[BUILD] New FET/RACK Official Help Thread - Please read first post!

[dbonin]

Not much to go wrong there.  Just those two resistors on the meter PCB.

- Get +4dBu (1.23VAC) signal across input output +/- and the meter should be at around 0  in +4.
- Get+8dBu (1.95VAC) signal  across input output +/- and the meter should be at around 0  in +8.

You can just use a 0dBu signal on the input like setting Qbias, then increase the output to get +4 and +8.  You'll need to be in GR off.  With GR on the signal will probably be too low.  That might be what you're seeing.

Mike

That was quick and easy.  Spot on again!
Thanks Mike.

 

[thecmarus]

Post back after you try that...I'll give you some other stuff to check.

Mike

Scrubbed with 99% isopropyl and a toothbrush, dried it with a cloth, repeated again twice. Issue is still going on. PCB looked pretty clean to begin with as I did this a couple times already.

 

[Echo North]

Scrubbed with 99% isopropyl and a toothbrush, dried it with a cloth, repeated again twice. Issue is still going on. PCB looked pretty clean to begin with as I did this a couple times already.

Set your unit to:
Input: 12 O’Clock
Output: 12 O’Clock
Attack: GR ON (Full CW)
Release: Full CW
Ratio: 20:1
Meter: GR
Signal: Sine 1kHz @ 0dBu (0.775 VAC across input + and -)

Using chassis as your ground reference, what is your DC V at TP21 for all 4 ratio settings.

1. What are those readings?
2. Do they change as you rotate the release?

Mike

 

[thecmarus]

Set your unit to:
Input: 12 O’Clock
Output: 12 O’Clock
Attack: GR ON (Full CW)
Release: Full CW
Ratio: 20:1
Meter: GR
Signal: Sine 1kHz @ 0dBu (0.775 VAC across input + and -)

Using chassis as your ground reference, what is your DC V at TP21 for all 4 ratio settings.

1. What are those readings?
2. Do they change as you rotate the release?

Mike

4        -1.606
8        -2.435
12    -3.265
20    -5.9

No change in DC as I rotate.

On another note after a thorough warmup after the last cleaning the problem seems to have lessened. It used to swing from 0 to -7 or -10 and now it's only going down to about 0 to -1.

Going to clean again a couple more times and see if it makes a difference. Thank you for your time Mike! This is making me want to rescrub all my Lola and Gold preamps I built!

 

[Echo North]

4        -1.606
8        -2.435
12    -3.265
20    -5.9

No change in DC as I rotate.

On another note after a thorough warmup after the last cleaning the problem seems to have lessened. It used to swing from 0 to -7 or -10 and now it's only going down to about 0 to -1.

Going to clean again a couple more times and see if it makes a difference. Thank you for your time Mike! This is making me want to rescrub all my Lola and Gold preamps I built!

Those readings look good and it's good they are not changing with rotation.

Report back after a deep scrub....don't be afraid to soak the back side and let it run off the edge of the PCB. Get the ratio board too.

Mike

 

[thecmarus]

Those readings look good and it's good they are not changing with rotation.

Report back after a deep scrub....don't be afraid to soak the back side and let it run off the edge of the PCB. Get the ratio board too.

Mike

Mike,

Bought a gallon jug of isopropyl from my local hardware store and went buck wild on all the PCBs. Still getting the 1db drop on the VU meter when I put the release on 1, which is still better than my original problem which was from 0 to -7 or -10. A few things I've noted are that it is definitely worse until the unit has been on for over an hour or more. The VU meter takes a substantial amount of time to reach 0db, and I know this is normal for an 1176, but this is more dramatic than my Rev D kit.

Maybe if I'm feeling adventurous tonight I'll give it another scrub.

 

[Echo North]

Mike,

Bought a gallon jug of isopropyl from my local hardware store and went buck wild on all the PCBs. Still getting the 1db drop on the VU meter when I put the release on 1, which is still better than my original problem which was from 0 to -7 or -10. A few things I've noted are that it is definitely worse until the unit has been on for over an hour or more. The VU meter takes a substantial amount of time to reach 0db, and I know this is normal for an 1176, but this is more dramatic than my Rev D kit.

