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

[kingofspain2]

The ground plane has continuity to the chassis?

Check your Q bias voltages...maybe one got shorted out in the solder job.

https://www.hairballaudio.com/uploads/pdf/fet_rack_d_v1.14_voltages.pdf

all Qs are within volt tolerances of 10%, q2 base however is reading 0.86 volts instead of 1.06 volts, there's also a hyper sensitivity to the amplitude of the noise when touching R8 or C1, signal ground and chassis are separate, entire chassis is continuous...  thoughts?

 

[Echo North]

I just noticed that I had put CN2 on the ratio PCB backwards!! #Facepalm (I thought I had triple-checked everything)
Which certainly explains some of the gremlins. I'm hoping everything will work properly once reversed. Unfortunately I don't have access to a solder sucker for a couple of days to fix it. I will report back once I have.

With that in mind, at the moment, these are my measurements from the troubleshooting diagnostics:

- Q bias range is 0 to -3.02 at fully cw via tp 18
Amp Stages
- TP1: 0.068
- TP15: 1.31
- TP17: 0.288
- Output Trnf BRN: 0.005
- Output XLR: 5.320 VAC
GR Control Amp:
- 20:1 = 0.009  Second day's measure: -1.0 and creeping (I think I measured incorrectly the first time)
- 12:1 = 0.009
- 8:1 = 0.008
- 4:1 = 0.003.  Second day's measure: -1.6 and creeping

GR Control Amp Output Voltages:
- All low
GR Control Amp Threshold Voltages:
- 4:1 = -1.194
- 8:1 = - 1.152
- 12:1 = -1.036
- 20:1 = -0.57 and dropping

Cheers,
Dan

Lots of weird stuff here.  Not even sure where to start.

So your +30VDC and -10VDC power supply voltages are correct?

You checked the amp stages with TP18 set to most negative?  -3VDC ish?

Mike

 

[Kroc]

Lots of weird stuff here.  Not even sure where to start.

So your +30VDC and -10VDC power supply voltages are correct?

You checked the amp stages with TP18 set to most negative?  -3VDC ish?

Mike

Hey Mike,

I'll have to wait till Monday to properly recalibrate and continue testing, as I now only have access to a very cheap DMM until then, which doesn't have enough V~ resolution to accurately tweak things.

However I desoldered CN2 and put it in the correct way, and that seems to have fixed the issues I was noticing. The signal sounds normal, and I'm not experiencing the same gremlins with ratio buttons in like distortion, pops on the ratio switch, output control noise, etc. I just did a very rough QBias calibration in the mean time, and things are now metering as expected (maybe not with total accuracy until I recalibrate, but anyway), and I'm getting gain reduction. It's actually sounding pretty great!

I'll report back once I have done the full recalibration, but things are already looking much better so far.

On a secondary note, I saw there's a new MathAudio THD Meter plugin (https://mathaudio.com/thd-meter.htm). Would it be possible to calibrate the THD using it?

Cheers,
Dan

 

[KurtisHofman]

So when I was testing the stud diode for the power supply, I bumped the silver metal casing surrounding the blue bit with the multimeter and it sparked. I got the correct readings from everything after and nothing seems to have blown. It can also spark upon contact after I have disconnected the main PCB from everything. Is this maybe normal and it's just discharging? Thanks

 

[Kroc]

Lots of weird stuff here.  Not even sure where to start.

Okay, so: I'm set up for testing again. I fixed the CN2 connector as mentioned and have just recalibrated everything properly. Calibration went fine and everything is working just as expected. Clean signal is being passed, and gain reduction is definitely occurring (I can go all buttons in and totally squash). Not experiencing any of the weird gremlins I had before.

However, to double check, I went through the troubleshooting page again and remeasured everything. The GR sections seem much better, but the amp stage readings are still out, and everything is consistently reading a little low.

After fix and successful bias:

Readings - [Expected readings]
Amp Stages
- TP1: 0.07 [0.129 VAC]
- TP15: 1.217 [2.20 VAC]
- TP17: 0.266 [0.314 VAC]
- Output Trnf BRN: 4.977 VAC [5.73 VAC]
- Output XLR: 4.909 VAC [5.40 VAC]

GR Control Amp:
- 20:1 = 0.436 [0.447 VAC]
- 12:1 = 0.213 [0.215 VAC]
- 8:1 = 0.129 [0.130 VAC]
- 4:1 = 0.060 [0.62 VAC]

GR Control Amp Output Voltages:
- 20:1 = 3.998 [4.16 VAC]
- 12:1 = 1.965 [2.08 VAC]
- 8:1 = 1.200 [1.30 VAC]
- 4:1 = 0.560 [0.650 VAC]

GR Control Amp Threshold Voltages:
- 20:1 = -6.574 [-6.78 VDC]
- 12:1 = -3.547 [-3.83 VDC]
- 8:1 = -2.409 [-2.72 VDC]
- 4:1 = -1.453 [-1.79 VDC]

Any thoughts?

