[BUILD] 1176LN Rev D DIY

[Matt C]

I have an 1176 that I built years ago that generally seems to be working well, but I recently measured the ratios and they are quite a bit higher than they should be.  4:1 was more like 7:1, 8:1 was 11:1, 20:1 was 33:1, etc. 

I checked the resistor ladders around the ratio switch and they all looked fine, which makes me think the issue is in the gain reduction control amp that is fed by the ratio switch.  But again no obvious issues there.  I did notice that the GR control amp loads down this resistor ladder quite a bit, so the signal levels at those switch nodes drop significantly compared to when GR is switched off, but maybe that's normal?  If not maybe the hfe of those GR amp transistors is too low?

Anyone run into this problem? I know I can just modify the ratio resistors to get the ratios I need, but when testing that out I had to change them so much that I'm suspicious there's a larger problem.

 

[Echo North]

I have an 1176 that I built years ago that generally seems to be working well, but I recently measured the ratios and they are quite a bit higher than they should be.  4:1 was more like 7:1, 8:1 was 11:1, 20:1 was 33:1, etc. 

I checked the resistor ladders around the ratio switch and they all looked fine, which makes me think the issue is in the gain reduction control amp that is fed by the ratio switch.  But again no obvious issues there.  I did notice that the GR control amp loads down this resistor ladder quite a bit, so the signal levels at those switch nodes drop significantly compared to when GR is switched off, but maybe that's normal?  If not maybe the hfe of those GR amp transistors is too low?

Anyone run into this problem? I know I can just modify the ratio resistors to get the ratios I need, but when testing that out I had to change them so much that I'm suspicious there's a larger problem.

I've always found the ratios to have a small sweet spot (3-8db GR maybe).  Too close to zero and you're hitting the knee, in this case your ratios look too low.  Too much GR and your FET saturates making the GR way higher.

Mike

 

[Echo North]

I'm working on a build of the Rev D that I started from scratch using my own boards. I used mnats Rev D schematic as reference and copied his layout quite a bit minus the power supply as I'm building a dual unit and have a single supply for both channels. I initially had no trouble with it and recall being able to complete the first calibration step but I've since tidied up the wiring a bit and now my preamp section doesn't seem to be preamping.
If I input a .776VAC 1Khz sine I get about 100mV at the positive input of the board with the input attenuator up to max. I'm using a stepped attenuator that I made myself so that may be why it's not 129mV at midpoint as suggested by the Hairball troubleshooting guide.
My input transformer is a Lundahl 1540 in 2:1 step-down configuration. With the input attenuator at max I get 776mV at the + transformer input and ~100mV at + output of transformer. Not sure if this is too low and the cause of the problem.

Following the troubleshooting guide but with input and output controls at max my readings are:

TP1 = ~100mV AC
TP15 = ~110mV AC
TP17 = ~110mV AC
Output TX Brown = 2V AC

So it seems like the Line Amp is working. I checked DC voltages for the preamp transistors and they all seem to be within tolerance

Q2 - C=1.77V,  B=1.02V,  E=0.49V
Q3 - C=12.52V,  B=1.77V,  E=1.13V
Q14 - C=30.11V,  B=12.52V,  E=11.89V

My Q-bias voltage at Pad 18 is -3.1VDC fully CCW and goes to 0VDC fully CW and affects the voltage at Pad 15.
Q1 gate voltage is -2.6VDC.
I'm thinking there's a problem with Q1 or my input transformer/attenuator but before I take everything apart to test it I thought I'd see if anyone had a second opinion.

Also I may have an issue with my grounding. I have the main pcb ground wired to the chassis at the IEC ground point and with power off I'm getting 0ohms from ground to chassis everywhere, but when powered on it's almost an ohm on the main board and goes negative at 0V on the psu. Is that normal?

Line amp seems broken. TP15 should be around 2VAC with GR off (0dB/1kHz input).  Pull Q1 see if that fixes TP15.

