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

[Randyman...]

AH HA!  I see it now.

Would that affect "Non-Linked" operation (Link Switch = "Down" where Output Pot feeds Ratio Board pad 15 directly?).  It would seem that the Stereo Link is out of the Sidechain at that point, correct?  I also un-hooked the paralleled "Loop" input from the Stereo Link PCB to make sure it wasn't shorting out the Sidechain somehow...

Thanks for working with me!  I think I'm getting close to a pair of working 1176's!

 

[Echo North]

Hard to say without looking at the schematic, but I would put it in there since it needs to be there and see what happens.

If that doesn't fix it, I would remove the stereo link circuit in one of your units and see if you can get it working without it.  Eliminate that as a wiring error.  Check where the (side chain) audio leaves the output pot and make sure the audio is making it to the side chain (ratio board/main PCB).  See if you can trace it through.

Mike

 

[Randyman...]

Will do.  Thanks for the suggestions.  I'll get to this a little later (after this place has gone to bed  ).

Have a good night  8)

 

[0dbfs]

If that's not it, I would also check grounding since you mentioned that it does work when you touch the pot. Is there an alligator-clip connection that you can make (instead of your finger) that will do the same thing so that it works hands free?

Sounds like at this point you can get it to fail and work. Both consistently. At least sort of.  :-\

Best,
jonathan

 

[Randyman...]

When I touch it - it still isn't responding to Sidechain audio - it's responding statically to my finger creating a circuit somewhere.  The gain reduction is basically on/off when I touch it (it doesn't follow the sidechain audio - it's a consistent amount of reduction when I touch it partially based on where the Release pot is set).

The front panel has a good connection to ground FWIW...

I'll have a good look at the PAD 7 connection and make sure all is well to the Release pot.  I've already verified that PAD 22 has continuity to the Ratio Green when the GR Bypass is disabled if that helps.

Thanks for the suggestions - more are welcomed!  I'll report back once I get home...

You guys rock!  ;D

 

[0dbfs]

I think the signal is fed into the sidechain from the point where the output pot connects up (at least on the schematic). Can you follow it with a scope through that section? Seems like there was a cap in there to get rid of any DC offset before hitting the diodes for turning into DC/CV. Was it C19 or C20? Don't remember now but I think that was an electrolytic after the transistors. Maybe if it was backwards or bad it would prevent audio from passing.

Best,
j

 

[Randyman...]

I believe PAD15 is what connects to the Output Pot, and daisy chains to the Ratio Board PAD15.  I'll double check that as well - but I'm pretty sure I already checked continuity.

I don't have an oscilloscope - but I'll peek at the schematic and see what components surround the PAD15 main PCB output and see if I goofed somewhere...

Looks like C7 (1uF) is right before PAD15 on the main PCB - I'll check it when I get home.  Are the orange Drop 1uF Caps polarized?  I didn't see any polarity markings on the PCB or on the Caps themselves - but they are installed the same way on both boards (could very well be the culprit!).

Q - Would having C7 in backwards still allow the unit to pass audio and have functional Output gain control?

Thanks!

 

[0dbfs]

--- EDIT ----
Ooops, see you posted while I was writing... Yes, Pad 15... If C7 was bad, you wouldn't get any audio through to the output amp.
--- EDIT ----


Yes. C19, C20... And check C17 too while referring to the schematic.

Actually, if you touch the pot and you get reduction then signal would theoretically be going through those caps into the gain-control-amp... Audio comes through the divider network before that from (pad 15?) at the top leg of the output pot. If you look at this section of the schematic you should be able to see how the audio signal after the pre-amp section goes into one half (left half on the schem) of the ratio-board/switch over to where the GR-OFF switch is.

Maybe you can disconnect your ratio assembly completely and hard-wire a test-jig with some spare resistors and jumpers to simulate both sections of the switch in a fixed 4:1 ratio setting and go from there?

Best,
j

 

[Randyman...]

I'll give that a shot as well.

Is C7 polarized?  If it was in backwards, would the unit still pass audio and have a functional output pot?  What about C10?  Just curious - as I thought the Orange Drop 1uF's (C7 & C10) weren't polarized (like C1 & C2) - but I think they might be looking at the Schematic.  What designates "+" on the Orange Drop Caps?