Thanks,
Dan

 

[Echo North]

Hey Mike,

I'll have to wait till Monday to properly recalibrate and continue testing, as I now only have access to a very cheap DMM until then, which doesn't have enough V~ resolution to accurately tweak things.

However I desoldered CN2 and put it in the correct way, and that seems to have fixed the issues I was noticing. The signal sounds normal, and I'm not experiencing the same gremlins with ratio buttons in like distortion, pops on the ratio switch, output control noise, etc. I just did a very rough QBias calibration in the mean time, and things are now metering as expected (maybe not with total accuracy until I recalibrate, but anyway), and I'm getting gain reduction. It's actually sounding pretty great!

I'll report back once I have done the full recalibration, but things are already looking much better so far.

On a secondary note, I saw there's a new MathAudio THD Meter plugin (https://mathaudio.com/thd-meter.htm). Would it be possible to calibrate the THD using it?

Cheers,
Dan

Never used it, but anything that can measure THD will work.

 

[Echo North]

So when I was testing the stud diode for the power supply, I bumped the silver metal casing surrounding the blue bit with the multimeter and it sparked. I got the correct readings from everything after and nothing seems to have blown. It can also spark upon contact after I have disconnected the main PCB from everything. Is this maybe normal and it's just discharging? Thanks

If you touch it with something with a ground potential it can certainly spark.  If it tests fine after it should be ok.

Yes that could be discharge from the large caps. Be careful.

 

[Echo North]

Okay, so: I'm set up for testing again. I fixed the CN2 connector as mentioned and have just recalibrated everything properly. Calibration went fine and everything is working just as expected. Clean signal is being passed, and gain reduction is definitely occurring (I can go all buttons in and totally squash). Not experiencing any of the weird gremlins I had before.

However, to double check, I went through the troubleshooting page again and remeasured everything. The GR sections seem much better, but the amp stage readings are still out, and everything is consistently reading a little low.

After fix and successful bias:

Readings - [Expected readings]
Amp Stages
- TP1: 0.07 [0.129 VAC]
- TP15: 1.217 [2.20 VAC]
- TP17: 0.266 [0.314 VAC]
- Output Trnf BRN: 4.977 VAC [5.73 VAC]
- Output XLR: 4.909 VAC [5.40 VAC]

GR Control Amp:
- 20:1 = 0.436 [0.447 VAC]
- 12:1 = 0.213 [0.215 VAC]
- 8:1 = 0.129 [0.130 VAC]
- 4:1 = 0.060 [0.62 VAC]

GR Control Amp Output Voltages:
- 20:1 = 3.998 [4.16 VAC]
- 12:1 = 1.965 [2.08 VAC]
- 8:1 = 1.200 [1.30 VAC]
- 4:1 = 0.560 [0.650 VAC]

GR Control Amp Threshold Voltages:
- 20:1 = -6.574 [-6.78 VDC]
- 12:1 = -3.547 [-3.83 VDC]
- 8:1 = -2.409 [-2.72 VDC]
- 4:1 = -1.453 [-1.79 VDC]

Any thoughts?

Thanks,
Dan

This is a Rev D? If so you're fine. We recently switched to a new blue t-pad that has less front end gain.  Your reading seem normal for it.  I need to update that part of the troubleshooting doc.

Mike

 

[Kroc]

This is a Rev D? If so you're fine. We recently switched to a new blue t-pad that has less front end gain.  Your reading seem normal for it.  I need to update that part of the troubleshooting doc.

Mike

It is a Rev D, yes. That's great to hear! So, specifically, the TP1 (0.07 VAC instead of 0.129 VAC) and TP15 (1.217 VAC instead of 2.20 VAC) readings are fine? Those were the ones with the biggest discrepancy.

If that isn't a problem, I'll close this baby up and stick her in my rack.

Thank you so much for all your help.

Cheers.
Dan

 

[Echo North]

It is a Rev D, yes. That's great to hear! So, specifically, the TP1 (0.07 VAC instead of 0.129 VAC) and TP15 (1.217 VAC instead of 2.20 VAC) readings are fine? Those were the ones with the biggest discrepancy.

If that isn't a problem, I'll close this baby up and stick her in my rack.

Thank you so much for all your help.

Cheers.
Dan

Yup..if you increase you tpad up 1 to 1 1/2 dots on the scale you should see the listed readings. That's the difference between the tpads.

 

[KurtisHofman]

If you touch it with something with a ground potential it can certainly spark.  If it tests fine after it should be ok.

Yes that could be discharge from the large caps. Be careful.

Thanks for clearing that up. I appreciate it. It really scared me for a moment haha!

