[BUILD] 1176LN Rev D DIY

[guitarrock04]

Unit is functioning, but not 100% correct.. Still have an issue with the 20:1 setting, but more critically, there is a persistent audible distortion on any audio. Kind of fizzy, almost HPF phenomenon. The unit functions and compresses on vocals and drums I tried. Output waveform is asymmetric, if it matters.
DC voltages in the pre and line amps are spot on to the Hairball troubleshooting guide. AC voltages with .1vac 1khz at transformer out are NOT in line with guide.. I get 0.656vac @ q2 base, 0.088vac @q3 base, 4.92vac @q14 base and 0.337v at the output pot.
I'm grounding pin 22, per mnats videos as a surrogate for GR off. However, I do note a change in the q2 base voltage when I switch ratio. Goes to 0vac in slam mode. Rotary ratio position shouldn't affect signal voltages when pin 22 is grounded??

Any help appreciated.

 

[gswan]

You should probably look to fixing the obvious faults (20:1 setting) first. The cause may be creating other artifacts in your audio. However the 1176 is not a clean compressor, it introduces a lot of second harmonic distortion. Did you feed a clean sine wave into the unit and probe it with a scope to see if there are any artifacts showing? When calibrating did you setup the symmetry setting for minimum distortion?

 

[guitarrock04]

I thought I had it, but no avail. I have a 6 pos 2 pole ratio switch. (note that this was called out on the original mnats BOM) It should only be 5 position - I labelled it to the most clockwise position as slam. If I work CCW to CW based on position, the DC values in GR section make perfect sense. It matches the table in section 6 perfectly. Previously what I thought was 20:1 was in fact SLAM, and since I used what is labelled 12:1 for calibration was in fact 20:1, I did as I should've. Everything was displaced by one switch position...

My AC voltages are still off. AC is low at Q2 base, high at Q3, Q14 and low at TP15. Low at TP22, (.033vac @4:1, to 1.54vac @20:1) Low at bases of q7 and q9 (1.56vac and 1.62vac, respectively) My DMM is unable to measure vac at Q8 and Q10 bases.. Low at CR3 anode (.310vac @4:1 through 1.44vac @20:1)

Other thoughts?

 

[gswan]

OK, that answers the 20:1 fault problem.
Now, if the DC bias voltage is OK around Q2 but the AC is low at Q2 base it may mean that you have the FET too far into conduction. What calibration technique did you use? Measure the AC signal on both sides of R26 (27K) and see.

 

[JW]

Anybody have a link to an analysis of what the original caps were (generally) in an older rev D?
Are we safe to assume that the mnats bom, when it has a part like, let's say a ceramic disc capacitor, that that is what was used in that spot? More than ceramic disc though, I'm interested in the film caps in the preamp and line amp. Some folks use Orange Drop (which series?) instead of Wima, that's all I know. Were orange drops used originally? In guitar amps, Mallory 150 series polyesters are often used in place of Orange Drops. Would like to know what is commonly found for film caps in an original? Other caps too. ..

 

[znjamznjam]

Hy
I have mount Cinemag CM3303 input transformer to PCB for C-3837-1 input transformer. After troubleshooting my setup a see that Cinemag Metal Can now have not connectie with grond. I see that Cinema mcan have another pin for grond then C-3837-1. Is er anybody that use cinemag C-3837-1 input trafo in these builds?

Thanks in advance

 

[guitarrock04]

OK, that answers the 20:1 fault problem.
Now, if the DC bias voltage is OK around Q2 but the AC is low at Q2 base it may mean that you have the FET too far into conduction. What calibration technique did you use? Measure the AC signal on both sides of R26 (27K) and see.

I get 0.070 vac on one side. My DMM can't measure the other. That was a trend on the higher (>3vac) voltage points elsewhere in the circuit. I can reduce frequency on the sig gen and get a reading, but it skews everything relative to the 1khz build guide voltages..

I used this process verbatim:
https://www.hairballaudio.com/blog/resources/build-guides/fetrack-v2-buildbrstep-4-calibration
Q bias trimpot wiper is -1.499v

 

[gswan]

I get 0.070 vac on one side. My DMM can't measure the other. That was a trend on the higher (>3vac) voltage points elsewhere in the circuit. I can reduce frequency on the sig gen and get a reading, but it skews everything relative to the 1khz build guide voltages..

I used this process verbatim:
https://www.hairballaudio.com/blog/resources/build-guides/fetrack-v2-buildbrstep-4-calibration
Q bias trimpot wiper is -1.499v

That Qbias value looks wrong. It looks like the FET is way into conduction. Did you map the curve for the FET prior to use (for matching purposes)? I would have expected somewhere between -0.2 and -0.5V depending on the FET transfer curve.

 

[JW]

Can you fit 1/2 watt carbon resistors into the latest mnats (rotary) rev d boards? Like this:
https://www.digikey.com/en/products/detail/ohmite/OF273JE/16838679

 

[guitarrock04]

That Qbias value looks wrong. It looks like the FET is way into conduction. Did you map the curve for the FET prior to use (for matching purposes)? I would have expected somewhere between -0.2 and -0.5V depending on the FET transfer curve.

I will re-calibrate.

Curve, no. I just used my cheapie transistor tester to match IDSS on a couple parts for q1 and q11

 

[gswan]

I get 0.070 vac on one side. My DMM can't measure the other. That was a trend on the higher (>3vac) voltage points elsewhere in the circuit. I can reduce frequency on the sig gen and get a reading, but it skews everything relative to the 1khz build guide voltages..

Can your DMM work at 1kHz? Some cheap DMMs do not. They are made for main frequencies (50/60Hz). Make sure it can handle audio frequencies accurately before you have any confidence in its measurements.

