1176 low gain..

[skipwave]

This may be unrelated to your issue, but I noticed on my PCB (which is MNats Rev J), that the three pads under the distortion trimmer R16 are all connected together. There are three pads just outside the silkscreen part footprint (perpendicular to the three under the silkscreen outline) that appear to be the correct pads. I haven't traced it yet, so that needs to be confirmed.

If that connection was left open, and no feedback applied to the emitter of Q2, you could be running Q2 at far too high gain, and thus distorting all of the devices after.

 

[line61984]

[quote author="skipwave"]This may be unrelated to your issue, but I noticed on my PCB (which is MNats Rev J), that the three pads under the distortion trimmer R16 are all connected together. There are three pads just outside the silkscreen part footprint (perpendicular to the three under the silkscreen outline) that appear to be the correct pads. I haven't traced it yet, so that needs to be confirmed.

If that connection was left open, and no feedback applied to the emitter of Q2, you could be running Q2 at far too high gain, and thus distorting all of the devices after.[/quote]

i dont see the pads under the dist trimmer. trid to trace from all of the pins on that trimmer to any of the pins on Q2 and Q3 and there was no connection between them. the distorting happens when regardless of the signal strengh unless it is very very low. it sounds hideous. Is there a way to test the resistors without having to pull them out to check designation to get to the value it should have. pots and trim pots dont seem to be affecting the audio at all either.

 

[skipwave]

I just checked the pcb, and sure enough, using the three vertical holes for R16 results in no feedback from Q4 to Q2. If you look at Q2, Q3, and Q4 as a "ring of three" type amplifier, it is capable of far more gain than needed. Feedback is taken from the emitter of Q3 to the base of Q2. Feedback is also taken from the emitter of Q4 to the emitter of Q2.

This feedback is vital to keeping Q2 in stable, low gain operation. Without it, Q2 could be distorting with very little signal applied to its base. Check if you have continuity from R21, which is just above Q4, to R15, with is next to the words "DIST. TRIM."

 

[line61984]

[quote author="skipwave"]I just checked the pcb, and sure enough, using the three vertical holes for R16 results in no feedback from Q4 to Q2. If you look at Q2, Q3, and Q4 as a "ring of three" type amplifier, it is capable of far more gain than needed. Feedback is taken from the emitter of Q3 to the base of Q2. Feedback is also taken from the emitter of Q4 to the emitter of Q2.

This feedback is vital to keeping Q2 in stable, low gain operation. Without it, Q2 could be distorting with very little signal applied to its base. Check if you have continuity from R21, which is just above Q4, to R15, with is next to the words "DIST. TRIM."[/quote]


yes im getting continuity fro R21 to R15

 

[line61984]

so sorry, I am connected to the 3 pads under the silkscreen. those are the correct ones from looking at the signal path on the board.

 

[skipwave]

Sounds like you are fine in that regard.

I will get mine running today or tomorrow, and hopefully I can offer some assistance then.

 

[line61984]

[quote author="skipwave"]Sounds like you are fine in that regard.

I will get mine running today or tomorrow, and hopefully I can offer some assistance then.[/quote]

ok now just found that R36 is blown and that connects to R35 on one end and on the other end is a fet transistors. tried replacing R36 and R35 just in case and powered up again but it blew out again. What could cause that? bet thats my problem.

 

[raysolinski]

Well..they connect your power (b+) to your output amp...My guess is q7 and q6 are installed incorrectly or are just blown..when you power up the resistors fry since the path is shorted by the bad or incorrectly installed transistors. Make sure to check your data sheet for your particualer brand of bc560's. Make sure you know which legs are the base, emitter and collector. When I first started I assumed ( ) that transistors would be standardized...Check your voltages as well..are you getting 30 volts on the b+ rail?..HAPYY THANKSGIVING! :guinness:

Ray

 

[line61984]

[quote author="raysolinski"]Well..they connect your power (b+) to your output amp...My guess is q7 and q6 are installed incorrectly or are just blown..when you power up the resistors fry since the path is shorted by the bad or incorrectly installed transistors. Make sure to check your data sheet for your particualer brand of bc560's. Make sure you know which legs are the base, emitter and collector. When I first started I assumed ( ) that transistors would be standardized...Check your voltages as well..are you getting 30 volts on the b+ rail?..HAPYY THANKSGIVING! :guinness:

Ray[/quote]

I will check those and get back to you. HAPPY THANKS GIVING TO YOU MAN!!!