Kurtis

 

[Nao_Idea]

Hello,

I built a second Rev F.  I thought it was fine as I was able to complete the calibration steps, Q Bias and -10db.  Then I found that the Release knob would lower the meter.  I checked all of the resistors and the marked ratings match what is specified in the build guide, along with every other value I could read off of the capacitors and other components mounted.  I looked at all the solder joints and sweated the ones that looked cold.  I have a feeling that something is going to ground and pulling down the voltage of the BRN wire (Q1?).  Also, during the build T1 did not sit flush against the board.  I pressed it in as far as I could get it.  I pulled it off once to see if anything was causing interference.  When I tried pushing it on a second time, it still wouldn't sit flush with the board and I couldn't get it free a second time so I soldered it in place. 

My power voltages were 30.5 VDC at CR8 and -9.8 VDC at CR9.

Following the Trouble Shooting Guide I measured the following voltages.

Q Bias
TP 18: 0 VDC to -2.725VDC

Amplification Stage
TP  1:  0.07 VAC
TP 15: 1.36 VAC
TP 17:  0.208 VAC
BRN: 1 mVAC
OUT XLR: 3.85

 

[Echo North]

Hello,

I built a second Rev F.  I thought it was fine as I was able to complete the calibration steps, Q Bias and -10db.  Then I found that the Release knob would lower the meter.  I checked all of the resistors and the marked ratings match what is specified in the build guide, along with every other value I could read off of the capacitors and other components mounted.  I looked at all the solder joints and sweated the ones that looked cold.  I have a feeling that something is going to ground and pulling down the voltage of the BRN wire (Q1?).  Also, during the build T1 did not sit flush against the board.  I pressed it in as far as I could get it.  I pulled it off once to see if anything was causing interference.  When I tried pushing it on a second time, it still wouldn't sit flush with the board and I couldn't get it free a second time so I soldered it in place. 

My power voltages were 30.5 VDC at CR8 and -9.8 VDC at CR9.

Following the Trouble Shooting Guide I measured the following voltages.

Q Bias
TP 18: 0 VDC to -2.725VDC

Amplification Stage
TP  1:  0.07 VAC
TP 15: 1.36 VAC
TP 17:  0.208 VAC
BRN: 1 mVAC
OUT XLR: 3.85

The Rev F output TX is a little different. Instead of BRN, you'd want to probe RED.  Your amp stages seem fine.

T1 also seems like it's fine.

Do you have a lot of flux on your PCB? Particularly around that release circuit and around Q1/11?

Mike

 

[Nao_Idea]

The Rev F output TX is a little different. Instead of BRN, you'd want to probe RED.  Your amp stages seem fine.

T1 also seems like it's fine.

Do you have a lot of flux on your PCB? Particularly around that release circuit and around Q1/11?

Mike

I  brought home my good meter to measure the RED.  I double checked the other test points for reference. 

These values are with the Q Bias set to lowest value, -2.725 VDC and 0.773 VAC at XLR In + and -.

TP1: 0.068 VAC
TP15: 1.34 VAC
TP17: 0.206 VAC
RED: 1.0 VAC

Out XLR: 3.82 VAC.
GR TP 22

20:1    0.447 VAC
12:1    0.282 VAC
8:1      0.218 VAC
4:1        0.143

#4 Anode CR2

20:1  0.677 VAC
12:1  0.740 VAC
8:1      0.859 VAC
4:1      1.004VAC

#5 TP21

20:1  0.2 mVAC
12:1  0.3 mVAC
8:1      0.3 mVAC
4:1      0.3 mVAC

Release still controls meter.
I can execute the Q Bias calibration and -10 db drop, the unit seems to behave through those processes. 

Before assembling the unit, I cleaned the circuit boards with 70% IPA and 2-Propanol, so they are free from flux.  There was less flux covering this build than the first one. 

I did re-flow the solder around Q1 and Q11.  I did not use the sockets for Q1 and Q11 like I did on my first Rev F, I ordered the wrong ones and didn't take the time to order the right ones.  Couldn't imagine it would actually come back to bite me. 

Thanks,
Mike

 

[Echo North]

I  brought home my good meter to measure the RED.  I double checked the other test points for reference. 

These values are with the Q Bias set to lowest value, -2.725 VDC and 0.773 VAC at XLR In + and -.

TP1: 0.068 VAC
TP15: 1.34 VAC
TP17: 0.206 VAC
RED: 1.0 VAC

Out XLR: 3.82 VAC.
GR TP 22

20:1    0.447 VAC
12:1    0.282 VAC
8:1      0.218 VAC
4:1        0.143

#4 Anode CR2

20:1  0.677 VAC
12:1  0.740 VAC
8:1      0.859 VAC
4:1      1.004VAC

#5 TP21

20:1  0.2 mVAC
12:1  0.3 mVAC
8:1      0.3 mVAC
4:1      0.3 mVAC

Release still controls meter.
I can execute the Q Bias calibration and -10 db drop, the unit seems to behave through those processes. 

