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

[Echo North]

I recently purchased a partial kit and the corresponding mousser cart of components. During calibration I have run into a problem with the attack and meter adjustments. I have gone back several times to make sure I have not missed a step and when it comes to the attack calibration with the trimmer R44 I just can not get any results.
With the attack cw to fully on I make the adjustments according to the hairball cal. guide, but I get no meter jump when the attack is switched off and no difference on my DMM reading.
Ive read that this version of the unit has very little possibility of wiring issues since the harnesses are premade and pretty dummy proof.
I did have a slight issue with a pad on the front of the board at Q3, the pad on the back was fine, could the copper pad on one side have caused the issue even if the pad on the back had a good joint?
Is there anyway to trouble shoot my transistors with a multimeter? My solder joints look good, I have touched up a few.


EDIT: I am working on a REV D v1.11 build

Sounds like issues with you GR amp/ratio section.  Double check those two sections.

 

[oskura]

Sounds like issues with you GR amp/ratio section.  Double check those two sections.

Should I be looking for anything specific as far as components go? Or just double checking solder joints?

I think I was on step 3 of the calibration section and when the attack knob is toggled there is no change on the meter.

 

[mnats]

During calibration I have run into a problem with the attack and meter adjustments. I have gone back several times to make sure I have not missed a step and when it comes to the attack calibration with the trimmer R44 I just can not get any results.
With the attack cw to fully on I make the adjustments according to the hairball cal. guide, but I get no meter jump when the attack is switched off and no difference on my DMM reading.

I'm not sure why you have posted to the old Rev D thread as well but it doesn't help so please try to keep your comments to the relevant thread.

To be clear there is no 'attack' calibration. I believe you are getting stuck on the Gain Reduction Meter Tracking calibration. Your symptom appears to be "no gain reduction". Can you confirm that there is no limiting of the audio signal? When the attack knob is fully counter clockwise we call this "gain reduction off". This is a separate function from attack even though the controls are on the same knob.

There have been many instances of no gain reduction so if that is your symptom then try using those search terms to see earlier troubleshooting threads.

Is there anyway to trouble shoot my transistors with a multimeter?

This is exactly what you are doing when you check transistor voltages against the chart linked from the FAQ which is in turn linked from the first post of this thread.

Should I be looking for anything specific as far as components go? Or just double checking solder joints?

You should look for any of the conditions listed in the FAQ. Let us know what you find so others can learn.

 

[Blahpstick]

Hey guys!  Sorry, Mike.  I sent you an email with all of this info before so you're probably going to end up reading this again...  :-\:-X

I was building a Rev A + active stereo link kit to completion.  I was attempting to calibrate the unit's Q Bias.  I got a solid reading at the input for the necessary 0 dBu source.  I get the output signal at the proper 2.75 VAC and while not touching anything else except for my probes to the output test points, the signal would begin to drop slowly but steadily.  It bottomed out around 500mV below the initial signal reading and then the voltage starts to surge seemingly randomly in little bursts of 50 - 100 mV at a time every second or so.  I tried reading the test points 10 + 11 for the meter tracking adjustment and same problem with no signal generator attached.  The meter follows this voltage drop and I can watch the needle slowly come down with the readings.

This only seems to be the case when 20:1 is selected.  4, 8, and 12 all have a solid position on the meter and don't seem to move at all even when left for 30 minutes.  I tried pushing in 4 and 20 at the same time and the needle goes back to the same position as it would be in 4:1 instead of going up as the usual All Buttons In mode would do.

Forgive my less than graceful description of the issue.  I will be glad to provide more details.

Thank you so much for your time!

 

[rockprocess]

I've completed the Rev D version and it works and functions fine except that I do have a pretty noticeable amount of hiss.  Is there anything I can check or consider to reduce the hiss?  The hiss is there with pretty reasonable levels, not cranking the input/output, etc.  I don't think its a gain staging issue.  Thoughts/suggestions?

 

[IndietownRecording]

I haven't built this yet, but what is the small hole in the faceplate (between the input and output knobs) used for?

 

[Potato Cakes]

I haven't built this yet, but what is the small hole in the faceplate (between the input and output knobs) used for?

That is for access to the meter zero trimmer.

Thanks!

Paul

 

[Hxcplayer]

They are coupling caps. The connect two sections of the circuit together and block unwanted DC from one section from moving to the next. Since they are in series with the signal and the audio passes through them they have noticeable impact on the sound.

Deeper:
http://diy.ecpaudio.com/p/some-notes-on-coupling-capacitors.html

Mike

Hey Mike,

Quick question about those coupling caps for the orange drops. 
Do you know if they affect the attack and release time as well or is there no change whatsoever but just a colorization difference to the audio output. 
Also, are all four caps connected in series? I'm asking because I'm writing a paper for my university about the influence that capacitors have on analog compressors.
If by any chance you have some cool sources about capacitors in compressors, it would be awesome if you could recommend me something.

Thanks!