EDIT - Oops - I see you already addressed that issue.  So C7 and C10 are likely installed OK if I'm passing audio?  Why does the Schematic list tham as being polar with a clear "+" indication when the Orange Drop 1uF caps themselves seem to be non-polar?  ???

EDIT2 - PPS - What should I be getting off the main PCB PAD15?  Sidechain audio?  Can I tap into it (PAD15 + Ground) and patch it into a line input on my ADC to see if audio is present there (since I don't have an oscilloscope)?

Thanks so much for your help!!!  I'll play with the Ratio Board stuff when I get home...

8)

 

[0dbfs]

Maybe they (C7 & C10) were electrolytic caps in the originals. The orange drops are not polarized so orientation wouldn't matter. I think electrolytic caps would pass audio either way (someone please correct me if I am wrong) but they need to normally be oriented such that they have a positive DC at the + side and a ground ref at the - side else they eventually pop but yes, if you get audio through the output amp then you should get audio at pad 15 through to the ratio network.

there is a link around here for a "poor man's scope/probe" for circuit probing. Just a cap in series to a lead plugged into your mixer or speaker and probe with that to "listen" at different points in the circuit. Helps to find where the audio goes away without a scope but I have never used it.

Cheers,
j

 

[Randyman...]

So I should be getting sidechain audio on PAD15 then?  I'll see if I can get a poor man's scope (cap and an ADC  ) working for some troubleshooting tonight...

I really did take my time on these boogers - and only made 2 mistakes (at least only 2 I found!).  I initially wired the XLR to the IP Tranny (instead of to the attenuator), and had a single electrolytic cap I had in backwards - but caught it before I soldered it in (seriously triple and quadruple checked everything as I went!).

I'll report back much later tonight once I get home and have a chance to get my poor man's scope working...

Toodles 

 

[Echo North]

Yes off the top of my head, pad 15 is where the audio enters the side chain.

Mike

 

[Randyman...]

Quick Q - Why is the Release Pot to PAD7 and Attack Pot DPST to PAD 22 using shielded unbalanced wire when none of the other wires feeding the Attack/Release pots are shielded (18, 19, and the "jumper" between pots)?  What is going through the Attack/Release pots?  Audio or control voltages?  Why only two shielded wires here?

Just curious how/why that is...

 

[Randyman...]

OK - I've done some more troubleshooting and I still can't track down my issue (no gain reduction - but when I touch the Attack Pot's connections with my finger - I get static/constant gain reduction of -20dB or more as long as the release pot isn't fully CW)

I removed the Stereo Link Board - no luck.  I bypassed S4(A) with a 46K resistor from Pad 15 to Pad 22 - still no gain reduction.

I poked around the DPST GR Disable switch, Ratio Pad 15, PCB Pad 15, PCB Pad 22, Ratio Green, and I have audio present at all points.


I'm a bit confused on how to completely bypass the Ratio board.  I get the Sidechain Audio part where I can put a 47K resistor (R78) between PCB Pad 15 and PCB Pad 22 for the 20:1 setting on S4(A), but the other side of the switch S4(B) that contains Pad 20 and 21 also interfaces with the Release pot (pad 18).

It appears that selecting 20:1 basically shorts main PCB pads 20 and 21, but what should I do with Ratio Board Pad 18 to the Release Pot?  It seems like that connection has all of the S4(B) Ratio resistors in series plus a 120 Ohm R58 back into Pad 20.  So should I connect the Release Pot (pad 18) to PCB Pad 20 through R58+R61+R62+R63 in series (approx 2.5K Ohms)?


Should I measure some of the transistor voltages on the GR section and report back? I don't know what voltages I'm looking for - is there a reference I can use?



Thanks for any more help here.  I think I partially answered my earlier question about the unbalanced wires - and it appears the wire that connects Ratio Board Green (22) to DPST GR Disable Switch should be a shielded wire as well to complete the unbalanced sidechain audio loop from PCB Pad 15 (via the Output Pot) to PCB Pad 22.  The fact that a shielded wire wasn't specified there was what threw me!  I still don't undestand what the Attack/Release is doing in the circuit - but I'll get there some day!  

Have a great day  8)

 

[Randyman...]