 

[line61984]

[quote author="raysolinski"]Well..they connect your power (b+) to your output amp...My guess is q7 and q6 are installed incorrectly or are just blown..when you power up the resistors fry since the path is shorted by the bad or incorrectly installed transistors. Make sure to check your data sheet for your particualer brand of bc560's. Make sure you know which legs are the base, emitter and collector. When I first started I assumed ( ) that transistors would be standardized...Check your voltages as well..are you getting 30 volts on the b+ rail?..HAPYY THANKSGIVING! :guinness:

Ray[/quote]

it worked. the transistors were backwards. opposite from the boards silkscreen. I assume they are blown because there is very little gain. i smelled something burning but no smoke. I checked all the parts and nothing melted from what i saw.Passes audio But no distortion. how do i tell if the q6 and q7 are blown?

 

[raysolinski]

Put in new transistors correctly and replace r36 and r36....once you powered them on in the wrong position you toasted them for sure..now I am off to TURKEY :green: :green: :green:

Ray

 

[line61984]

[quote author="raysolinski"]Put in new transistors correctly and replace r36 and r36....once you powered them on in the wrong position you toasted them for sure..now I am off to TURKEY :green: :green: :green:

Ray[/quote]

ok will replace those and get back to you. thanks for the help bro. really appreciate it.

Derrick

 

[bluezplaya]

I have been reading this thread and guess I might chime in and ask for some help. I too am having some issues with my new 1176. I have had the board stuffed for a month or so now and had it mostly wired and ready to fire up. Anyway, finished wiring the IP and OP jacks today, turned it on for the first time. Here is what it is doing:

LOUD constant hum...input and output knobs seem to work properly (increase and decrease signal) signal is passing through, obviously...NO signs of compression using the ratio, attack and release knobs. AND R89 (220R) is slowing cooking. The bias trimpot increases and decreases the signal as I turn it. I have not done a star ground as of yet and that will be my next thing to do, just need to find out why R89 is slowly burning.

Any help is certainly appreciated.

Adam

 

[dagoose]

I'm using my stereo 1176 for about a year now and still love it but there is one thing that i sometimes miss, it's the gain.. Sometimes i need just a little bit more gain, my output pots ar fully open all the time but sometimes i need let's say, a potmeter that goes all the way to about 15 instead of 10 :grin:
There is a topic about rewiring the lundahl LL5402 trafo for more gain (http://www.groupdiy.com/index.php?topic=1261) but i'm not sure what to do, is it possible to rewire or is the mNATS rev F board (think i have that one, bought it somewhere in october 06) allready wired that way?
It's quite hard to check right now because it's in my rack and beacuse of the wiring hard to get to the trafo's so before i start doing things i'd like to now more.

More pics here: http://www.dagoosemusic.nl/diy/1176/

 

[bluezplaya]

LOUD constant hum...input and output knobs seem to work properly (increase and decrease signal) signal is passing through, obviously...NO signs of compression using the ratio, attack and release knobs.

After reading MNATS 1176 page I discovered that using replacement output transistors (which I did...BD135, BD136), I did not need the two jumpers wired. I had them wired and thought they were supposed to be but that is only for BD517 and BD518. I removed the jumper and now the hum is not as loud but it is still constant and the signal is cracking a bit and is not as loud.

More to come...

 

[raysolinski]

R89 is part of the feedback for your voltage regulator..how were/are your voltages? Is your regulator in right? Are your caps c23 and c25 in correctly in terms of polarity?...with those jumpers connected you definately toasted your output transistors..replace those..they are first in line in terms of the b+rail..with those in incorrectly it explains r89 doing a slow burn..before you put in those new transistors check those voltages..maybe you killed your regulator as well...as for dagoose did you do the resistor mod that sets the threshold higher? That and the 1:2 output tranny should have you swimming in gain..I have never had my gain past 7...

Cheers,
Ray

 

[raysolinski]

PS:
I had to laugh at your avatar Bluezplaya...I live about 1/2 mile from Peter Frampton! :thumb:

Ray

 

[bluezplaya]

Oh yeah! I heard he lived in Cincinatti! Well R89 makes a little sense now. So I guess i will get together another Mouser order and replace those transistors. Thanks for the advice.

 

[skipwave]

I just fired up my '76 for the first time and listened to some old tracks through it. Holy shit! Astoundingly good sounding, and more gain than I know what to do with (the input pot never went up past 3). I can see why a stepdown transformer on the input would make sense. I'm using the electronically balanced in, no money for transformer right now. A mysterious military transformer made by Nytronics on the output, impedance ratio spec'ed at 135:600 ohms.

 

[line61984]

thats rad man. Made a parts list and should be on track by mid week. cant wait