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

[Echo North]

That's really weird. Changing the AC resistors should have no effect on the DC threshold. The threshold is set by the 5 resistors I mentioned before off the -10V DC rail.  It has no relation to the rest of the circuit.  It's just a simple 5 resistor voltage divider.

Testing ratios on the Rev A is pretty tricky. 1) you have to avoid the knee because that will lower your ratio and 2) there is no source resistor on Q1 in the Rev A, so the FET saturates super quickly which also lowers the ratio. So at the higher ratios there is a very small 4dB sweet spot where you actually see the correct ratio. 

Also the Rev A is quick to saturate, that's it's "charm". It's definitely the most interesting revision because it's so "clippy". There is a reason the only made like 150 of them then redesigned it.

I should get to testing that mod mid week. I'm worried you have something else going on.  Have you test your voltages per the troubleshooting guide?

Send me an email, we're going to switch back to the old TPads we used on the Rev A.  They have a little better bottom range. You can try one of those if you like and report back.

Mike

 

[cdeboda]

That's really weird. Changing the AC resistors should have no effect on the DC threshold. The threshold is set by the 5 resistors I mentioned before off the -10V DC rail.  It has no relation to the rest of the circuit.  It's just a simple 5 resistor voltage divider.

Testing ratios on the Rev A is pretty tricky. 1) you have to avoid the knee because that will lower your ratio and 2) there is no source resistor on Q1 in the Rev A, so the FET saturates super quickly which also lowers the ratio. So at the higher ratios there is a very small 4dB sweet spot where you actually see the correct ratio. 

Also the Rev A is quick to saturate, that's it's "charm". It's definitely the most interesting revision because it's so "clippy". There is a reason the only made like 150 of them then redesigned it.

I should get to testing that mod mid week. I'm worried you have something else going on.  Have you test your voltages per the troubleshooting guide?

Send me an email, we're going to switch back to the old TPads we used on the Rev A.  They have a little better bottom range. You can try one of those if you like and report back.

Mike

I'll shoot you an email about those t-pads, maybe it'll work out so that I don't need the mod as much.

I have gone through the voltages with the guide and don't remember anything crazy, but I'll go through them again  and write them all down. I'll post them after I finish, thanks again.


Edit: Is it okay to go through the troubleshooting guide with the x5 resistors in there or should I swap them back out before?

 

[alex_dempsey]

Absolutely. It's covered in the trouble shooting guide #2 "Amplification Stages".

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

Start from the beginning to get the set-up right. TP1 is directly after the input tx. Note that everything after TP15 is pretty dependent on the output pot tolerance so it can be +/- 20%.  I keep forgetting to add that.

Shoot me an email about that TPad.

Mike

I went through the troubleshooting steps and I have a couple questions. At TP 1, I'm reading .153VAC, is that too much? There isn't a margin of error given and its not super super close so I want to make sure its right. And you said theres about a 20% margin of error for the output transformer reading and I think Im just barely inside that. Im reading about 7.4VAC there. What do you think about those?

Alex

 

[alex_dempsey]

I went through the troubleshooting steps and I have a couple questions. At TP 1, I'm reading .153VAC, is that too much? There isn't a margin of error given and its not super super close so I want to make sure its right. And you said theres about a 20% margin of error for the output transformer reading and I think Im just barely inside that. Im reading about 7.4VAC there. What do you think about those?

Alex

Is it possible there is a capacitor that failed short somewhere and it is letting a lot of signal through the circuit? With my input nob set to just under 48, Im reading about 100mVAC at the output of the box. Then I just blow on the input nob and the voltage reading at the output shoots to 1 or 2 VAC and continues to climb dramatically as you increase the input gain. That sounds to me like a capacitor isn't functioning correctly somewhere.