 

[guitarrock04]

I went and bought a new meter, just to be sure. I got no change in readings.
I swapped a different Q1, and now re-calibrating, this time get ~ -1.1v at qbias wiper. CR3 anode voltages are closer to guide; 2.78vac @20, 0.57vac @4:1.

BUT - new meter tells meI get 0vac at OTX brown and Q10 base with new meter. That's chassis reference. If I measure between brown and black leads, I get ~3.6vac. Could that be an issue with decoupling cap somewhere? Perhaps an issue with Q9 or Q10?

 

[gswan]

I went and bought a new meter, just to be sure. I got no change in readings.
I swapped a different Q1, and now re-calibrating, this time get ~ -1.1v at qbias wiper. CR3 anode voltages are closer to guide; 2.78vac @20, 0.57vac @4:1.

BUT - new meter tells meI get 0vac at OTX brown and Q10 base with new meter. That's chassis reference. If I measure between brown and black leads, I get ~3.6vac. Could that be an issue with decoupling cap somewhere? Perhaps an issue with Q9 or Q10?

Can you provide more details that 'brown and black leads'? This has no meaning. Use circuit references to be accurate. No idea what 'OTX brown' is either.
Q10 is a JFET, no base. Only gate, drain and source. Unless you are using a different schematic than the one I have.

So you have confirmed that all DC steady-state conditions are correct? This is usually a good indication that you have transistors operating and correct resistors in place. After this comes an attempt at calibrating the FET bias voltage so that it is just into conduction. You have done this OK?

Now you are addressing metering. You should be able to see the signal VU level when switched to +4 or +8 positions. If not then you may need to add diodes to your meter. Once the meter is actually operating as a VU meter than you can calibrate the gain reduction metering.

 

[guitarrock04]

Can you provide more details that 'brown and black leads'? This has no meaning. Use circuit references to be accurate. No idea what 'OTX brown' is either.
Q10 is a JFET, no base. Only gate, drain and source. Unless you are using a different schematic than the one I have.

So you have confirmed that all DC steady-state conditions are correct? This is usually a good indication that you have transistors operating and correct resistors in place. After this comes an attempt at calibrating the FET bias voltage so that it is just into conduction. You have done this OK?

Now you are addressing metering. You should be able to see the signal VU level when switched to +4 or +8 positions. If not then you may need to add diodes to your meter. Once the meter is actually operating as a VU meter than you can calibrate the gain reduction metering.

Brown and Black are the primary wires (leads) on the Output Transformer (OTX).
Q10 is a 2n3707. Perhaps you're thinking of Q11, which is the FET..
All DC voltages are 'in spec.'

I'm walking through this step by step. Sections 3, 4 & 5 are where my unit starts to deviate.
https://help.hairballaudio.com/hc/en-us/articles/360045942974-FET-RACK-Troubleshooting-Guide

 

[gswan]

OK, it seems your schematic varies from the original 1176LN-F circuit, which has Q1 and Q10 as FET devices.
I don't see any reference to Q10 base in these test instructions either. Do your AC voltages match the Signal Line Amp tests for Q5, Q6, Q7, Q8, Q9?

 

[guitarrock04]

This is the Rev D build thread..
If I set Point 17 to achieve 0.2000VAC, I get 0.199vac @q4 base, 0.287vac @q5 base, 0.286vac @q6 base and 3.678vac at OTX brown wire.
I finally got a reading on the brown wire by manually setting the DMM to a higher range.

 

[znjamznjam]

IMPORTANT TO ALL CINEMAG USERS:

I finally put my units (REV A and REV D) in the rack in the studio and we checked the PHASE and realized the units were both outputting out of phase...

Obviuosly we checked the XLRs and the Patchbay and it's all good... so we had to swap the polarity on the INPUT trafos - so now the Yellow and Orange are reversed AND THIS FIXED THE ISSUE !

So if you used the CINEMAG TRAFO you MUST WIRE THE WAY SHOWN IN THE PICTURE:

NOW IT SOUNDS AND GREAT AND IN PHASE

sage

Hy witch cinemag as input you use?

 

[JW]

Is it okay to connect the power transformer AC secondaries to two of these boards in series? Like the unit in the adjustments video on the mnats site appears to be? Will the voltages on each unit be fine as long as the power transformer is highly enough rated?

And in this case, how would you go about grounding the boards?

 

[JW]

Ah jeez. I powered up with C25 backwards. Do I need to change out the regulator now? And for that matter, if the cap got really hot but didn't blow, is it at all safe to power up with the same cap again with polarity corrected?
Assuming I'll replace it soon. .

 

[jasonallenh]

I went and bought a new meter, just to be sure. I got no change in readings.
I swapped a different Q1, and now re-calibrating, this time get ~ -1.1v at qbias wiper. CR3 anode voltages are closer to guide; 2.78vac @20, 0.57vac @4:1.

BUT - new meter tells meI get 0vac at OTX brown and Q10 base with new meter. That's chassis reference. If I measure between brown and black leads, I get ~3.6vac. Could that be an issue with decoupling cap somewhere? Perhaps an issue with Q9 or Q10?

Sorry to drop in, but I had an almost identical issue with the qbias- I couldn't get it negative enough. The issue turned out to be a broken trace at one of the pads for the ratio switch (though I am troubleshooting a Hairball PCB). I would recommend checking the actual continuity of your traces in that area and make sure those connections are actually being made. On the Hairball PCB specifically, I saw several good looking solder joints that weren't actually connected to the trace that was supposed to terminate under it! Once I found this issue, my Qbias when from like -1.3 to the more normal -2.5 on the D version.