[BUILD] 1176 Rev A - Back to the beginning...

[Echo North]

Double check your transformer output transformer wiring.  If you have the color codes covered with terminal blocks on the PCB download the MNATS doc for your Revision and look at the overlay to confirm it's wired correctly.  make sure orange and yellow are connected to each other and not anything else.

Tripple check wring agains the MNATS guide.

J209's and 2N5457's are JFETS.  They have a Gate, Drain, and Source.

I have identified that the sound is coming from the VU meter! ...And only when there is an input, and, only when the meter registers in red and beyond. Hmmm...

That's wacky.  If you disconnect the meter is all ok?

 

[Curran]

almost done with my 1176 Rev A!!!!! it has been a fun and intense build!!! What is the bare wire coming off of the frame of the output transformer used for? I already have the green ground wire fashioned to the bolt of the transformer, then going to  the ground pad on the PCB power section. thanks for the help!

 

[K.March]

That's wacky.  If you disconnect the meter is all ok?

This is really wacky, I think it might be sympathetic resonance. I was completely stuck until I changed the frequency I was inputting. I moved my oscillator to 995hz, the sound disappeared, I tried at 1008, it disappeared. I doubled it to 2000, there it was; same with 4000. 3000 it was still present but not as overwhelming.

So I verified it wasn't the lamp and then very gently took the meter apart. I removed the tape off of those little diodes (I think they're diodes... they're just rectifying ac to dc right?), clipped and cleaned some gnarly solder joints and reapplied the input with the meter disassembled. The sound was back but maybe not as powerful, I couldn't tell if it was the diodes or the meter motor itself, so I just re-taped the diodes and put em back in a little tighter and reapplied the original proscribed 1000 Hz. The sound was there, but I ignored it, and just went ahead with the calibration. Q-bias: check. Discrete Meter: check. Gain reduction: check. I then (very amateurishly) compared the signal with the other build I had help on (also a rev A), they're basically identical (given the uncontrolled setup). A/B-ing them, I can't tell the difference, both sound great.

I think it was a 1 in a million sympathetic resonance issue. It works great, and doesn't have anything wrong with it, when I'm using it... just that strange feed-backish hum/noise high pitch thing going on when I apply that pure 1000 Hz signal, which after fully calibrating, is almost un-noticable, although, still there faintly if you really know what to look for-- do you think it could be a sympathetic resonance thing?

 

[Echo North]

Really odd.  If you swap the meter with your other unit is it still there?  If it's not, toss that meter into the fires of hell and let me know to send you a new one.

Mike

 

[AntonSwe]

Ok, so now I have done som testing. It turned out the voltage levels started to differ almost immediately. I sent +1 dBu to the compressor and at the input XLR the voltages were the same for 50 Hz and 1kHz. But on the output of the bournes 600 Ohm T-pad that changed. For example, when I got 390 mV out of it at 1kHz it only gave me 190mV at 50 Hz! Strange right?

The only time it gave me about the same values was when I had it fully open (at 0 on the front panel), otherwise the 50Hz signal was always considerably lover.

When I did this measurement I  had disconnected the input transformer from the PCB.

I don´t know what to think. Both of my 1176 compressors show the exact same frequency response, I measured my newly built SA-3A right after using the same outputs, levels and cables and it measured fine. It seems very unlikely that BOTH of my input T-pads are broken, but I have gone through the MNATS input wiring page over and over and I can´t se that I have done anything wrong.

Are the T-pads very sensitive to heat? Was thinking maybe I overheated them when I soldered them, although I believe I was careful.

I can't imagine how the t-pad would cut frequency.  It's just resistance.

This was without the the PCB attached?  You should keep it attached to the PCB to keep the input transformer properly loaded.

Check all of the input cap/r values near the input.

Mike

Yes I thought that was strange to.  I connected the main PCB and measured again an got the following values:

                                                  50Hz                1kHz
XLR in (between pin 2 and 3):    0.738 V            0.735 V
PCB in (between chassi and +):  31.3 mV          52.3 mV
C7 (between chassi and -):          1.137 V            1.882 V
XLR out (between pin 2 and 3):  0.530 V            0.872 V

I have carefully checked all the values of the caps and the resistors near the input and they are correct. I have done the q-bias adjustment again (using a DMM following one of your later descriptions) and that works perfectly but I still have the same strange/high passed frequency response.

