500 series blue face 1176 rev A - Help thread

[slegg]

Dbonin,
I'm delighted to read your answer. My english is not so good, and I can't explain my gratitude easily.... Thank you very much. I love this website. Slegg

 

[Unit7]

I just ordered this kit. Studying the build manual in advance and there's one detail I don't get, regarding the Q bias setup. Manual says:

"From your DAW, insert a 1kHz -18dBFS tone (0dBu, 0.775V). Turn up the input pot until in your DAW you read -17dBFS (+1dBu, 0.869V). You can do this using either the DAW meters or a plug-in VU meter is handy.

What we now need to do is turn the Q-bias pot CW until we read the output as dropping 1dB back to -17dBFS (+1dBu, 0.869V)"


I'm assuming that I could send a tone from one channel in my DAW into the U76, and then monitor the output of the U76 on another channel in the DAW.

I'm also assuming that by "Turn up the input pot until in your DAW you read -17dBFS" we're talking about moving the input pot on the U76 until the input on my DAW is getting -17.

If my assumptions are correct, if I'm already seeing -17dBFS from the the output of the U76, I can't see how I could trim ("to -17dBFS"). Is this a typo or am I missing something?

EDIT: Got this because an older version of the manual was uploaded for a while. It's fixed now

 

[Dr Gris]

Congrats to your buy, I'm in envy.

Your assuptions are right, and it is indeed a typo. You want a one dBu drop (back to -18dBFS)
When I calibrated my 1176's I didn't trust my DAW, I just used my DMM set to read AC RMS.

Please share your thoughts.

Love your work and studio!
Best regards
from a fellow Swede

 

[Unit7]

Good info, AND a pat on the back!
That made my day.

Tack!
Pål

 

[Unit7]

I'm doing the calibration right now. Find it VERY hard to trim the input knob to get -17dB back to my DAW. It's super sensitive. After a few minutes of just touching the pot back and forth I was finally able to get -16.8dB. Is this how it use to be? I mean that the input knob is so sensitive.

 

[buildafriend]

Hi,

I'm highly interested in purchasing one of these rev A 500 series kits. I just have to ask if there are any bugs or if they are still being worked out. This kit looks so awesome!

 

[Unit7]

Hi,

I'm highly interested in purchasing one of these rev A 500 series kits. I just have to ask if there are any bugs or if they are still being worked out. This kit looks so awesome!

Mine just ready. No problems. Fun to build, fun to use! Compared with my Purple MC77 sounds a bit dirtier and perhaps a tad bit thinner, otherwise very similar. I'll probably get another one, so...recommended!

 

[JeromeMason]

Payment sent!

Can't wait to get it in just to look at the case 

Next week I can buy all the parts from Hairball and Mouser. This is gonna be so much fun to build!

 

[JeromeMason]

Success!!!

Great sounding blue stripe..... I'm amazed how close you got this thing David. It's smooth as silk when you're just using it normally, but when you slam it, boy does it ever put out some grit. I put it on the bass.... loved it, put it on the vocal.... loved it. So, I'm gonna have to build another i guess..... truly an accomplishment David, well done sir. Please make more stuff!

Maybe...... a DBX 160 VU 500 perhaps  ;D that would be sweet.

As far as the build goes, Bobby Baird helped me understand a few things and was there to answer any questions I had, great guy, thanks Bobby! This build I would say isn't EASY but it's definitely not hard at all. The PCB's are very well labeled, what looks to be complicated will turn out to be quick and easy. Everything is placed perfectly and at least for my unit, looks like it came out of a factory. The calibration is very simple, and again, everything is laid out so it's very easy to get to everything to. Just make sure to read all the directions in the manual, follow them step by step, and you will find it an semi easy build.

 

[bobbybrave]

Hey guys, this is my first post.


I've reached the setting up stage in my U76a build, and it is passing audio fine, but am having problems with the q bias set up.

I cant seem to get any gain reduction at all when passing through the -17db test tone and adjusting the Q bias trimpot.

I tried searching on the forum for answers and tried a few things but i'm still stuck!

Ie;

Adjusting the Q bias trimpot slowly.