One more quick question:  Should I have audio at PCB Pad 7 that connects to the Release pot?  It's a shielded wire that appears to come form the preamp section.  When I connect my poor man's probe (Soundcard through a 47uF cap) into Pad 7 , I don't hear audio and I get full gain reduction.  Does that sound like it might be the issue?

Thanks again - sorry for blowing up the past 2 pages!  :-[

 

[0dbfs]

Do you get audio at the junction of C20 & CR3 or C19 & CR2 or just before those two caps?

Pad 7 has control voltage from the release pot. It feeds to both Q1 and Q11 for the GR in the preamp and the meter driver.

Pad7 should have a negative DC on it.

To hard wire a fixed 4:1 and bypass the ratio switch try this:

- Disconnect the ratio assembly from the PCB. Pads 15 & 22. 18, 20, & 21.

- 1.5k + 470R + 470R (2.4k) between pad 20 and pad 21.
- 10M into pad 21. Both sides of 10M into pad 21.
- 120 ohm between pad 21 and pad 18.

- 47k + 68k + 56k + 56k (227k) between pad 15 and pad 22.
- 47k between pad 22 and ground.

That 10M in there appears to not be doing anything like that. Not sure what's up with that but it's on the schematic. You could probably leave it out for the test.

Like this:

Cheers,
j


PS.
Did you set the Q bias propoerly? You should get compression once that is set properly without calibrating the GR meter.

 

[Randyman...]

Do you get audio at the junction of C20 & CR3 or C19 & CR2 or just before those two caps?

Pad 7 has control voltage from the release pot. It feeds to both Q1 and Q11 for the GR in the preamp and the meter driver.

Pad7 should have a negative DC on it.


PS.
Did you set the Q bias propoerly? You should get compression once that is set properly without calibrating the GR meter.

Yep - Audio at C20/CR3 and C19/CR2 (audio at the anode, but not the cathode of CR2/CR3).  Pad 7 has about -1.25v DC.  Q Bias was set (-1dB) as was Null Adjust (both units were calibrated).

I'll have to buy some resistors to try bypassing the Ratio Board (thanks for the diagram!).  Is that all that's left at this point - or could something still be wrong on the PCB?  It's gotta be something I did on both units  ???  Unless I got a pair of bunk switches or something?  How should I go about testing the ratio board in the meanwhile?

I'm fairly confident in the wiring overall.

I think I figured out that the GR Control circuit simply controls the amount of gain in the Preamp Stage - correct?  I thought the GR stage was a separate signal gain stage, but it just tells the preamp how much gain to "take away" for reduction - is that right?  Learning little by little by probing around   ...

Thanks for your valuable time  8)

 

[0dbfs]

If you are getting audio up to those diodes that's where its rectified and sent over to the ratio board.

Check this out:
http://www.nrgrecording.de/html/how_it_works.html

If you adjust the qbias pot, that -1.25 moves right? And if you are listening to audio through the unit that voltage should reach a point where reduction engages and moving the qbias pot then makes it more loud or quiet...

Say for a sec that you ground the cable plugged into pad 15. This would disable audio to the sidechain. If you then send audio through the unit and adjust the qbias up/down you should get to a spot where you can control the audio level with the qbias pot. I believe the qbias cal gets you right on the edge of that point. Then reconnecting pad 15 sends audio back into the sidechain and the audio is rectified at the diodes and added to that -1.25 V making it a variable -1.25 bias voltage... That is pad 7 going to the gates of Q1/11 which control the compression in the preamp and the meter GR circuit.

So, if you can use the qbias like that you can confirm that Q1 is seeing and reacting to the CV signal without sidechain audio. Eliminates the possibility of a bad Q1.

If your GR meter is already cal'd then you should see the same thing on the GR meter. Seeing it there eliminates the possibility of a bad Q11 (or at least confirms that it's reacting as expected).

Q11 controls the GR meter. Q1 controls the audio. Can you swap those two easily?

Cheers,
j

 

[Randyman...]

If you are getting audio up to those diodes that's where its rectified and sent over to the ratio board.

Check this out:
http://www.nrgrecording.de/html/how_it_works.html

Cool link!  Thanks.

If you adjust the qbias pot, that -1.25 moves right? And if you are listening to audio through the unit that voltage should reach a point where reduction engages and moving the qbias pot then makes it more loud or quiet...

