[BUILD] 1176LN Rev D DIY

[kazper]

Recently I was asked to debug and calibrate several DIY 1176LN's with the mnats PCB, and discovered what I think is a fairly significant error. In the preamp section, R12 is specified as 920 ohms. I suggest that people use 1.8K instead.

The original UREI Rev D and E schematics show 910 ohms for R12, but many examples actually use 1.8K, and all Rev F  units use 1.8K. Basically, I think the 910 ohm value was a mistake that crept into certain D's and E's. (Some of them do use 1.8K, which makes me think that UREI had a service bulletin or upgrade procedure that addressed this.) All UREI Rev F units use 1.8K, as do the UA 1176LN reissues.

If you use 910 or 920 ohms instead of 1.8K, DC bias in the Q3 / Q14 section will be incorrect, and you'll lose about 8 dB of headroom in the input section. Besides the headroom loss, you'll also have 8 dB less limiting available. (Since Q3 / Q14 precede the sidechain circuit, having less gain before clipping at this point forces you to run the input pot lower, reducing the amount of possible limiting before onset of distortion.)

The error in the value of R12 is understandable, since 910 ohms is shown on the old schematics. But these units will work and sound much better with 1.8K. I can't imagine any benefit to using 910 but if you disagree, please speak up.

(I wasn't sure whether I should start a new thread or post on the main 1176LN Rev D DIY thread, but decided on a new thread because the topic seemed deserving of attention.)

http://www.groupdiy.com/index.php?topic=44909.0

 

[tazman]

Interesting...  My next step will be to re replace the transistors.  Then we'll see..

 

[tazman]

Ok so i replaced all of the 2nxxx components except for the 5457's as I don't have any rePlacements yet.  If I bypass it all sound well, once I start compressing I get distortion.  It seems that the higher the ratio the worse the distortion.

Can anyone suggest where I should look next?  The +30 and -10 voltages are correct.

Any suggestion would be appreciated.

Thanks

Rob

 

[MicDaddy]

Have you tried tweaking the Q-bias adjustment while compressing, to see if it has any effect on the distortions you are experiencing?  Send a loop through the compressor, get just enough gain reduction to where you are noticing some distortions, adjust the trimmer clockwise a turn or two, or more and note any change.  It may have no affect on the distortions, just trying to help you get sorted.

 

[tazman]

I have and it does affect it to some degree.

BTW, I took voltage readings on the transistors with ratio 20:1 and attack set to off.  Is there a chart I can check against?  I have a website from a friend of mine with some values, but it also shows a Q15, whwch my Rev D does not have.  The following is what I got:


b     c       e
q2 0.46   1.76    0.57
q3 1.76   12.12  1.16
q4 0.432  3.94    0.44
q5 3.33     25.2    2.8
q6 27.4     2.8      2.27
q7 4.5   14.8    3.9
q8 14.8   30.1    14.2
q9 3.4   17       2.9
q12 -0.79  11.08  -1.36
q13 -0.92  11.43  -1.46
q14 12.14  30.1 11.5


Thanks,

Rob

 

[tazman]

Could someone be so kind to post the voltage readings at the the transistors?  I'm comparing mine to the ones here http://www.axtsystems.com/index.php?option=com_content&view=article&id=57:1176lnproblems&catid=34:1176ln&Itemid=62

but the schematics are different.  I'd like to know the ones for the Rev D.

Thanks,

 

[colombian]

Hello All,

First I want to Thank you for this great forum and all the contributors!!

Second, I have an issue you may help me fix. I just finished building a Hairball kit audio using the BOM and mnats rev 2.2 board. My PCB is labeled "v2.2 26.01.09".  the board shown in the "v2 building" document on mnats Rev D page is dated 30.04.08. 

Since i knew in advance about the famous R32 burning out, I checked and double checked the Output Transformer wiring were correct and belonged to rev 2  board + diagrams....  I turned on the compressor and the R32 GONE in 10 Seconds!  very strange......  since I use rev 2 and all color match the wiring guides from, MNATS it, it is pretty straight forward but yet.... R32 is burned... 