Maybe if I'm feeling adventurous tonight I'll give it another scrub.

You shouldn't have to scrub that much.

We need to figure out where the voltage is fluctuating to get to the root of it. I would have expected Pad 21, but no dice.

Scrub that attack/release PCB too!  Confirm all those values.

Mike

 

[Jec909]

Hi everyone,

I've sent a mail to hairball but I thought I'd write here as well as Mike answers in this thread as well.

I've built two Rev D . nothing to report regarding the built, well documented, really top notch.

Seems my device is not even passing audio

On both of my FETs when calibrating Qbias, I get no AC reading on the output between OUT + and OUT -.
Also I understand that when the device is on OFF,, it should be in pass trough. Is that right? Because I also get no AC reading on the output between OUT + and OUT -. in OFF

I do get a reading on the output between OUT + and OUT - but only when i turn up and down the output control then it briefly reads between 0 VAC to 1.5 VAC or so but drops right ways to 0 VAC

To this question:

Set your unit to:
Input: 12 O’Clock
Output: 12 O’Clock
Attack: GR OFF
Release: Full CW
Ratio: 20:1
Meter: GR
Signal: Sine 1kHz @ 0dBu 0.775 VAC

With your DMM measure your AC at the following test points using the chassis as you reference (black probe).

-Input XLR + = FET1: 0.387 VAC / FET2 0.383 VAC
-TP1 = FET1: 0.110 VAC/ FET2 0.139 VAC
-TP15 = FET 1 0VAC / FET2 0VAC
-TP17 = FET 1 0VAC / FET2 0VAC
-Output Transformer BRN = Where is it?
-Output XLR + =  FET 1 0VAC / FET2 0VAC

The issue is consistent on both device, I need some heads up

 

[Echo North]

Hi everyone,

I've sent a mail to hairball but I thought I'd write here as well as Mike answers in this thread as well.

I've built two Rev D . nothing to report regarding the built, well documented, really top notch.

Seems my device is not even passing audio

On both of my FETs when calibrating Qbias, I get no AC reading on the output between OUT + and OUT -.
Also I understand that when the device is on OFF,, it should be in pass trough. Is that right? Because I also get no AC reading on the output between OUT + and OUT -. in OFF

I do get a reading on the output between OUT + and OUT - but only when i turn up and down the output control then it briefly reads between 0 VAC to 1.5 VAC or so but drops right ways to 0 VAC

To this question:

Set your unit to:
Input: 12 O’Clock
Output: 12 O’Clock
Attack: GR OFF
Release: Full CW
Ratio: 20:1
Meter: GR
Signal: Sine 1kHz @ 0dBu 0.775 VAC

With your DMM measure your AC at the following test points using the chassis as you reference (black probe).

-Input XLR + = FET1: 0.387 VAC / FET2 0.383 VAC
-TP1 = FET1: 0.110 VAC/ FET2 0.139 VAC
-TP15 = FET 1 0VAC / FET2 0VAC
-TP17 = FET 1 0VAC / FET2 0VAC
-Output Transformer BRN = Where is it?
-Output XLR + =  FET 1 0VAC / FET2 0VAC

The issue is consistent on both device, I need some heads up

Your signal is dying in your first amp stage. The signal pre amp.

Set your compressor controls:
Input: 12 O’Clock
Output: 12 O’Clock
Attack: GR OFF
Release: Full CW
Ratio: 20:1
Meter: GR
Signal: None

With your DMM measure DC at TP18 using the chassis as you reference (black probe). Rotate the Qbias pot (R59) and note the DCV at each extreme of the pot. What are those 2 reading?

Mike

 

[Jec909]

Your signal is dying in your first amp stage. The signal pre amp.

Set your compressor controls:
Input: 12 O’Clock
Output: 12 O’Clock
Attack: GR OFF
Release: Full CW
Ratio: 20:1
Meter: GR
Signal: None

With your DMM measure DC at TP18 using the chassis as you reference (black probe). Rotate the Qbias pot (R59) and note the DCV at each extreme of the pot. What are those 2 reading?