Yes - Adjusting Q Bias changes Pad 7's voltage, and adjusting Q Bias further does bring down the audio output level as well.

Say for a sec that you ground the cable plugged into pad 15. This would disable audio to the sidechain. If you then send audio through the unit and adjust the qbias up/down you should get to a spot where you can control the audio level with the qbias pot. I believe the qbias cal gets you right on the edge of that point. Then reconnecting pad 15 sends audio back into the sidechain and the audio is rectified at the diodes and added to that -1.25 V making it a variable -1.25 bias voltage... That is pad 7 going to the gates of Q1/11 which control the compression in the preamp and the meter GR circuit.

So, if you can use the qbias like that you can confirm that Q1 is seeing and reacting to the CV signal without sidechain audio. Eliminates the possibility of a bad Q1.

If your GR meter is already cal'd then you should see the same thing on the GR meter. Seeing it there eliminates the possibility of a bad Q11 (or at least confirms that it's reacting as expected).

Q11 controls the GR meter. Q1 controls the audio. Can you swap those two easily?

Cheers,
j

Working on that next.  Doesn't pad 22 feed the GR circuit, and Pad 15 is the output of the preamp stage?  When calibrating, I had the "GR Disable/OFF" DPST engaged and had Pad 22 shorted to ground.  I confirmed Pad 22's audio went silent with the GR Disable set to "OFF".  If I re-engage GR, Audio is then present at Pad 22 (GR Control Amp Input) and C19/C20.

Also, when I touch the Attack Pot or Pad 7 & Ground with my finger, I see gain reduction on the GR meter, and I get a reduction in output level.  Does that verify that Q1 and Q11 are working?  I didn't socket them, and would prefer to avoid de-soldering them.

It seems like everything should be working, but the sidechain audio just isn't making it to the GR Control part.  Would Q7/Q8/Q9/Q10 have any effect on this - and what voltages should I be seeing on them?  What about the Attack/Release?  What should I be seeing going through there?

Thanks again - reading the 1176 "how it works" now  8)

 

[0dbfs]

Q7,8,9,& 10 set up an inverted polarity and non-inverted for side chain audio through C19 and C20 to the two diodes. The diodes only pass positive waveforms so they chop off the bottom have of each inverted/non-inverted sidechain and mix those two so that compression follows both positive and negative peaks.

Since you were able to hear audio at C19 & C20 this confirms that it's getting through Q7,8,9,10. I'm not sure what the reading should be though in that area. Maybe someone else can confirm that?

Did you use 2n3707's or other in there? I guess if there wasn't enough signal in that section for some reason then that may affect something.

Interesting about touching the attack pot. Is something in that section shorted to GND? How do you have the ratio board grounded? Is there only one path to ground? Can you change the grounding of that part around to see how it changes things?

I eventually ended up wiring that section point to point n the back of some switches instead of using the board.

No audio is expected after the diodes since it is only a positive going waveform at that point. What are your compressor input/output settings? Can you turn input up and output down with no changes (for more compression)?

Yes - Adjusting Q Bias changes Pad 7's voltage, and adjusting Q Bias further does bring down the audio output level as well.

Does the GR meter show reduction (like the audio does) when you adjust the Q bias pot with the sidechain bypassed (GR-OFF or pad-22 to gnd)?

Seems like you have confirmed that the audio/cv makes it to the attack pot. Maybe you can meter at diodes cathode (pad 19) and look for similar readings at the attack pot (AC and DC?). Seems that I remember this signal jumping through the ratio board too. Maybe that's a potential spot for soldering issues? I remember those boards didn't take the solder as well. Not nearly as well as the main PCB. I had to chuck mine and go with some other switches I had

You could also bypass the attack/release pots and hard-wire a couple fixed resistors and a cap to see if it changes the behavior.

The attack/release pots set up the time constants with that capacitor (c27) which releases the positive CV into the -1.25V line to pad7 (bringing it towards 0V and back towards -1.25 based on sidechain-audio/cv peaks). R55 (attack pot) adjusts how fast the cap charges (which offsets the -1.25 qbias) and R56 adjusts how fast the cap drains to gnd (via pad18/Q bias path).

Cheers,
j