnats board but my pushbuttons were the four pole (FPDT) that SSLTech mentioned, so I had to do some...errr...extending. I was going to share the design a while back, but I wanted to make sure they worked right first! :roll:

Joel
 
It took me about maybe 30 minutes to wire 4 dpdt's into the configuration and then maybe another 20 mintues to do the wiring to the pots and boards, push comes to shove you could always hack that together to test the limiter and make sure the problem is on your switch pcb, Im guessing it is.

dave
 

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