I have also cheked all my DC voltages according to mnats typical voltages pdf and they all check out with only minor discrepancies. I have -1.238 VDC on the gate of Q1 and -1.103 VDC on the gate of Q11.  The only place I had a bigger difference was at Q4 where I had:
Base:      12.94 VDC (mnats has 13.72)
Emitter:  4.181 VDC (mnats has 6.6)
Collector:  1.991 VDC

I also remembered that I got two capacitors left when the build was finished. They are small red WIMA 0.001 µF 100V film caps which I believe only go at C13 but I already have two of the exact same kind in there so I´m guessing Mouser just sent me two extra by misstake.

I really have no idea what to do or check next.

Do you have any suggestions?

Thanks for all you help and patience!

EDIT: sorry i made a typo,  the correct input voltage at the XLR was:
                                                  50Hz                1kHz
XLR in (between pin 2 and 3):    0.738 V            0.735 V

 

[Curran]

ok- the unit powers up, no smoke!! yeah!!! I am sending a 1k signal from my recordings console into the unit. it passes signal, BUT the input t-pad is NON functioning. CCW I get full signal level, CW same thing, at high noon on the kbob the signal drops about 2db. Soooooooooooo I am stuck at the qbias part of the calibration. What should I be looking for here? the wiring has been double checked and looks correct. I checked TP10(17.5v), and TP11(-1.4v).

 

[mnats]

What should I be looking for here?

The first line of the first post of this thread. Read the link, then report back with your findings.

 

[Curran]

Yes, I did go through the FAQ, I have checked my parts to the schematic, I have also checked voltages of all the fets against the voltage schematic. I reviewed the wiring instructions and everything looks correct, The unit passes the 1k signal, but the Tpad is acting goofy as I said. any help will be awesome

 

[Echo North]

If you're worried about the t-pad.  Disconnect it from the input transformer.  Then feed a 1K signal into it (at the xlr) and measure the output at the leads that are now disconnected from the input tx. 

 

[mnats]

Yes, I did go through the FAQ, I have checked my parts to the schematic, I have also checked voltages of all the fets against the voltage schematic. I reviewed the wiring instructions and everything looks correct, The unit passes the 1k signal, but the Tpad is acting goofy as I said. any help will be awesome

As mentioned in the FAQ a signal tracer is the tool you need here.

 

[Curran]

oh my goodness!!! I have a tracer for pedals to trouble shoot and I should have thought of that! DUH!!!!!

 

[agoraphobia]

Hi There,

I'm experiencing some crackling and popping of audio whenever I turn the input knob. It doesn't happen when I'm not turning it and the unit for all other intensive purposes works as it should, just this little issue is a little bit of a bother. any ideas what it could be just to put me in the right direction of rectifying it?

Cheers!

 

[Echo North]

Do you have any suggestions?

Tripple check your load resistor R4 270Ω and R5 (27K), C1/C2 (0.15uF).

Mike

 

[damian]

Hi,

I have a problem with my 1176 I just built, audio does pass through but it wont calibrate or compress, When I checked for errors in my build I noticed that I had C9 (6.8uf  electrolytic the wrong way around) I put it the right way around but its still the same,

What damage could I of done by this?

Thanks,

Damian

 

[Echo North]

Hi,

I have a problem with my 1176 I just built, audio does pass through but it wont calibrate or compress, When I checked for errors in my build I noticed that I had C9 (6.8uf  electrolytic the wrong way around) I put it the right way around but its still the same,

What damage could I of done by this?

Thanks,

Damian

We'll it won't compress UNTIL it's calibrated.  So really you have one issue right now, setting your qbias.  What is you DCV at the gate of Q1?  What happens to that voltage when you rotate the Qbias.

I doubt C9 is your issue.

 

[AntonSwe]

Do you have any suggestions?

Tripple check your load resistor R4 270Ω and R5 (27K), C1/C2 (0.15uF).

