[BUILD] 1176LN Rev D DIY

[ChrioN]

Perhaps a strange question, but: Why did they use odd resistors like 38.3k, 44.2k and 7.68k when the resistors used had a tollerance of 5%? Were these values more common and standardised back in the days?

 

[ChrioN]

Anyone? 

 

[3nity]

Wow...
I was stuffing my board and i couldnt find any 38K3 resistor and the same question crossed my mind!
i ended up using 2 resistors...1 x 36K + 2.2K...

 

[mnats]

Perhaps a strange question, but: Why did they use odd resistors like 38.3k, 44.2k and 7.68k when the resistors used had a tollerance of 5%? Were these values more common and standardised back in the days?

If you look at the original schematic you'll see that they didn't use these values as they weren't at all common.

 

[ChrioN]

you're right thats quite embarrassing. How come you didn't use the values from the rev A schematic?

Perhaps a strange question, but: Why did they use odd resistors like 38.3k, 44.2k and 7.68k when the resistors used had a tollerance of 5%? Were these values more common and standardised back in the days?

If you look at the original schematic you'll see that they didn't use these values as they weren't at all common.

 

[mnats]

you're right thats quite embarrassing. How come you didn't use the values from the rev A schematic?

Because it's the Rev D?

Seriously, the reason I took the sidechain from the later version was primarily to save having to select resistors to tweak the circuit.

 

[DrFrankencopter]

Does anyone have a DC voltage chart for the 1176LN rev D? Or, if you've got a working one on the bench, can you measure B,C,E voltages WRT ground?

I'm building a Purple MC76 (which is closest to the D), and I'd like to troubleshoot some problems I've got with the meter section (I can't get it to null R74).

Cheers

Kris

 

[edanderson]

do you have r44 disconnected?  the MC77 PCB included a jumper for connecting and disconnecting it.  during the calibration, you may have to do each step a few times until everything is correct.

does your front panel meter zero adjust control work?  can you get the needle to point to 0dB?

ed

 

[DrFrankencopter]

Hi Ed,


R44 is disconnected.

Here are my symptoms:
- The front panel meter adjust seems to work backwards CCW brings the needle up the scale. Is this normal? It seems counter intuitive, so I'm guessing that it's not normal.
- When passing audio through the unit, it will compress, and when compressing the GR needle moves up the scale (like into the +ve range), instead of down the scale. It's like the whole meter is working backwards for the GR circuit. I've flipped points 28 & 29 and it makes no difference.

If I try to null using the itterative procedure (while metering R74), I keep winding both R71 and R75 to the ends of their range and have 2.5 V across R74.

It's weird....

Cheers

Kris

 

[Biasrocks]

R44 is disconnected.

Are you putting the meter at 0VU before you adjust meter NULL?

Mark

 

[DrFrankencopter]

R44 is disconnected.

Are you putting the meter at 0VU before you adjust meter NULL?

Mark

Yup... I used R71 (front panel adjust) to get the VU to read 0 in GR mode. Bringing this pot CCW raises the needle position on the VU....but every time I adjust R75 from there the VU needle keeps falling, and I need to keep turning R71 CCW to bring it back up. Eventually I get R71 all the way to the CCW stop, and I still can't null R74.

On your build, when you turn the front panel pot CCW, which way does the VU meter needle move?

Cheers

Kris

 

[dmp]