Alex

 

[saqeef]

Hello, I recently bought a FET/RACK Rev F. I'm wrapped up with all of the soldering, and I'm on the calibration stage. I'm sending a 1Khz signal out of my DAW into the input of the 1176, and have adjusted for it to read .775 VAC  at input + and input -. However, when I check the output +/-, I'm not getting the desired reading. More of something like 0.002-0.003 VAC. I noticed I had put the 2K zero adjust trimmer on the top side instead of the bottom, so I de-soldered it and re-soldered it to the bottom side, hoping that would fix it, but still nothing.

The unit does receive power and will turn on when I engage the GR button, but I can't get that read, and the meter won't move when adjusting the input and output.

One thing I'm not sure about is that I can't really get the desired readings for the cathode (30V DC)and anode (-10V DC). For the cathode, I'm getting something along the lines of 0.004V DC on the cathode, and then nothing too solid on the anode. Surprisingly, however, the unit, as I mentioned before, is receiving power and turns on. Do you have any advice for what the issue might be? Thanks in advance!

 

[Spadehead]

Hey Mike - I've seen the mod you posted a few pages ago to pad the input to the REV A 1176 - I'm taking it that these 4 resistors or the numbered ones on the Ration board, right?

If so is this something that can be done to the older REV D comps too?

I have 1 REV A and 2 REV D and they all operate a around 9am on both input and output - wondering whether this is something you only recommend for the older REV A boards?

Thanks in advance...

 

[compaintdept]

So I just finished putting together a rev A. Not the first project I've ever done but maybe a bit more ambitious than anything I've done before. Anyways, I ran into a problem right off the bat. I was hoping not to have to make a post but I'm really scratching my head here. I'm getting 0.066 VAC at TP1 instead of 0.113 VAC as specified in the troubleshooting guide. Everything seems fine up until the transformer. I think? Am I missing something really dumb?

1k sine wave to the input (0.775 VAC measured at (+) & (-) XLR input)
Input and output halfway @ "24"
Attack off (full CCW) / Release (full CW)
Ratio: 1:20 / Meter @ GR

Measuring w/ chassis as my reference
I'm getting 0.385 VAC going up into the tpad.
The VAC on 9 pads on the tpad read (looking from the bottom):
      7                  8                9 
(0.263)    (0.211)    (0.212)
    4                  5                6
(0.223)    (0.009)    (0.385)
    1                  2                3
(0.383)    (0.329)    (0.208)

On the input transformer I'm getting:
Pin 1 - 0.385
Pin 4 - 0.263
Pin 5 - 0.000
Pin 6 - 0.000
Pin 8 - 0.066
And TP1 is 0.066 VAC

I reflowed the solder, checked that all three L brackets were screwed in tight. There was a post very early on in this thread where someone had a sort of similar problem and I believe Mike had him check the voltage after removing Q1. I tried this too and got the same results. Any thoughts here? Thanks in advance!

 

[IanB]

Hello.
I have 2 Rev A kits that are “finished” but both have problems. I’ll start with 1 then get to the other later:

It’s passing great clean audio (sounds great as a pre; input & output knobs produce healthy gain) but *not compressing.
My attack/release board looks fine.  I guess the attack knob could be faulty/damaged but it looks & clicks on fine etc.

DC voltages are weird at :
Q10 C & Q8 C: at both these pins my multimeter reads: “1”!!
All other DC readings on Q pins in the GR Amp section are perfect.
Please help me what to do next.
Thanks!
Ian

 

[Echo North]

Hello.
I have 2 Rev A kits that are “finished” but both have problems. I’ll start with 1 then get to the other later:

It’s passing great clean audio (sounds great as a pre; input & output knobs produce healthy gain) but *not compressing.
My attack/release board looks fine.  I guess the attack knob could be faulty/damaged but it looks & clicks on fine etc.

DC voltages are weird at :
Q10 C & Q8 C: at both these pins my multimeter reads: “1”!!
All other DC readings on Q pins in the GR Amp section are perfect.
Please help me what to do next.
Thanks!
Ian

You've been through this guide?

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

Check the pad 22, Pad 21, and GR amp output tests.