Mike

I have now checked the input circuit again. Just to be sure nothing was broken I desoldered R4, R5, C1 and C2 and measured them with my DMM and they all worked and were the right values.

I also disconnected the T-pad from the input transformer and sent a 50Hz and 1kHz signal through the T-pad and measured on its output and of course it worked as it should.

The change in level between treble and bass occurs after the input transformer, on the input of the main PCB. I also checked the transformer PCB and even resoldered the input transformer to it (in case of bad solder points) but nothing changed. None of the pins on the transformer are shorting to the chassi except pin 6 as it should.

All of the grounding is in order (audio ground on the PCB, input transformer shield to pin 6 and then compressor chassi, input and output XLR, chassi to IEC inlet) with very low resistans, <1 ohm.

Is there a way to measure only the input transformer with the proper load on its input and output? I have a bunch of resistors laying around that I can use.

I know it seems very unlikely that both of my input transformers would have the same problem but I am all out of ideas now...

Thanks again!
/Anton

 

[Curran]

oK- I got the tpad to work correctly, the output from the Tpad to the transformer was not grounding I scraped the powder coating from the chassis but ended up running a wire from the negative line of the input tranformer to the screw hole on the chassis to get the ground to connect. no the volume goes up and down. Still not getting the qbias to adjust at all. as I turn the trim pot CW the signal level decreases, CCW the volume increases. the voltage reading fully CCW(never stops turning though) on the Q bias  for Q1 gate is (-.800). as I turn the trim pot CW the voltage slowly increases to (0). the source and drain read (0) the entire time. I have also successfully pulled the solder pad clean off of the input transformer pcb for the ground wire. damn it. I may need to buy a new pcb for the input transformer. with the sound probe I can pass signal all the way to the gain reduction section of the circuit. the GR circuit passes nothing so far(probably because it isn't set up yet with the q bias.)

 

[damian]

Hi,

I have a problem with my 1176 I just built, audio does pass through but it wont calibrate or compress, When I checked for errors in my build I noticed that I had C9 (6.8uf  electrolytic the wrong way around) I put it the right way around but its still the same,

What damage could I of done by this?

Thanks,

Damian

We'll it won't compress UNTIL it's calibrated.  So really you have one issue right now, setting your qbias.  What is you DCV at the gate of Q1?  What happens to that voltage when you rotate the Qbias.

I doubt C9 is your issue.

at the gate of Q1 I get a range from 0.003 up to 2.360 when turning Q bias

Damian

 

[Echo North]

oK- I got the tpad to work correctly, the output from the Tpad to the transformer was not grounding I scraped the powder coating from the chassis but ended up running a wire from the negative line of the input tranformer to the screw hole on the chassis to get the ground to connect. no the volume goes up and down. Still not getting the qbias to adjust at all. as I turn the trim pot CW the signal level decreases, CCW the volume increases. the voltage reading fully CCW(never stops turning though) on the Q bias  for Q1 gate is (-.800). as I turn the trim pot CW the voltage slowly increases to (0). the source and drain read (0) the entire time. I have also successfully pulled the solder pad clean off of the input transformer pcb for the ground wire. damn it. I may need to buy a new pcb for the input transformer. with the sound probe I can pass signal all the way to the gain reduction section of the circuit. the GR circuit passes nothing so far(probably because it isn't set up yet with the q bias.)

Seems like your Qbias adjustment is working as it should.  What is the issue with setting it?

Mike

 

[Echo North]

Hi,

I have a problem with my 1176 I just built, audio does pass through but it wont calibrate or compress, When I checked for errors in my build I noticed that I had C9 (6.8uf  electrolytic the wrong way around) I put it the right way around but its still the same,

What damage could I of done by this?

Thanks,

Damian

We'll it won't compress UNTIL it's calibrated.  So really you have one issue right now, setting your qbias.  What is you DCV at the gate of Q1?  What happens to that voltage when you rotate the Qbias.

The level is correct, however it should be a negative value.  Is it negative?

I doubt C9 is your issue.

at the gate of Q1 I get a range from 0.003 up to 2.360 when turning Q bias

Damian

The level is correct but it should be negative.  Is it a negative value?