Mike

 

[Mikolaj]

I'm building a dual 1176 rev d compressor rotary version using Hairball printed circuit boards.
On step three calibrating the meter I  short pad 22 to ground as per video and zero the meter. When i remove the ground i would expect a -10 db drop on the meter. The needle does not move.
Pad  22 has .22 volts ac. and pad 21 has -6.9 volts
Can any one help with a starting point to solve  this problem
Thanks
Mickey

 

[Mikolaj]

Solve the problem. qbias was off

 

[Shattersignal]

Start buy measuring the DC voltage at pad 18 as you rotate the qbias. It should go from 0VDC to some negative value like -2.2VDC.

Use the CT/0V of the power supply as your common.

Mike

Hi there!  I'm helping a buddy with a Rev A build.  We are having difficulty with calibration step 1.  We are seeing a bias voltage range of between -1.67v and past -3v with the sweep of the Q Bias pot--there seems to be no way to set this bias correctly, as we cannot achieve the correct voltage within the range provided by the pot.  No voltage reduction is observable on the output VU meter as the bias is adjusted.  Here's what we've tried, so far:

--We've installed a 5k pot in R59,
--We've installed new, and HFE matched transistors in Q12 and Q13,
--We've re-flowed all the solder joints in the Attack and Release signal path.

Are there other areas we ought to explore to get the GR circuit engaged and the bias point set correctly?

Thanks a ton!!

 

[Echo North]

Hi there!  I'm helping a buddy with a Rev A build.  We are having difficulty with calibration step 1.  We are seeing a bias voltage range of between -1.67v and past -3v with the sweep of the Q Bias pot--there seems to be no way to set this bias correctly, as we cannot achieve the correct voltage within the range provided by the pot.  No voltage reduction is observable on the output VU meter as the bias is adjusted.  Here's what we've tried, so far:

--We've installed a 5k pot in R59,
--We've installed new, and HFE matched transistors in Q12 and Q13,
--We've re-flowed all the solder joints in the Attack and Release signal path.

Are there other areas we ought to explore to get the GR circuit engaged and the bias point set correctly?

Thanks a ton!!

This is the Rev D thread.  What version is your Rev A PCB?

Mike

 

[Shattersignal]

This is the Rev D thread.  What version is your Rev A PCB?

Mike

Thanks  ton, Mike.

Search took me here while looking for folks with the same issue as me.  Sorry if this isn't the correct thread--it seemed logical to me to inquire here. 

We have a FET Rack v. 1.12

Cheers from LA!

 

[Shattersignal]

This is the Rev D thread.  What version is your Rev A PCB?

Mike

I'll copy my last post to your FET RACK thread.  Thanks again, Mike.

 

[Zebra_PD]

Hi, I'm starting to collect parts for my rev D build. Came across some UTC H-8 input transformers that are 600ohm, respons 50 to 100k cps.

Do you guys think these would work as  substitute for the 0-12 transformer?

 

[warmtape]

Hello all, I completed the build of the 500 series FET/D, but when powered on the gain reduction LEDs are pulsing (with pulse rate controlled by the release potentiometer) and when passing audio through the device, the audio clicks at the rate of the LED pulsing. Last night I was able to investigate with a tech and we discovered that the negative supply pulses when the gain reduction knob is out. When pushed in, we do not see pulsing [which is the reverse of #4 of the FAQ for common issues with this device]. We are not sure why this may be happening and are wondering if anyone may have any advice on the issue?
Many thanks in advance

 

[Echo North]

Hello all, I completed the build of the 500 series FET/D, but when powered on the gain reduction LEDs are pulsing (with pulse rate controlled by the release potentiometer) and when passing audio through the device, the audio clicks at the rate of the LED pulsing. Last night I was able to investigate with a tech and we discovered that the negative supply pulses when the gain reduction knob is out. When pushed in, we do not see pulsing [which is the reverse of #4 of the FAQ for common issues with this device]. We are not sure why this may be happening and are wondering if anyone may have any advice on the issue?
Many thanks in advance

Moved to FET/500 thread:

https://groupdiy.com/index.php?topic=57192.0

 

[myers1st]

Hi-

So I'm working on an older dual rev D with the mnats power supply.  I had the PSU working and trimmed to +30 and -10.  I finished one channel and applied power to test it, but forgot to disconnect the unfinished second channel from the psu and there must have been a short somewhere.  LEDs on the PSU went on briefly, heard a pop, lights out, I killed power after about two seconds.  Nothing smoked or looks fried.

