[BUILD] 1176LN Rev D DIY

[beatnik]

Okey
I took my 1176 to an electrical engineer i got in contact with to try to finish it up. We looked over it for some hours and got stuck at the Gain reduction meter tracking adjustment at the point where you put pad 22 back in circuit and there is supposed to be a -10 db drop. However no drop at all occurs when we do so. What we found out were that the attack trimpot were bad, so we swapped that one out for a good one, thought this would solve the problem but NO. All solder joints seemed fine and the typical voltages from mnats schematic matched. The wiring seemed like it was fine too. Do you guys have any clue what could be wrong here? Where should we look especially? Does the transistors effect the VU Meter somehow?

Kind regards
Isak

hey Isak, i'm in the same situation as yours. Have you managed to solve that?

 

[beatnik]

i noticed that when i do the first step of the third video "apply 1kHz 0dB in VU mode and adjust the output control until the meter reads 0VU"

in the video the output control is being turned counter-clockwise.

i need to turn it clockwise instead!

then when switching the compression on, i get no drop.

something is wrong at this point but i don't understand what

 

[fluxofpinkindians]

In the meter lamp wiring section it is recommended to  "Slip a piece of insulation over one side of your selected power resistor."

Just wondered what I should use that is safe to insulate this. Is there a specific product ?

with many thanks

 

[Bobby Baird]

Use some 3/16th inch heat shrink.

 

[fluxofpinkindians]

Use some 3/16th inch heat shrink.

sorry, my post was not clear. I mean't heat insulation as the resistor will dissipate 2 watts and get very hot!

 

[Bobby Baird]

When he says slip a piece of insulation over selected resistor he's referring to heat shrink on exposed wire as not to short anything. As to the resistor you will mount it to the meter in a way that it won't touch anything else because it dissipates so much heat. I used 5 watt film resistor before, but prefer wire wound resistors now. Try http://www.mouser.com/ProductDetail/Xicon/280-CR5-220-RC/?qs=%2fha2pyFaduif4w6q%2fiuX8R7TalNUFMYBVmkn3tIECUSQB0ixGNn3rA%3d%3d 

 

[fluxofpinkindians]

thank you very much for your help 

 

[Sillen]

Hey Beatnik did you ever solve the problem? I didn't : /. Anyone else that has a clue what could we could be wrong?
Also Mike of Hairball Audio, in the last kit I bought from you the attack pot was broken. : / I don't know  but maybe you could ask Alpha for a new one or should I do it myself?

/Isak

 

[ln76d]

If you guys have some problems with the circuit, and calbration videos don't solve the problem, if you didn't check this before - it's a good source of knowledge:

http://www.jblproservice.com/pdf/Vintage%20JBL-UREI%20Electronics/UREI-1176LNmanual.pdf

 

[fluxofpinkindians]

I am having trouble calibrating the step 3 GR Meter tracking. It is compressing way too much. The unit passes audio and is compressing.  I have done the Q bias and the Discrete Meter Circuit successfully. I have made sure that r44 is in circuit.

I have calibrated the meter in GR mode to equal 0VU.

Flipping back to +4, I am sending a 1khz sine wave at -16dbu from AVID IO which is measuring 0.79 VAC at pin 2 of the input XLR with respect to ground. I am adjusting the input and output to get a 0VU reading with the GR circuit off.

Flipping the GR circuit back on, I cannot achieve a gain reduction of LESS than -20db. Even when the input is set to max CCW. The input control has a negligible affect on the gain reduction and, in fact, when I turn the input CW the gain reduction gets slightly LESS rather than more!?

(I should note that the Input attenuator will not travel full CCW to infinity and stops at 48. Is this normal ?)

would really appreciate any help

[EDIT: I can get it to "behave" by sending -24dbu from PT which corresponds to a .314 VAC at pin 2 of the XLR with respect to ground but surely this is not right!!!!]

 

[Echo North]

I am having trouble calibrating the step 3 GR Meter tracking. It is compressing way too much. The unit passes audio and is compressing.  I have done the Q bias and the Discrete Meter Circuit successfully. I have made sure that r44 is in circuit.

I have calibrated the meter in GR mode to equal 0VU.