Measuring the voltage at Q1's gate while adjusting the Q trimpot.

Fully CCW Q1's gate voltage is at -.76V and and when I rotate the Q bias trimpot fully CW it goes back to zero.

Are these the right voltages?

Any ideas?

Thanks 

 

[Bobby Baird]

Are you using a vu meter or volt meter to see a drop in voltage at the output as you turn the trimmer for the q-bias? If you know the input is .775v coming from your sound generator out of your daw. After biasing you should see .690v at output of u76. http://www.sowter.co.uk/decibels.php

If you can achieve the bias and still have a problem it could be that your fets are not matched correctly. Do you hear gain reduction happening, but don't see it on the meter? As far as Q1 voltage check this out. http://mnats.net/files/1176REVA_125_VOLTAGES.pdf

 

[JeromeMason]

Hey guys, this is my first post.


I've reached the setting up stage in my U76a build, and it is passing audio fine, but am having problems with the q bias set up.

I cant seem to get any gain reduction at all when passing through the -17db test tone and adjusting the Q bias trimpot.

I tried searching on the forum for answers and tried a few things but i'm still stuck!

Ie;

Adjusting the Q bias trimpot slowly.

Measuring the voltage at Q1's gate while adjusting the Q trimpot.

Fully CCW Q1's gate voltage is at -.76V and and when I rotate the Q bias trimpot fully CW it goes back to zero.

Are these the right voltages?

Any ideas?

Thanks 

Is the control amp connected to the correct molex pin? In the picture it looks like you're supposed to put the resistor leg in the "left" side of the socket, but it's actually the "right" side of the socket. I had the same problem, and found I had the CA and Main PCB connected in the wrong side of the molex. Check that and make sure it's in the "right" side of the socket and see if you get gain reduction.

 

[bobbybrave]

Hey guys,

Looks like I got it working!

I had a good break for a few days and after checking everything, came back to adjusting the Q bias. Looks like I was still turning too quick and missing the tipping point for Q1 to start conducting!

I guess my nerves were a bit frayed that day, amazing what coming back to a project with a fresh mind can do.

Thanks for your help and suggestions guys, sorry if I have wasted your time.

Merry Christmas Everyone!

 

[TheJames]

A couple of notes on some experiences I've had...

I noticed that the polarity on these was inverted.  In an email exchange with David, this was confirmed and the action to be taken is to swap the red and blue wires from the output transformer.

I also noticed an odd problem with getting 20-30db of gain reduction while in 4:1 or no ratio at all, even with the GR switch out.  I was able to trace this back to the 6.8uf tantalums.  I have new ones coming in, but replacing these with some electrolytics resolved the problem for now.  Seeing as how this is DC blocking from the control amp stages to the FET gate, I don't think there's any audible difference to be gained from using the tantalums.  That said, I'm keeping with the original design and replacing like for like.

Happy New Years all!

And again, David, thanks for your projects.  These sound awesome!

James

 

[rob_gould]

So I finally got to finishing this build and testing it tonight.

Straight away I knew something was up.  With GR bypassed, the unit barely passes audio - both input and output pots have to be up full to hear a signal at a reasonable volume, and the sound sounds like it is high pass filtered almost.  Also, the meter is pegged all the way over to the right.  Is this normal before the matched FETs at Q1 and Q11 are put in place?

I tested voltages between main board and control amp board and they were correct : -10.00V and 29.89V.

I get the correct 'squeak' for a second just after turning it on, and then the meter slams to the right and audio starts passing through, albeit the fizzy high passed version I described earlier.

I've checked that all the transistors are in the right positions, and I've made sure that the output transformer is wired correctly.  The dot on the sticker on the input transformer matches the dot on the PCB so that should also be correct.

Does this sound familiar to anyone?  I'll continue to check all component orientation etc, but it'd at least be good to know if the meter should be pegged all the way to the right with Q1 and Q11 removed from circuit.

Cheers

Rob


Edit : The heatsink on Q6 is also very hot.  I can't touch it for more than 3 - 4 seconds.  Is that normal?