 

[RR25]

If anyone has any ideas what might be causing this I'd be very appreciative
Thanks in advance.

Thanks for your help in the previous posts Mike and mnats. I'm back from a quick break and getting closer to finishing this rev A.

I fixed the issue i was having with a low amount of signal and not being able to compress 10dB to finish the calibration steps that I mentioned in previous posts.
(turns out I just wasn't paying enough attention and I had two resistors in each others places in the signal preamp section)

I have now finished the calibration and everything went great, although when I try and run audio through it I get a very weak and filtered signal, almost sounds like a bandpass at about 1KHz.

I have checked all of the resistors again and they are all correct (now). All of the voltages checked out too.

Any help in narrowing down what might be the culprit would be great.

Thanks again

 

[mnats]

If anyone has any ideas what might be causing this I'd be very appreciative
Thanks in advance.

In my experience there are no issues that are not caused by one of more of the conditions listed in my FAQ linked from the first post. As also mentioned there, when you have an audio fault a signal tracer might be the best tool.

Once you've built it, connect the ground clip to the chassis. Inject audio into pins 2 and 3 of the input XLR or IN+ and IN- on the Main PCB. Start measuring audio from Test Point 1, then 15, 17 and OUT+. Keep in mind that the input and output controls have an influence at the various points on the circuit. Come back with the results.

 

[vtr]

Hey all,
Just finished assembling a complete kit REV A (v1.12) and active stereo link. I’m onto the first calibration step (Q Bias) and having an issue. I am applying the test input 1k signal at .775 VAC. I have the unit’s controls configured as described in the Q Bias section. When I measure the output signal, I’m only measuring .14 VAC. If I max out the INPUT and OUTPUT controls on the front of the unit, the output I measure is 2.8 VAC.

With the unit’s controls setup for the Q Bias (input and output set to mid), I measure the following at the test points below:

TP1:
.114 VAC

TP15:
.049 VAC

TP17:
.025 VAC

Hope this information helps. Please advise. Thank you in advance for your assistance.
Matt

 

[RR25]

Thanks mnats! I'll give that a try tomorrow.

In my experience there are no issues that are not caused by one of more of the conditions listed in my FAQ linked from the first post. As also mentioned there, when you have an audio fault a signal tracer might be the best tool.

Once you've built it, connect the ground clip to the chassis. Inject audio into pins 2 and 3 of the input XLR or IN+ and IN- on the Main PCB. Start measuring audio from Test Point 1, then 15, 17 and OUT+. Keep in mind that the input and output controls have an influence at the various points on the circuit. Come back with the results.

 

[mnats]

When I measure the output signal, I’m only measuring .14 VAC. If I max out the INPUT and OUTPUT controls on the front of the unit, the output I measure is 2.8 VAC.

With the unit’s controls setup for the Q Bias (input and output set to mid), I measure the following at the test points below:

TP1:
.114 VAC

TP15:
.049 VAC

TP17:
.025 VAC

Looks like the Signal Preamp stage isn't working properly. Find out where the voltages are different from the ones in the chart within that stage, then look for one or more of the conditions listed in the FAQ. Please report back what you found that was wrong.

 

[RR25]

In my experience there are no issues that are not caused by one of more of the conditions listed in my FAQ linked from the first post. As also mentioned there, when you have an audio fault a signal tracer might be the best tool.

Once you've built it, connect the ground clip to the chassis. Inject audio into pins 2 and 3 of the input XLR or IN+ and IN- on the Main PCB. Start measuring audio from Test Point 1, then 15, 17 and OUT+. Keep in mind that the input and output controls have an influence at the various points on the circuit. Come back with the results.

Hi mnats, thanks for the help.

I've built a signal tracer and been through the board. The signal at the XLR input is strong (exactly 0.775 VAC) and I can follow it clearly to the attenuator with the signal tracer but the position of the input knob did not change the level of the signal through the signal tracer which seemed a bit weird.

The signal died down to 0.02 VAC at the output of the input transformer so would I be correct to assume that the issue lies in the signal preamp section of the PCB? I have also checked my voltages for Q2 and Q3 and they are well within spec.

These readings were done with the controls set for calibration using the chassis as ground:

IN + = 0.775 VAC
TP 1 = 0.02 VAC
TP 15 = 0.02 VAC
TP 17 = 0.01 VAC

Thanks again!

 

[mnats]

Hi mnats, thanks for the help.

I've built a signal tracer and been through the board. The signal at the XLR input is strong (exactly 0.775 VAC) and I can follow it clearly to the attenuator with the signal tracer but the position of the input knob did not change the level of the signal through the signal tracer which seemed a bit weird.

The signal died down to 0.02 VAC at the output of the input transformer so would I be correct to assume that the issue lies in the signal preamp section of the PCB? I have also checked my voltages for Q2 and Q3 and they are well within spec.