I triple-read this forum but it mainly references to incorrect OT wiring from rev 1 to rev 2.... nothing else.  which is NOT my case.

I took out white/red wire and measure +30V  DC  between that wire and the ground (violet wire)  so no wonder the 39ohm resistor burns.....  any idea?

I looks like the Hairball output transformer came damaged?  is there any way to test it?  what else could i check?

I added a few pictures to show wiring.

Thanks a lot to all of you!!!
God Bless

Ludwing

 

[colombian]

another picture

 

[tazman]

I am running the scope with a 1Khz sine wave.  Should I be getting a sine wave at B, C or E on Q12 and Q13? 

 

[tazman]

Here's something else.  Scoping the output XLR, Pin 2 shows a hotter sin wave than Pin 3.  I assume this is NOT right, they should be balanced!?! 

The unbalance is also present at connection X and Y on the GR/+4/+8/off switch (I use the push button version)

If it is wrong, could someone suggest what I should check to see why?  Output trafo?

Any suggestions would be greatly appreciated.

 

[s2udio]

Here's something else.  Scoping the output XLR, Pin 2 shows a hotter sin wave than Pin 3.  I assume this is NOT right, they should be balanced!?! 

You should be measuring between 2/3 for balanced.
Pin 3 is grounded for unbalabced,
Simply measuring 2 or 3 to ground is not correct and will give misleading results.
Also check the GR switch as it only feeds the meter for output indication and gr....
Should have no influence over the output unless the meter circuit is at fault.

 

[tazman]

Ok.  I will test it this morning. 

 

[tazman]

Here's something I found that I'm a bit confused about.  When I check the sine wave across R5, in bypass mode the wave looks good on both sides, when I turn on compression, the wave seems to get "fatter" at the bottom on one side of R5.  Could this be showing me the distortion?  If so, I'm not sure where to look next, i.e., what could be causing this.  After R5 you have Q1 and C1.

Could I have a bad FET Q1 2N5457?

Any suggestions would be highly appreciated.

Thanks,

Rob

 

[s2udio]

Take a step back and check all your work carefully...ie board interconnections,tracks and component
values, against the circuit diagram............you may be,and probably are missing something simple.
This can be achived with a simple DMM, and no power applied....I find 90% of faults this way
Imho A scope is only brought in when all else fails,and experience calls for it..........and i think you are in to deep at this stage ,so a recheck is essential, as all you may be doing is chasing your Tail !
Read this thread carefully.
http://www.groupdiy.com/index.php?topic=646.0

 

[tazman]

Thanks s2udio.  I did that last week.  I went checked all the caps, resistors, which i had to do based on color coding as in the circuit they don't read correctly with a DMM (is there as way to get an accurate reading in circuit?).

I checked the wiring and it appears to all be correct.  I will go through it all again as  I've just been spending long hours trying to troubleshoot this and am getting very frustrated, so maybe I missed something!!! LOL.

 

[s2udio]

I went checked all the caps, resistors, which i had to do based on color coding as in the circuit they don't read correctly with a DMM (is there as way to get an accurate reading in circuit?).

Caps, no you need a cap meter out of circuit.
Resistors can be tested in circuit if there are no parrallel lower resistance paths...If in doubt lift one
end !
Check the schematic to see the lie of the land !
Diodes you test on the semiconductor junction TEST function on the DMM...
You can also test transistor junctions for PNP,NPN type
Until you can troubleshoot with a basic DMM, Forget the scope.

 

[tazman]

I tested all the transistors and they seem ok.  I'm checking again the resistors by color.  If in doubt I will lift one side.
As for the caps,  I checked the markings and all seem ok.  I don't have a meter that will test them. I guess I can start replacing them one by one  :-[ 

Thank you

Rob

 

[s2udio]

I don't have a meter that will test them.

Then time to buy one ......honestly ....no one takes someone faffing around without tools seriously
Do you get your car repaired by a plumber. ???

 

[tazman]

I was wrong, it does.  I guess it may be time to start removing them and testing.

 

[s2udio]

You don't need to remove all trasistors/ resistors ......read above , and learn to read schematics,
And  Get a DMM...........the scopes for later .
For starters read some online electonics basics....theres zillions.