[BUILD] 1176LN Rev D DIY

[Echo North]

Ah, whoops. I think i'm measuring it wrong, as I'm seeing 0V DC on Q1 and pad 18. Q1 has the labels G, S, and D, which one of those am I measuring? And to measure the voltage on pad 18 using my multimeter, which probe (positive or negative) would I put on the pad?
Sorry for all the (probably dumb) questions, I'm pretty new at this stuff. But thanks for the help!

You can do a google search on how to use a DMM.  There is a ton of info, I suggest you look at a tutorial before continuing.

Do a wiki search on "Field Effect Transistor" (FET).  The G,S,D label will make more sense.

Then measure your DC on each side of R35 and report back.

My guess is that you over-rotated the the qbias trimmer and it broke.

Mike

 

[rockmonkey731]

OK, so on the FET, I should be measuring the DC voltage from S to D? In other words, have the positive probe on S, and negative on D? And the DC voltage on R35 is measuring at 8.5 Volts DC.
How can I test the trimmer to see if I broke it?

 

[Echo North]

I'm more than happy to help but please read carefully.

Check the DC voltage between your Q1 gate and ground. 

Also,  I need to know the DC on each side of R35.  Black probe an ground and test the DC on either side of R35.

To test the trimer you must remove it.  Place your DMM on resistance mode (Ω) and place your probes on the two outer pins.  You should see the value of your trimmer +/- 5%.  If it's a 2K trimmer, then 2K.  If it measures infinity or zero it's broken.  Now measure between one of the outside pins and the wiper.  As you rotate the wiper you should see it move from 0Ω - 2KΩ.  If it measures infinity or zero it's broken.

Mike

 

[rockmonkey731]

Ah sorry about that, that makes sense.

The DC voltage on the Q1 gate is measuring -.09.
Pad 18 is also about -.09 volts.
For R35, the side closest to the Q bias trimmer reads -9.62 Volts DC, and the further side reads -1.163 Volts DC.

I'll start working on getting the trimmer out and let you know what I find out.
EDIT: Just tested the trimmer, it is working properly.
          And since I've put it back, I'm getting 0 volts from the Q1 gate and pad 18.

 

[Echo North]

0vdc? 

Is your trimmer oriented correctly?  Perpendicular to the footprint?

Mike

 

[rockmonkey731]

The trimmer is perpendicular to the footprint. Just checked again and I'm getting .018 Volts DC. Is that normal? It seems pretty low.

 

[Echo North]

Ya it's low. 

Triple check your ratio PCB and attack/release pot wiring.  Also for the Qbias to work properly you need to have a ratio button activated.

Mike

 

[rockmonkey731]

It looks like everything is hooked up properly. Oddly enough, now that I put the ratio board back into place, the q bias is now adjusting the Q1 gate voltage, as you said it should be. It's going from 0 vDC to about -2VDC. Maybe I nudged something into place.
So now that that seems to be working, what should I check next? When I put a signal into the unit, the meter doesn't move at all.

 

[Echo North]

It looks like everything is hooked up properly. Oddly enough, now that I put the ratio board back into place, the q bias is now adjusting the Q1 gate voltage, as you said it should be. It's going from 0 vDC to about -2VDC. Maybe I nudged something into place.
So now that that seems to be working, what should I check next? When I put a signal into the unit, the meter doesn't move at all.

Sounds like you had a bad joint.

Set your qbias now as described earlier.

 

[rockmonkey731]

Ok, the output voltage is at a constant .066 Volts AC regardless of the q bias trimmer, input/output levels, etc.

 

[Echo North]

Then try going through this:
http://www.groupdiy.com/index.php?topic=20058.msg639168#msg639168

 

[rockmonkey731]

The voltages in the meter section on the voltage guide are dependent on the unit being calibrated correctly.

Right now if you can't pass audio, let's tackle that first.  Set you qbias to -2 VDC or more by placing your DC volt meter between the Q1 gate and ground.  Now turn your GR off by setting the attack to "off".  The should effectively disable all GR and Q1 turning your 1176 into a 2 stage mic pre with about 40 dB of gain.

If you're not getting any output like this, there is an issue in you preamp stages.  Feed a 1khz 1VAC signal into the input and set the in/out knobs to the middle.  Now let's see where the signal dies.

Measure the AC voltage at these points using GROUND as the reference.

1. Input XLR pin 2
2. + input at the PCB (by C1)
3. - of C7
4. Output XLR pin 2

Ok, my AC voltages are as follows:
1) .286
2) .052
3) 0
4) .159

Seems like I'm losing a lot in my input stage.

 

[Echo North]

The voltages in the meter section on the voltage guide are dependent on the unit being calibrated correctly.

Right now if you can't pass audio, let's tackle that first.  Set you qbias to -2 VDC or more by placing your DC volt meter between the Q1 gate and ground.  Now turn your GR off by setting the attack to "off".  The should effectively disable all GR and Q1 turning your 1176 into a 2 stage mic pre with about 40 dB of gain.