Hello,
I have a few observations that I thought I would share with the group. I may be way off since I am building a pair of 1176's for the first time.  And they are certainly not working the first time! But I am having a great time trying to figure them out.
- In the calibration video, mnats says the Qbias trimpot should be adjusted all the way to ground, and then dialed back to bring the fet into conduction. I think this is reversed actually, the Qbias should actually be adjusted fully negative, since the fet Vsource is equal to ground. The fet becomes more conductive as Vgs approaches zero, I believe, i.e. when Vg is more negative the the resistance of the fet is high, i.e. no gain reduction of the input signal. I discovered this because I accidently used a 560 ohm for r9 instead of 560k - which made for a huge reduction of the input signal no matter what the fet was doing! Really a fun gremlin to try to find.
- The setting of the input attenuator (from hairball) affects the input signal noticably. When feeding the 1176 with a function generator with output impedance of 600 ohms, the signal was being lugged down depending on the T-attenuator position. If the T-attenuator was constant 600 ohms impedance, it would not show this behavior. I took some measurements of the T-attenuator, and calculated the impedance (assuming 600 ohm output impedance: Z = R1+1/((1/R3)+(1/(R2+Zout)) ))
The results showed that the impedance of the T attenuator went from 520 ohms at fully ccw to 382 ohms at fully cw. I hooked this up like the picture earlier in this thread, and I cannot think of a different way to wire it. The two attenuators I have showed appx the same behavior (which is good since I am trying to make matching units for dual mono / stereo). I'm curious if I am doing something wrong? I know a lot of work went into finding a source for this component, and I do not mean to offend anyone.

Thanks to everybody who participates in this forum & especially Mike at Hairball Audio for the great parts and Mnats for the great boards!

Dan

 

[Biasrocks]

Yup... I used R71 (front panel adjust) to get the VU to read 0 in GR mode. Bringing this pot CCW raises the needle position on the VU....but every time I adjust R75 from there the VU needle keeps falling, and I need to keep turning R71 CCW to bring it back up. Eventually I get R71 all the way to the CCW stop, and I still can't null R74.

On your build, when you turn the front panel pot CCW, which way does the VU meter needle move?

Cheers

Kris

I got a little confused when I adjusted the meter on my build.

This is what worked for me.

Take R44 out of circuit.
Center Null Pot (R75)
Volt meter across R74, on my build TP10/11

Meter Calibration

A) Zero VU with FP Meter Adj (R71)
B) Adj Null Pot (R75) for .00 volts across R74

Repeat A&B until both conditions are met.

Put R44 back in circuit, then do your 10db GR adjustment

Done.

I would verify the resistor values around the meter circuit if you can't get
it calibrated; R73: 680R, R74: 2M7, R76: 8K2, etc. Also check for shorts.

My FP adj works as follows: CW: VU Needle up, CCW: VU Needle down

You may have to swap the leads on your FP adj pot.

Mark

 

[ilfungo]

ciao
Can you tell me  a potentiometer ( easy to find) for  the  FP 0 adjusting?
Mouser code?
Farnell code?
Thanks 

 

[DrFrankencopter]

Made some serious progress. Did a ton of probing around the meter circuit and ultimately found that Q12 had a dead short between emitter and collector. The short wasn't at the transistor, it was at R74 and R44. The pads on my MC76 PCB were really close together in this area, and I had to do a little bit of cutting to clean it up.

The meter zero's & nulls now.

Cheers

Kris

 

[Biasrocks]

The meter zero's & nulls now.

Great!  8)

Mark

 

[julien31]

I made 3 rev D versions and for all of them i had to change R44 for proper calibration. The 1K trim pot had to be changed for a 2K to achieve the 10dB calibration. Has any of you had the same problem? Is it the way to go?
Thanks in advance
Julien

 

[DrFrankencopter]

I need to set  the T&C resistor that attaches to the 20:1 ratio switch to set the threshold of compression....what range should I be looking at? Currently I have no resistor installed, and I get no compression in 20:1...

Also, is there a schematic for Mnats 1176D? I realize its based on the Urie schematic & part #'s, but there are differences, and I can't seem to find a schematic anywhere. I'd like to reference it for my MC76 build...

Also, on the Mnats 1176D Is the 20:1 ratio threshold resistor on the main PCB, or are people hanging it off the switch? I'm curious because there's no place for it on the Hairball audio ratio PCB...

Cheers

Kris

 

[kilmister]

Also, is there a schematic for Mnats 1176D?

Yes there is: http://mnats.net/1176_revision_d.html

-Paavo

 

[DrFrankencopter]

Also, is there a schematic for Mnats 1176D?

Yes there is: http://mnats.net/1176_revision_d.html

-Paavo

Many thanks....

A note for Mnats, the link to the 1176 rev d page from this page: http://mnats.net/1176.html does not work (it needs .html added to the end).

Cheers

Kris