Mike

Thanks for the quick answer Mike
I get -00.3 mVDC with r59 full CCW and -2.97 VDC with r59 full CW

 

[Echo North]

Thanks for the quick answer Mike
I get -00.3 mVDC with r59 full CCW and -2.97 VDC with r59 full CW

Set it to -2.97 VDC then try those readings in your first post again.

Lets just stick to unit 1 for now.

Mike

 

[Jec909]

Set it to -2.97 VDC then try those readings in your first post again.

Lets just stick to unit 1 for now.

Mike

Sure!

With r59 at  -2.97 VDC full cw and 1000 Hz 0 dBu (0.775VDC)  signal to the input

-Input XLR +  = 0.383 VAC
-TP1 = 0.139 VAC
-TP15 = 0 VAC
-TP17 = 0 VAC
-Output Transformer BRN = Where is it?
-Output XLR + =  0 VAC

 

[Echo North]

Sure!

With r59 at  -2.97 VDC full cw and 1000 Hz 0 dBu (0.775VDC)  signal to the input

-Input XLR +  = 0.383 VAC
-TP1 = 0.139 VAC
-TP15 = 0 VAC
-TP17 = 0 VAC
-Output Transformer BRN = Where is it?
-Output XLR + =  0 VAC

Don't worry about BRN right now.

Your signal is dying in the Signal Preamp stage marked on the PCB.

Try removing you Q1 FET carefully and see if you get signal at TP15.

Mike

 

[Jec909]

Don't worry about BRN right now.

Your signal is dying in the Signal Preamp stage marked on the PCB.

Try removing you Q1 FET carefully and see if you get signal at TP15.

Mike

I unsoldered Q1 FET

And signal do not reach tp15

No VAC reading between chassis and 15

 

[Echo North]

I unsoldered Q1 FET

And signal do not reach tp15

No VAC reading between chassis and 15

You need to search through that Signal Preamp section marked on the PCB.  Triple check all component values, solder joints, and continuity. Something is a miss in there.

Check you transistor values against this chart in that section:
http://mnats.net/files/1176REVD_VOLTS.pdf

Make sure your power supply voltages are still good (+30VDC and -10VDC).

Mike

 

[Jec909]

You need to search through that Signal Preamp section marked on the PCB.  Triple check all component values, solder joints, and continuity. Something is a miss in there.

Check you transistor values against this chart in that section:
http://mnats.net/files/1176REVD_VOLTS.pdf

Make sure your power supply voltages are still good (+30VDC and -10VDC).

Mike

thanks mike. will do and report

 

[Echo North]

thanks mike. will do and report

Also make sure all of your L brackets are secure in place.

Good luck! 

Mike

 

[Jec909]

Mike, feeling a bit confused.

On these test points, I should see DC or AC current?

I've been measuring  AC all the time :S

 

[Echo North]

Mike, feeling a bit confused.

On these test points, I should see DC or AC current?

I've been measuring  AC all the time :S

Depends on what your looking for.

In the first step we did (TP1, TP15) we're looking for actual audio signal, which is AC.  You input your signal (0dBu @ 1Khz) and trace that AC through the gain stages.  Your signal is measuring fine at TP1 which is after the T-Pad and input transformer and at the beginning of the signal preamp.

For a Revision D you should then see about 2 VAC (8 dBu) at the output of the signal pre amp which is TP15.

You have nothing there so your signal is dying in that signal preamp section. You have a wrongly paced component, damaged component, or bad solder joint/pad.  Measuring the DC V at the transistor leads as shown in that schematic with voltages, can help us narrow down the issue further.

Mike

 

[Jec909]

I found the problem. I deserve a big slap . The issue was a missing resistor  LOL  :-[ (r8, which was hidden below the orange drop cap. )

I mostly wasted your time but I did learn a lot!  Many many thanks for your extremely valuable help and time.