Flipping back to +4, I am sending a 1khz sine wave at -16dbu from AVID IO which is measuring 0.79 VAC at pin 2 of the input XLR with respect to ground. I am adjusting the input and output to get a 0VU reading with the GR circuit off.

Probably what you're sending is -16dbfs from your AVID which is in the area of 0dbu.  There is no direct conversion, it depends on the spec of your A/D converter.

You want 0dbu (0.775 VAC) between pin 2 and 3.  Not ground.

Try again using pin 2 and 3.

Mike

 

[beatnik]

Hey Beatnik did you ever solve the problem? I didn't : /. Anyone else that has a clue what could we could be wrong?
Also Mike of Hairball Audio, in the last kit I bought from you the attack pot was broken. : / I don't know  but maybe you could ask Alpha for a new one or should I do it myself?

/Isak

I didnt't. My unit is not compressing, but the meter is bouncing like it should! And if i switch between the ratios, 8 and 12 show more gain reduction than 4 and 20 (without changing input and output controls) which is not normal

It's difficult for me because all the correct voltages on the transistor. If I would have anything obviously wrong, I could have a starting point, at least!

If you guys have some problems with the circuit, and calbration videos don't solve the problem, if you didn't check this before - it's a good source of knowledge:

http://www.jblproservice.com/pdf/Vintage%20JBL-UREI%20Electronics/UREI-1176LNmanual.pdf

Thank you, I read the troubleshooting pages and there are some suggestions.

In my case, no limiting, I narrowed the possibilities to:

- Q bias misadjusted: I think this is impossible, I can perform the procedure correctly, as the video, and I even tried two different pair of matched FETs

- Wire from attack control to R7 is open: I checked all the continuity between every wire and everything is ok, but I will repeat

- GR control amp defective: I think this is most likely where the problem is, but since the transistors voltages are ok, I don't know what to check.

Is there any additional measurement/test I can perform? Please let me know, I will do everything to help you in helping me 

 

[fluxofpinkindians]

I am having trouble calibrating the step 3 GR Meter tracking. It is compressing way too much. The unit passes audio and is compressing.  I have done the Q bias and the Discrete Meter Circuit successfully. I have made sure that r44 is in circuit.

I have calibrated the meter in GR mode to equal 0VU.

Flipping back to +4, I am sending a 1khz sine wave at -16dbu from AVID IO which is measuring 0.79 VAC at pin 2 of the input XLR with respect to ground. I am adjusting the input and output to get a 0VU reading with the GR circuit off.

Probably what you're sending is -16dbfs from your AVID which is in the area of 0dbu.  There is no direct conversion, it depends on the spec of your A/D converter.

You want 0dbu (0.775 VAC) between pin 2 and 3.  Not ground.

Try again using pin 2 and 3.

Mike

Thanks for your help. Really appreciated. The unit is still compressing way too much.

I have gone back and made sure the test voltage is .775 VAC rms between pin 2 & 3. I have redone the q bias test.

I am still concerned there is something up with my input attenuator. To properly set the meter at +1VU for the q bias test I have to turn the OUTPUT down to almost -48 (this is with the input attenuator as far CCW as it will go - it doesnt go all the way to infinity - with the knob put on so that the dot hits 0 at it will only turn CCW to just over 48).

The unit passes audio and is compressing but I am concerned how little the input knob affects the amount of compression in comparison to my Retro 176 which will fully attenuate the input to off.

A sine wave a -22dbu (0.775 VAC rms) will compress to a MINIMUM of -10db with the input set full CCW at 4:1!!!!

I can only get 330 ohms resistance out of the t-pad with my DMM but I have not desoldered it and perhaps I need to.

thanks for any suggestions / advice. Apologies for my lack of experience 

 

[sr1200]

This may or may not help, but calibration works best if the trimmers are all set to about 50% of their max value.  If you set the trimmers to min or max, things like that can happen causing all sorts of issues.  Are you sure you got the correct value of the input pot?  (you didnt switch the tpad and the input right?)

 

[fluxofpinkindians]

This may or may not help, but calibration works best if the trimmers are all set to about 50% of their max value.  If you set the trimmers to min or max, things like that can happen causing all sorts of issues.  Are you sure you got the correct value of the input pot?  (you didnt switch the tpad and the input right?)