If you're not getting any output like this, there is an issue in you preamp stages.  Feed a 1khz 1VAC signal into the input and set the in/out knobs to the middle.  Now let's see where the signal dies.

Measure the AC voltage at these points using GROUND as the reference.

1. Input XLR pin 2
2. + input at the PCB (by C1)
3. - of C7
4. Output XLR pin 2

Ok, my AC voltages are as follows:
1) .286
2) .052
3) 0
4) .159

Seems like I'm losing a lot in my input stage.

Actually I think .05 VAC is normal at the input with the differential->single ended -> attenuation -> step down transformer.

Seems like your signal is dying in the first amp stage maybe.  You should be able to get 0->2VAC or something at the wiper of the output pot as you turn it.  If not check your components and DC voltages agains the MNATs voltage chart.

Mike

 

[rockmonkey731]

Alright, I've been testing voltages throughout the board, and most things look OK, but I've noticed that there's a few capacitors/resistors that have voltage on one side, but nothing on the other. Is that normal? Or can I assume those components are bad?

EDIT: Ah Ha! The center leg of Q4 wasn't making a good connection with the board. I re-soldered it, and the unit seems to be working as it should, I'm moving on to calibration now.

 

[jplebre]

Hello guys!

Lurking for a while, never posted. So YEYYY post one!

Well, just finished my 1176LN (rev D) build. Thanks to the guys at Hairball (Mike) and Mnats and every single one of you that put input and effort making this kit SOOO easy to build.


The unit is working as it should. Don't have an original 1176 to compare but it's doing pretty much what I remember.

Some weird things:
- GR meter goes out of whack constantly. Think its getting better now (unit on for 1h + and consecutive "0" set adjustments)
- GR meter goes WILD on slam mode (all buttons in, or just the necessary buttons in as I recently found). As in, needle get's pinned hard right.
- Output pot gets scratchy at about 6 (on the oringinal enclosure). Also, at 6 and all the way up there's this weird AC like noise w/ oscilation PSCHOUM, PSCHOUM. Any ideas?

Thanks guys

I have HONESTLY tried to read this thread, but after page 20 or so it's too much for the brain. Most things being discussed require a focused mind to read and it's hard to focus for more than like 20 pages full of so much information!

 

[jplebre]

Pictures or it didn't happen

 

[Echo North]

- GR meter goes out of whack constantly. Think its getting better now (unit on for 1h + and consecutive "0" set adjustments)

The discrete meter circuit takes a good hour to "warm up".  Sounds normal.

- GR meter goes WILD on slam mode (all buttons in, or just the necessary buttons in as I recently found). As in, needle get's pinned hard right.

This is normal.  Same in the vintage units.

- Output pot gets scratchy at about 6 (on the oringinal enclosure). Also, at 6 and all the way up there's this weird AC like noise w/ oscilation PSCHOUM, PSCHOUM. Any ideas?

I get this too on mine.  Never looked into it because I would never have a reason to have the output that high.  Not sure if anyone has found a fix for it.

 

[jplebre]

Never looked into it because I would never have a reason to have the output that high.

Sooooo true. I don't think I EVER had the o/p higher than 18ish (14 o'clock) ever. And that's slamming and clipping everything else

I never noticed the behaviour of the needle on slam. I guess I'm being overly careful as it's something I built and want it to be working in top shape

 

[mnats]

- GR meter goes out of whack constantly. Think its getting better now (unit on for 1h + and consecutive "0" set adjustments)

That is the reason why the original and the Hairball case has a hole to adjust the meter. Thanks for the question; I'll add it to the FAQ.

- GR meter goes WILD on slam mode (all buttons in, or just the necessary buttons in as I recently found). As in, needle get's pinned hard right.

This is covered in the FAQ linked from the first line of the first post of this thread. No need to read 20 pages.

- Output pot gets scratchy at about 6 (on the oringinal enclosure). Also, at 6 and all the way up there's this weird AC like noise w/ oscilation PSCHOUM, PSCHOUM. Any ideas?

I've read similar descriptions before but - like this one - missing any information about what source, if any, and what the output is plugged into. With either of my builds of the Rev A or D plugged into the balanced line input of the Mackie 1402VLZ that sits on my test bench and using a signal generator feeding the balanced input pins 2 and 3, I have never observed the oscillation or feedback that others have described even when the output is all the way up. I'm guessing that it must be a symptom caused by external factors or a common wiring fault.

 

[jplebre]

Hi Mnats

Re: slam.... UUPSSS
sorry really missed that one :S

Re: noise - any way I can provide you with info to see if this issue can be narrowed down (future generations of DIY'ers

I had the issue happening with the unit plugged in the in/out of a DIGI192, DIGI96, RME ADI8DS and RME UFX.
I usually use the hardware as "hardware interfaces" in Protools.

Will try and put it on the insert of a NEVE shortly will let you know if problem persists.