Now the power trafo is acting funny.  Before, I was getting 25v between secondary ends and center tap.  Now I measure 0V between secondary ends and center tap, but I measure 75V between secondary ends and chassis.  Could I have fried the power transformer?  I should be able to read the correct transformer output voltage without the psu connected right? 

Thanks for any help.

 

[weiss]

hey guys, can you answer a quick question for me?
if the rotary meter switch is set to GR off, should pad 22 show continuity to ground?

thanks

 

[Echo North]

hey guys, can you answer a quick question for me?
if the rotary meter switch is set to GR off, should pad 22 show continuity to ground?

thanks

Yes.

 

[Alexwfm]

Hello!

Weird thing happening to my RevD 1176: without any input signal the output self oscillates bring the output volume towards fully clockwise.

Things I found out:
-Disconnecting the output XLR or using a short 1 metre XLR of stops the oscillation.
-Different XLR produce different frequencies that vary 25Khz - 150Khz  about 4Vpp.
-Grounding output + doesn't do anything while grounding the output - produces the same high freq oscillation.
-Lifting the PCB and moving it around I could vary the frequency and amplitude.

Checked transformer wiring multiple times..

For the rest the 1176 works well, calibration was fairly straightforward. Tested earth points with multimeter and all earthing was good.

I'm desperate as I'm not sure what else to check!

Any help will be greatly appreciated!
Cheers!

ps. if anyone fancies the challenge there's another paranormal situation needing debunk:  if I disconnect the wire going to pin 17 -basically disconnecting the return signal from the output volume pot- the signal still passes through the line amp section, lower in volume and in low end but I can't understand how that's possible as that circuit is not connected to the other sections in any way but for that wire?

 

[Echo North]

Hello!

Weird thing happening to my RevD 1176: without any input signal the output self oscillates bring the output volume towards fully clockwise.

Things I found out:
-Disconnecting the output XLR or using a short 1 metre XLR of stops the oscillation.
-Different XLR produce different frequencies that vary 25Khz - 150Khz  about 4Vpp.
-Grounding output + doesn't do anything while grounding the output - produces the same high freq oscillation.
-Lifting the PCB and moving it around I could vary the frequency and amplitude.

Checked transformer wiring multiple times..

For the rest the 1176 works well, calibration was fairly straightforward. Tested earth points with multimeter and all earthing was good.

I'm desperate as I'm not sure what else to check!

Any help will be greatly appreciated!
Cheers!

ps. if anyone fancies the challenge there's another paranormal situation needing debunk:  if I disconnect the wire going to pin 17 -basically disconnecting the return signal from the output volume pot- the signal still passes through the line amp section, lower in volume and in low end but I can't understand how that's possible as that circuit is not connected to the other sections in any way but for that wire?

That's usual caused by the wires from the meter PCB to the XLR output.  Keep them away from the PCB and/or shield them.

 

[Alexwfm]

I see! Though I get that issue also when the meter is on GR? I’ll try anyway!

 

[Echo North]

I see! Though I get that issue also when the meter is on GR? I’ll try anyway!

Yup you will. 

 

[Alexwfm]

Hello Hairball Audio,

I've tried removing the wire from the XLR to the meter but it didn't help.
Though what resolved the issue was rewiring both of the XLRs ground to the main PCB  star ground instead of using the wire coming out of the output transformer chassis. It seems so strange how that could be affecting the circuit, as the transformer chassis is physically connected to the case as well as the star ground just a few cm away!

Cheers