Mike

 

[IanB]

I have read it and gone through it.
With the build that is passing signal but not compressing:
I’m getting very low ACV (with 0.775VAC input) at TP22 (4:1=0.009, 8:1=0.012, 12:1=0.018, 20:1=0.043 which is about 1/10 of what they should be and the meter is correct........readings are also 1/10 of what they should be for all ratios at the Anode of CR2).
I’m getting correct DC voltages at Q7 - 10 so it must be attack knob/board???
My other Rev A (which isn’t passing signal but I’ll sort that 1 next)  is producing Output-knob-level-responsive hiss & weirdly I can hear it compressing with the attack knob engaged. I’ll try swapping the 2 attack boards and see if that sorts it!
Good plan for tomorrow? Going to bed now.

Re The other build which isn’t passing signal (I’m doing great eh! ): AC voltage is correct at TP1 then very low thereafter so the problem is in the input section after the T-pad and input Tx which is a good thing!). DC is correct at Q4 then low at Q5 (B=1.2, C=0.7, E=0.1). So the problem is somewhere between Q4 & Q5 or maybe Q5 is faulty/fried? I’m going to gut this part of the board. Continuity is fine. Ok plan?

Thank you SO much for your time & help! I kinda feel like I’m close.
Cheers.
Ian

 

[Echo North]

I have read it and gone through it.
With the build that is passing signal but not compressing:
I’m getting very low ACV (with 0.775VAC input) at TP22 (4:1=0.009, 8:1=0.012, 12:1=0.018, 20:1=0.043 which is about 1/10 of what they should be and the meter is correct........readings are also 1/10 of what they should be for all ratios at the Anode of CR2).
I’m getting correct DC voltages at Q7 - 10 so it must be attack knob/board???
My other Rev A (which isn’t passing signal but I’ll sort that 1 next)  is producing Output-knob-level-responsive hiss & weirdly I can hear it compressing with the attack knob engaged. I’ll try swapping the 2 attack boards and see if that sorts it!
Good plan for tomorrow? Going to bed now.

Re The other build which isn’t passing signal (I’m doing great eh! ): AC voltage is correct at TP1 then very low thereafter so the problem is in the input section after the T-pad and input Tx which is a good thing!). DC is correct at Q4 then low at Q5 (B=1.2, C=0.7, E=0.1). So the problem is somewhere between Q4 & Q5 or maybe Q5 is faulty/fried? I’m going to gut this part of the board. Continuity is fine. Ok plan?

Thank you SO much for your time & help! I kinda feel like I’m close.
Cheers.
Ian

If you're not getting significant signal to TP22, but you have the correct signal at TP15, there is an issue between those to points and that is a linear path in the schematic:

http://library.hairballaudio.com/docs/fet_rack_a_doc_v1.12.pdf

If you look at the schematic, all that is between TP15 and TP22 is:

- The relay
- R19,20,21,22, and 78
- The ratio switch
- Ratio switch cable and headers
- The attack pot switch

You've got a bad pad or point in there somewhere.  Continuity testing should help sort that.  Once you get signal to TP22 I think you'll see your signal at the GR Amp output will be fine.

#2 sounds like a good plan.  You can pull the transistors and test them with a diode tester.  Google something like "Testing BJTs with a DMM". 

Mike

 

[IanB]

If you're not getting significant signal to TP22, but you have the correct signal at TP15, there is an issue between those to points and that is a linear path in the schematic:

http://library.hairballaudio.com/docs/fet_rack_a_doc_v1.12.pdf

If you look at the schematic, all that is between TP15 and TP22 is:

- The relay
- R19,20,21,22, and 78
- The ratio switch
- Ratio switch cable and headers
- The attack pot switch

You've got a bad pad or point in there somewhere.  Continuity testing should help sort that.  Once you get signal to TP22 I think you'll see your signal at the GR Amp output will be fine.

#2 sounds like a good plan.  You can pull the transistors and test them with a diode tester.  Google something like "Testing BJTs with a DMM". 

Mike

Thanks so much Mike!
Legend!