Thanks for your help. I have attached a pic of the input. I wired as per the instructions and think I got it right! The XLR input is wired through the T-PAD (it says 500 ohms 1k on it - from the mouser BOM) to the input transformer to the PCB.

I have tried setting all trim pots to 50% but to no avail.

 

[audiophreak]

ok , trying to understand ... If my Q1 gate voltage ( Bias voltage ?  )  is not changing with input signal, how am I getting measured GR at the output XLR ?

You cannot have gain reduction if the gate voltage remains the same. So which condition isn't true?

  OK ... I have measured Q1 gate with and without signal and it does not change = condition TRUE
            I have measured  1.18vac @ input at Input XLR and 1.08vac @ output XLR in GR ByPass - when put in to GR mode I measure 0.40vac @ Output XLR = condition TRUE

    ... how do I test each further to see which one is FALSE

  another thing I noticed is that originally I was getting -1.44vdc at Q1 gate before I started de-soldering and double checking components , removing and reinstalling main PCB - I may have damaged something ... getting ready to say F***k it , write off the loss and move on...

 

[sr1200]

http://mnats.net/1176_reva-d_hairball_wiring_input.html
As long as you followed this entire link to a tee, you should be good with everything.  Perhaps just run through it quick and trace your wires to make sure its all done properly.

 

[audiophreak]

http://mnats.net/1176_reva-d_hairball_wiring_input.html
As long as you followed this entire link to a tee, you should be good with everything.  Perhaps just run through it quick and trace your wires to make sure its all done properly.

  I'm using the rotoary switches , will trace all wiring , Thanks

 

[mnats]

ok , trying to understand ... If my Q1 gate voltage ( Bias voltage ?  )  is not changing with input signal, how am I getting measured GR at the output XLR ?

You cannot have gain reduction if the gate voltage remains the same. So which condition isn't true?

  OK ... I have measured Q1 gate with and without signal and it does not change = condition TRUE
            I have measured  1.18vac @ input at Input XLR and 1.08vac @ output XLR in GR ByPass - when put in to GR mode I measure 0.40vac @ Output XLR = condition TRUE

    ... how do I test each further to see which one is FALSE

I should have said that you cannot have gain reduction from the FET if the gate voltage doesn't change.

Pull the gain reduction FET (you did socket it, right?) and switch GR on and off. Still getting a drop in the output?

If yes, then something else is pulling down the signal. Use a signal tracer as suggested in the troubleshooting FAQ and find out where the signal is dropping.

If no, then it IS the FET that is reducing the gain and your gate measurement wasn't accurate.

Earlier you mentioned that the GR Metering FET is changing with input signal. Note that both FETs are driven from point 7 on the schematic. If one gate voltage is changing and the other isn't that points to a problem with the path from point 7 to the affected FET gate. Short? Open circuit? Faulty component? Should be an easy thing to verify with a multimeter.

  another thing I noticed is that originally I was getting -1.44vdc at Q1 gate before I started de-soldering and double checking components , removing and reinstalling main PCB - I may have damaged something ... getting ready to say F***k it , write off the loss and move on...

How has the gate voltage changed since then?

Note: it's usually better to put something aside when frustration starts to get the better of you. Come back to it later with a clear head and things will go more smoothly.

 

[audiophreak]

    I didn't realize the Q1 gate voltage changed from -1.44vdc to -2.08vdc till after I had swapped ratio PCBs and and then swapped back , changed 3707s to 5088s and had de-soldered all resistors in both GR meter driver and GR control amp sections to verify values , so I'm not sure when it actually changed.
    I had not sockets for the FETs , but the FETs legs are small in diameter so easier to remove than other components.

  Thank you for the clear trouble shooting direction and circuit description, I had made that signal tracing tool when I did the Rev.Js

  I'd like to think I have a pretty good project assembly ethic, but have some how missed the mark on this one.

I'm out tomorrow for a local one off , then back to this on Wednesday.

Mnats and Hairball , Thank you both for your EXTREME Patients with my ignorance with this  project .