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

[wmjohnson]

hi, I'm calibrating my Rev F and I have two questions. in regards to these steps:

"-Now let's set the meter to display this 10dBu drop.  When the attack is "off" (full CCW) and you see +10dBu at the output, set the meter to 0 by using the front panel zero adjust trimmer.

-When the attack control is "on" (full CW) and you see 0dBu at the output, use R44 to set the meter to -10."

In my case with the Rev F.... is the "zero adjust trimmer = to the Tracking Adj trimmer on my main pcb?
also, what would R44 be equivalent to on the Rev F? Because for me R44 is a regular, non-variable resistor on the meter pcb.
thanks

The zero adjust is the trimmer on the front panel and the Tracking adjust is the tracking adjust. There's no equivalent to R44 because the meter circuit is completely different. As I understand it, you can just skip the meter null calibration step.

 

[Echo North]

Update:

figured out I can pull q11 and recalibrate the Q Bias and it seems to behave like it should. the control voltage doesn't move like crazy when i move the release knob at least.

ordered a few more 5457s a few days ago, hopefully swapping one in will get the meter to work right, otherwise would anyone have any ideas about what might be going on in that section? seems like it's draining more current to drive the gr meter than it should be?

Hmmm. Not sure.

I would start with that FET replacement. If that doesn't fix it, we can move on from there.

Mike

 

[Echo North]

The zero adjust is the trimmer on the front panel and the Tracking adjust is the tracking adjust. There's no equivalent to R44 because the meter circuit is completely different. As I understand it, you can just skip the meter null calibration step.

Correct.

 

[wmjohnson]

I would start with that FET replacement. If that doesn't fix it, we can move on from there.

FETs came in today. went through recalibration and no cigar. It works fine whenever the release knob is full CW, but then the CV drops whenever I rotate it CCW.


This wouldn't be true to the original, but i could raise that resistor value and recalibrate, see if that keeps the voltage from falling?

 

[aste]

Hi, all.

I'm building the Rev A and I've managed to build and test the power supply, yay! I'm ready to move on to populating the PCB, but I'm having a tough time finding details on what value resistors, caps, etc go where. Is there a schematic that everyone follows, or do you just try to copy what is in the pictures in the build docs?

Thanks,
Andrew

 

[wmjohnson]

Hi, all.

I'm building the Rev A and I've managed to build and test the power supply, yay! I'm ready to move on to populating the PCB, but I'm having a tough time finding details on what value resistors, caps, etc go where. Is there a schematic that everyone follows, or do you just try to copy what is in the pictures in the build docs?

Thanks,
Andrew

There was a comprehensive parts list with values and part #s in my kits, and that's what I used to stuff the boards. It was inside the "Rev X Parts" box.

 

[aste]

There was a comprehensive parts list with values and part #s in my kits, and that's what I used to stuff the boards. It was inside the "Rev X Parts" box.

Perfect. Thanks!

 

[Echo North]

FETs came in today. went through recalibration and no cigar. It works fine whenever the release knob is full CW, but then the CV drops whenever I rotate it CCW.


This wouldn't be true to the original, but i could raise that resistor value and recalibrate, see if that keeps the voltage from falling?

If it's falling you have an issue somewhere. I would not alter components.

How much is your meter fluctuating?

Mike

 

[saqeef]

We'll let's start with your power supply.  Did it originally test fine? meaning after you stuffed the power supply and before you stuffed the rest of the PCB.

Mike

To be quite honest, I think I actually skipped testing just the power supply before stuffing the rest of the PCB. I was overeager.

EDIT: Having thought about it, I do remember stuffing the power supply, then attaching the PCB to the chassis before stuffing the rest of the board as instructed in order to test the power supply, but I really don't remember what the results were. Having said that, what's the next step?

 

[wmjohnson]

If it's falling you have an issue somewhere. I would not alter components.

How much is your meter fluctuating?

Mike

* I guess technically it's rising -- it gets closer to 0v.

By the time the release knob is at "5" the meter is pinning left and and I'm dropping a ton of signal.

 

[Echo North]

* I guess technically it's rising -- it gets closer to 0v.

By the time the release knob is at "5" the meter is pinning left and and I'm dropping a ton of signal.

Stumped on that one.

So as you turn the release how much is the pad 18 DCV changing?

I wonder if you have a fried FDH333.

Mike

 

[Echo North]

EDIT: Having thought about it, I do remember stuffing the power supply, then attaching the PCB to the chassis before stuffing the rest of the board as instructed in order to test the power supply, but I really don't remember what the results were. Having said that, what's the next step?

How do you know it's powered?  Is the meter lighting?

 

[wmjohnson]

So as you turn the release how much is the pad 18 DCV changing?

Pad 18 doesn't change at all, only pads 19 and 7.

 

[Shattersignal]

Resuming conversation from the Rev. D thread:

<Quote from: Hairball Audio on March 04, 2016, 12:51:59 PM>
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 <endquote>

Hi there!  I'm helping a buddy with a Rev A (FET RACK v1.12) 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]

Resuming conversation from the Rev. D thread:

<Quote from: Hairball Audio on March 04, 2016, 12:51:59 PM>
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 <endquote>

Hi there!  I'm helping a buddy with a Rev A (FET RACK v1.12) 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!!

You have one of the new Rev A PCBs. They do not QBias down to 0. Have a look here:

http://www.hairballaudio.com/blog/resources/post/fetrack-troubleshooting-guide

Pad 18 should have a bias range of -1 VDC to -1.78 VDC.  If you are not getting this range you could:

- Be measuring incorrectly.
- Wrong value at R60 or R83
- Wrong value at R61 or R64 or R58

Mike

 

[Echo North]

Pad 18 doesn't change at all, only pads 19 and 7.

Does the -DC at pad 21 change?

 

[Shattersignal]

Pad 18 should have a bias range of -1 VDC to -1.78 VDC.  If you are not getting this range you could:

- Be measuring incorrectly.
- Wrong value at R60 or R83
- Wrong value at R61 or R64 or R58

Mike

Thanks, Mike.  I'll have him double check resistor values at those locations. 

 

[wmjohnson]

Does the -DC at pad 21 change?

No, not when i rotate the release.

Only by switching ratios. At 20:1 it's sitting at -6.6vdc.

 

[saqeef]

How do you know it's powered?  Is the meter lighting?

Yes, the meter does light up, and the needle bounces from left to right, then settles back to the left.

 

[Echo North]

Yes, the meter does light up, and the needle bounces from left to right, then settles back to the left.

Are you sure you are measuring the supply test points correctly?  Measuring for DC V and using the chassis as your common? Also make sure that your PCB is mounted to the chassis with ALL 3 L brackets.

Mike