These readings were done with the controls set for calibration using the chassis as ground:

IN + = 0.775 VAC
TP 1 = 0.02 VAC
TP 15 = 0.02 VAC
TP 17 = 0.01 VAC

Thanks again!

Didn't ask for a measurement at IN+ but it brings up something I neglected to mention. But first, what was your reference when measuring IN+? Since both the input and output are galvanically isolated by the audio transformers you'll need to measure your input signal between IN+ and IN-; similarly the output has to be measured between OUT+ and OUT-, not between OUT+ and ground as my previous post implied.

If the signal isn't making it to TP1 then there is something wrong with the circuit to the left of the test point, not necessarily to the right. Although a dead short at the test point to ground would also cause problems and might be seen as an issue with the Signal Preamp if you're feeling pedantic.

 

[junkface]

Having a meter calibration issue with one of my new Rev D builds. (v.1.12) Q bias went fine. Then I was having troubles with the null adjust, there didn't seem to be enough resistance to get the meter zeroed with 0VDC at the test points. Subbed out the 2k for a 5k and was able to get that step ok. But now in the tracking adjust calibration step, with GR on (-10db) the meter is pinned to the left. Tracking adjust doesn't do anything. Replaced the tracking adjust 2k trimmer with a 5k I'd tested just to be safe there. Pulled and double checked that the HFE was matching on Q12/Q13, also tried replacing with another matched pair of 2n3708s I had lying around. Meter is still pinning left, even at lower gain reduction values.
Should I get another pair of 2n3707s in there, or is there some other likely avenue to look into first?

 

[mnats]

Having a meter calibration issue with one of my new Rev D builds. (v.1.12) Q bias went fine. Then I was having troubles with the null adjust, there didn't seem to be enough resistance to get the meter zeroed with 0VDC at the test points. Subbed out the 2k for a 5k and was able to get that step ok. But now in the tracking adjust calibration step, with GR on (-10db) the meter is pinned to the left. Tracking adjust doesn't do anything. Replaced the tracking adjust 2k trimmer with a 5k I'd tested just to be safe there. Pulled and double checked that the HFE was matching on Q12/Q13, also tried replacing with another matched pair of 2n3708s I had lying around. Meter is still pinning left, even at lower gain reduction values.
Should I get another pair of 2n3707s in there, or is there some other likely avenue to look into first?

Is the shunt back in the "Normal" position? When you write "pinned to the left" do you mean the needle is moving past the -20dB point? What effect does the front panel zero adjust have? Understanding that the unit is still not calibrated, what do the voltages of the transistors look like in relation to the ones on the schematic with voltages? Most crucial is a ˜0.7 drop between base and emitter.

To answer your question, if you are able to measure the Hfe of your transistors they are unlikely to be faulty.

 

[frederickalonso]

Great thread, thanks

 

[junkface]

Is the shunt back in the "Normal" position? When you write "pinned to the left" do you mean the needle is moving past the -20dB point? What effect does the front panel zero adjust have? Understanding that the unit is still not calibrated, what do the voltages of the transistors look like in relation to the ones on the schematic with voltages? Most crucial is a ˜0.7 drop between base and emitter.

To answer your question, if you are able to measure the Hfe of your transistors they are unlikely to be faulty.

So ya I put the shunt back into normal position after the meter zero step, the front panel zero adjust does it's thing fine in that step with the shunt NOT in normal. The meter then jumps way way left to -20 when I put the shunt back to normal to the point where the front panel zero adjust barely moves it to the right even when it's turned full CW. I can get it centered if I set the tracking adjust full CCW, but testing the gain reduction (with a 10db drop) will pin it to -20 and I'm unable to both center the meter with 0db GR and get a remotely accurate reading of -10db.
Here's some readings:
Q13 E -2.6dcV    C 13dcV  B -2dcV
Q12 E -2.5dcV    C 13dcV  B -1.94dcV
Q11  D 13.75dcV  S  -1.95dcV    G  -1.66dcV
Q1 ,Q2,Q3 all pretty spot on with schematic
Q4  B 1 dcV      C 4.88 dcV    E 0.41 dcV
Q5 B 3.25 dcV    C 24.6 dcV    E 2.7 dcV
Q6 C 27.1dcV  E 2.1dcV
Q7 B 4.4 dcV  C 14.7dcV    E 3.9dcV
Q8 C 29.6dcV  E  14.1dcV
Q9  B 3.4dcV    C 16.9 dcV  E 2.8 dcV
Q10  C 29.6 dcV    E 16.2 dcV
Q14 B 11.4 dcV    E 10.8 dcV

So ya all of these are reasonably in line with the schematic, give or take, and the base to emitter drop is around .6 in every case.

 

[mnats]

So ya I put the shunt back into normal position after the meter zero step, the front panel zero adjust does it's thing fine in that step with the shunt NOT in normal. The meter then jumps way way left to -20 when I put the shunt back to normal to the point where the front panel zero adjust barely moves it to the right even when it's turned full CW.

Remove the shunt then measure R44, the trim pot, while at either extreme.