Edit 2 :  I started looking at the main rev A thread to look for troubleshooting tips, and came across this :

http://mnats.net/files/1176REVA_125_VOLTAGES.pdf

Should this be accurate for this project?  I see that the type of FETs and transistors used is not the same, so should the voltage measurements be similar?  Because mine appear to be way off for all the FETs and transistors in the signal preamp and signal line amp portions of the circuit.

 

[Unit7]

The heatsink on my working unit gets very hot too.

I had, kind of, the same issue with mine at first. Found that the pins on the input pot had been bent during assembly (due to stiff and tightly packed cables at that spot) and shorted.

Don't remember if the meter pegged before installing Q1/Q11 though...

So I finally got to finishing this build and testing it tonight.

Straight away I knew something was up.  With GR bypassed, the unit barely passes audio - both input and output pots have to be up full to hear a signal at a reasonable volume, and the sound sounds like it is high pass filtered almost.  Also, the meter is pegged all the way over to the right.  Is this normal before the matched FETs at Q1 and Q11 are put in place?

I tested voltages between main board and control amp board and they were correct : -10.00V and 29.89V.

I get the correct 'squeak' for a second just after turning it on, and then the meter slams to the right and audio starts passing through, albeit the fizzy high passed version I described earlier.

I've checked that all the transistors are in the right positions, and I've made sure that the output transformer is wired correctly.  The dot on the sticker on the input transformer matches the dot on the PCB so that should also be correct.

Does this sound familiar to anyone?  I'll continue to check all component orientation etc, but it'd at least be good to know if the meter should be pegged all the way to the right with Q1 and Q11 removed from circuit.

Cheers

Rob


Edit : The heatsink on Q6 is also very hot.  I can't touch it for more than 3 - 4 seconds.  Is that normal?

Edit 2 :  I started looking at the main rev A thread to look for troubleshooting tips, and came across this :

http://mnats.net/files/1176REVA_125_VOLTAGES.pdf

Should this be accurate for this project?  I see that the type of FETs and transistors used is not the same, so should the voltage measurements be similar?  Because mine appear to be way off for all the FETs and transistors in the signal preamp and signal line amp portions of the circuit.

 

[rob_gould]

Thanks for the info!  You might be onto something here, but I'm not sure what the correct behaviour is.

I get continuity at the card edge input pins when the input pot is fully clockwise.  As soon as I move the pot CCW, I lose continuity as resistance increases.

I always get continuity where the pot connects to the board at inpot + and -, no matter where the pot is set.

There aren't any visible shorts on the pot itself, and I've verified that it is wired correctly.  So unless the pot is faulty, I guess this behaviour is normal.

If anything strikes you as strange, please let me know though.

 

[Unit7]

If your input pot doesn't short I'm afraid I'm not onto anything 
Just wanted to share that thing b/c my unit behaved as you described. I guess DON will have to take over from now
Good luck!

Thanks for the info!  You might be onto something here, but I'm not sure what the correct behaviour is.

I get continuity at the card edge input pins when the input pot is fully clockwise.  As soon as I move the pot CCW, I lose continuity as resistance increases.

I always get continuity where the pot connects to the board at inpot + and -, no matter where the pot is set.

There aren't any visible shorts on the pot itself, and I've verified that it is wired correctly.  So unless the pot is faulty, I guess this behaviour is normal.

If anything strikes you as strange, please let me know though.

 

[mutterd]

hey guys,

just wondering if any knew if this would fit in this build?

http://www.studioelectronics.biz/sunshop/index.php?l=product_detail&p=1160

I got the rest of the kit from Hairball but am up against a bit of a time crunch and might need to source something else out if Mike doesn't get the EA-5002's in time...

just got word from studio electronics - the transformer is "3 1/4" L x 2" H x 1 3/8" W measuring from end to end - so its about the same as the listed dimensions on the hairball site...

any info would be great - thanks,
Tim

 

[Sparqee]

I've been looking for an explanation for this "alternative" resistor but not finding any info on it.  Mouser sent me one of each.  When/why might I want to use the "alternative" 5.6R in place of the original 5.49R?

Any help would be appreciated.

Thanks.