 

[masterolsen]

Just finnished my 1176 REV-F build.
Everything went ok but I have one question.
C12 is a ceramic cap and is supposed to be 10pF, my kit packing list it says this correctly but the value is marked as "101" (100pF) capasitor and that`s what i got, so the 100pF is whats in there now. Can anyone tell me what impact that does to the sound?
I have to wait for the New part to arrive.
regards.

 

[Echo North]

Just finnished my 1176 REV-F build.
Everything went ok but I have one question.
C12 is a ceramic cap and is supposed to be 10pF, my kit packing list it says this correctly but the value is marked as "101" (100pF) capasitor and that`s what i got, so the 100pF is whats in there now. Can anyone tell me what impact that does to the sound?
I have to wait for the New part to arrive.
regards.

May not have any, but I would get the proper value in there.  Not sure where it is, but ofter it might just equate to more HF rolloff that may be out of the audible range anyway.

Fill-out a missing parts form to get the right part.

Mike

 

[Harpo]

.. is supposed to be 10pF, my kit packing list it says this correctly but the value is marked as "101" (100pF) capasitor ...

??? ?
101 = 101 = 10 pF.  (102 = 102 = 100pF), so you already got the correct part.

 

[scott2000]

??? ?
101 = 101 = 10 pF.  (102 = 102 = 100pF), so you already got the correct part.

+1

Looking at a bag of them now...

This is a pretty decent reference ......

https://tubeamplifierparts.com/handy-amp-info/capacitor-code-chart.html

 

[Echo North]

+1

Looking at a bag of them now...

This is a pretty decent reference ......

https://tubeamplifierparts.com/handy-amp-info/capacitor-code-chart.html

That's not how these TDK FK series are labeled. Third number is a multiplier like a resistor band.

100 = 10pF
101 = 100pF

Datasheet:
https://www.mouser.com/datasheet/2/400/eadmlcc_conventional_fk_en-844010.pdf

Mike

 

[nu-tra]

Hey DIY People!

I looked for answers on the hairball site and I tried the search function here but couldn't find info for my issue.  I built two hairball Rev D kits with Cinemag CM-3303 (surface mount) input and CM-96731 output transformers. I calibrated both units perfectly. I used a friends neutrik test equipment which I've used many times over the years. When I turn my gain CW from 0 on the input knob the impedance gradually drops  from about 480 to 120 ohm at the end of the turn. It should be cruising with slight fluctuation at around 600 ohms. I tested it against an older UA unit which stayed consistent at around 600 ohms when I turned the input CW. Any thoughts on what I could've done wrong on both Hairball Rev D? i could understand if I messed up somewhere on one but two?

Thank you for your time!

 

[Echo North]

Hey Party People!

I tried the search function but couldn't find info for my issue.  I built two hairball Rev D kits with Cinemag CM-3303 (surface mount) input and CM-96731 output transformers. I calibrated both units perfectly. I used a friends neutrik test equipment which I've used many times over the years. When I turn my gain CW from 0 on the input knob the impedance gradually drops  from about 480 to 120 ohm at the end of the turn. It should be cruising with slight fluctuation at around 600 ohms. I tested it against an older UA unit which stayed consistent at around 600 ohms when I turned the input CW. Any thoughts on what I could've done wrong on both Hairball Rev D? i could understand if I messed up somewhere on one but two?

Thank you for your time!

The original TPad is a very hard design to recreate at a reasonable cost. We use the same Tpad as Purple/Serpent, it's a 3 deck modular Tpad that does a nice job oh getting close to the original specs. We're always re-evaluating it. Inherently , they are imperfect at the extreme ends of the track beyond that normal operational range.

I think we're slowly making the switch back to the Bourns part for this.  It has a more consistent end of track impedance, but is more prone to develop track noise. Everything is compromise.  However, the nominal track impedance will still be lower than you may expect.

Mike

 

[nu-tra]

Can I purchase the bourns and will it work with this board? Which model?  Thanks