Before assembling the unit, I cleaned the circuit boards with 70% IPA and 2-Propanol, so they are free from flux.  There was less flux covering this build than the first one. 

I did re-flow the solder around Q1 and Q11.  I did not use the sockets for Q1 and Q11 like I did on my first Rev F, I ordered the wrong ones and didn't take the time to order the right ones.  Couldn't imagine it would actually come back to bite me. 

Thanks,
Mike

Pad 21 should be measured for DC.

 

[Echo North]

- Are all 3 legs on your release pot still connected? The can break off easily.
- Do you have any ACV on the negative side of CR3/CR4?

Mike

 

[Deejsirois]

Well, I am stumped. I have finished building a Rev D. I was going through the calibrations. Q bias went as planned. Same with the Null adjust. The tracking adjust has got me confused. I am sending in a 1k Hz signal at 0 dbu, both confirmed at input. I have 0.775 vac at output when attack is fully cw. I have 2.44 vac when attack is off. I can set the meter to zero when attack is off, but I cannot adjust enough when the attack is fully CW in order to get -10 on the meter....I have maxed out the trimmer and it's still pegged at -22. What's got me even more confused is that the only way I can get it to zero on the meter when attack is off and -10 when it is fully CW is to have 0.775 showing on my DMM whether attack is off or on!! Meaning I am reporting -10 on my meter when attack is fully CW even though no GR has occurred. Bad trimmer?

 

[Nao_Idea]

- Are all 3 legs on your release pot still connected? The can break off easily.
- Do you have any ACV on the negative side of CR3/CR4?

Pad 21 should be measured for DC.

Mike

TP21

20:1 -6.49 DCV
12:1 -4.377 DCV
8:1    -3.586 DCV
4:1      -2.921 DCV

I do not have voltage at the negative of CR3/4.

All three legs are attached.  I removed the pot and I think I put too much solder on the joint and it might flowed to the other side and been jumping the pins.  When I re-installed the Release Pot, the drop on the VU meter was significantly less.  Originally the drop was -10 to -20 on the VU meter.  After I removed the release pot and re-installed it, the drop is -0.5 to -1 on the VU meter (it seems to decrease as the unit warms up). 

I measured the resistance of the release potentiometer and I found the following values while it was off of the board. 

Pot Full CCW
Pin 1 to 2:  0.2 Ohm
Pin 1 to 3:  5.46 MOhm
Pin 2 to 3:  5.46 MOhm

Midway
Pin 1 to 2:  2.8 MOhm
Pin 1 to 3:  5.5 MOhm
Pin 2 to 3:  2.8 Mohm

Full CW
Pin 1 to 2:  5.5 MOhm
Pin 1 to 3:  5.5 MOhm
Pin 2 to 3:  0.2 Ohm

Up Date: I left the unit on for a while so it would warm up, I wanted to see how much the meter changed.  The needle was reading a little over +1 on the VU meter.  I zeroed the meter and tried the Release knob to see the change.  It was maybe a needle width.  I can live with this.  It it seems to be normal and functioning properly. 

Thanks again,
Mike

 

[scott2000]

I have 0.775 vac at output when attack is off.

Set controls as follows:

Input = "24" mid rotation
Output = "24" mid rotation
[glow=red,2,300]Attack = full CW[/glow]
Release = full CW
Compression ratio = 20:1
Meter mode = "GR"

  1.Apply a 1 KHz, 0dBu signal, confirmed with your DMM at IN + and IN - at the input XLR (0.775 VAC).
  2.Move your DMM to OUT + and OUT - at the output XLR.
3.Set output control for 0dBu (0.775 VAC) at the output.

. I have 2.44 vac when attack is fully  CW.

4.Set attack full CCW (off position). Set input control for +10dBu (2.44 VAC) at your output XLR.


Are you sure you are describing what steps you are taking???

 

[Deejsirois]

Set controls as follows:

Input = "24" mid rotation
Output = "24" mid rotation
[glow=red,2,300]Attack = full CW[/glow]
Release = full CW
Compression ratio = 20:1
Meter mode = "GR"

  1.Apply a 1 KHz, 0dBu signal, confirmed with your DMM at IN + and IN - at the input XLR (0.775 VAC).
  2.Move your DMM to OUT + and OUT - at the output XLR.
3.Set output control for 0dBu (0.775 VAC) at the output.


4.Set attack full CCW (off position). Set input control for +10dBu (2.44 VAC) at your output XLR.


Are you sure you are describing what steps you are taking???

I have modified my description. Still the same issue though