[BUILD] CAPI FC526~500 Series~FET Limiter Kit~Official Support Thread

[jsteiger]

.....Sending out a 0db signal with 0db=-13dbfs gives me the .775 reading, that's what I want right? So I want -30dbu below that being -43dbfs correct? I can do the rest of the math myself just wanted to make sure thats good!

Yes this sounds correct.

 

[jsteiger]

Just checking in, little confused and wanna get these babies all ready for a session tomorrow 8)

I am swamped packing orders today.

I read your post twice and I'm not sure I follow what you are saying. You need to have 7dB of gain reduction and if you are not close with the pot settings I have specified...then something else is messed up. I just use "+7dBu to 0dBu" because its easy. Make sure you have the FET biased properly for the 1dB of loss.

 

[jsteiger]

Sorry jeff, pack away get to me when you can no problem I completely understand you're one busy man!

I guess I'm just confused as to what is 7db of gain reduction for step 9 of the Meter calibration:

With the input at 10ish and output at 1ish I get  the signal to exactly  +7dbu  but  when I flip the GR toggle in step 6 of the gain reduction meter calibration , I get no difference of output level. Is this correct or should there be a difference?

You've gotten dyslexic with the controls. If you look again at step #4 and then #11 under the **Gain Reduction Meter Calibration** section, you'll see that the input should be around 1:00 and the output around 10:00. If you are at these positions and truly have a +7dBu signal at the input and don't have close to 7dB of gain reduction, then something else on your build is jacked up.

 

[geardude]

You've gotten dyslexic with the controls. If you look again at step #4 and then #11 under the **Gain Reduction Meter Calibration** section, you'll see that the input should be around 1:00 and the output around 10:00. If you are at these positions and truly have a +7dBu signal at the input and don't have close to 7dB of gain reduction, then something else on your build is jacked up.

Jeff,what a wizard you are! I did switch that input output...ugh this is what happens when I'm walking the plank with math and reread everything a million times  ;D ;D ...GOT THAT DIALED in and the rest of the calibration was a breeze. THANKS for popping my calibration cherry and helping me out. TREMENDOUS!

 

[jsteiger]

Jeff,what a wizard you are! I did switch that input output...ugh this is what happens when I'm walking the plank with math and reread everything a million times  ;D ;D ...GOT THAT DIALED in and the rest of the calibration was a breeze. THANKS for popping my calibration cherry and helping me out. TREMENDOUS!

Yeah glad you got there. It is really super easy but folks hear the word "calibration" and get all freaked out!

 

[Bowener]

After building nearly a dozen CAPI kits for myself and others, I find my self stumped for the first time.    My FC 526 build looks good to me.  Clean.  No solder bridges.  It passes audio just fine in bypass mode, but when I engage the compressor, everything sounds like its passing through a high pass filter.  The bottom end just drops out, and the overall level drops, regardless of whether GR is on or off.  The sound is not distorted, just lacking in bottom end and gain.  I've gone over the PC board very methodically with the BOM and check all the resistors.  They appear to be in all the right places.  All the diodes are correctly oriented.  It's not the opamp, as the same thing happens with my pre-built Red Dots (I'm using the gar 2520s that I built from a kit.)  Jeff suggested that I post on this forum instead of messaging him privately, so here I am.  Could I have a faulty FET?  I had that happen in a Hairball Lola kit I built, and replacing the FET totally fixed the low gain/distortion issue.  Faulty relay?  If anyone has any ideas, that would be most helpful.    Many thanks in advance.

 

[jsteiger]

After building nearly a dozen CAPI kits for myself and others, I find my self stumped for the first time.    My FC 526 build looks good to me.  Clean.  No solder bridges.  It passes audio just fine in bypass mode, but when I engage the compressor, everything sounds like its passing through a high pass filter.  The bottom end just drops out, and the overall level drops, regardless of whether GR is on or off.  The sound is not distorted, just lacking in bottom end and gain.  I've gone over the PC board very methodically with the BOM and check all the resistors.  They appear to be in all the right places.  All the diodes are correctly oriented.  It's not the opamp, as the same thing happens with my pre-built Red Dots (I'm using the gar 2520s that I built from a kit.)  Jeff suggested that I post on this forum instead of messaging him privately, so here I am.  Could I have a faulty FET?  I had that happen in a Hairball Lola kit I built, and replacing the FET totally fixed the low gain/distortion issue.  Faulty relay?  If anyone has any ideas, that would be most helpful.    Many thanks in advance.

My first guess would be something is wrong in the audio path. I suggest injecting signal and following it thru the circuit via the Test Points doc. We need to narrow down where things go south or we're simply shooting in the dark.

It most likely won't be the FET. Each and every VVR or MTR FET is tested/graded in an actual FC526 circuit. We monitor freq response and THD to make sure there are no weird issues.

 

[Bowener]

So here's my measurements of the audio path:

with .0258 VAC appearing at the input:

TP1 - 5.7  mVAC
TP2 - 110.3 mVAC
TP3 - 882.3 mVAC

The first tow seem in the zone, but the last measurement seems low to me.

All measurements take  with
input - full CW
output - full CW
attack - full CW
release - full CW
switched to Active
GR on

 

[jsteiger]

I would dbl check the output transformer wiring especially if you are using the Litz version.

 

[Bowener]

I would dbl check the output transformer wiring especially if you are using the Litz version.

Thanks Jeff.  I'm pretty sure I ordered the EA version.  My invoice says "2 x FC526-XFMR Entire Bundle".  The transformer says "2623-1      605-1634" on it.  Is that the Ed Anderson, or Litz?  From the first set of numbers, it looks like an EA transformer.  If that number is an EA version, then I can confirm that the colour codes of the wiring are correct, and that the solder joints look good.

 

[jsteiger]

OK, to quickly rule out the output transformer, desolder the Red lead from the PCB and see if your TP3 value changes, following the same procedures as you did in your previous audio path test.

I forgot to ask, have you turned the Q_BIAS trimmer CW so no level changes are seen? Take it out of its conductive range.

 

[Bowener]

OK, to quickly rule out the output transformer, desolder the Red lead from the PCB and see if your TP3 value changes, following the same procedures as you did in your previous audio path test.

I forgot to ask, have you turned the Q_BIAS trimmer CW so no level changes are seen? Take it out of its conductive range.

  I did forget to readjust the QBIAS full CW.

Bit of a face palm moment I'm afraid.  I've spent a few days agonizing over this, and it turns out that the issue was a faulty slot in my patchbay!  I moved the test jug over one slot to the right, and all was well!  It's now calibrated and sounding glorious with all the bottom end and gain a fellow could ask for!  On the one hand, I'm glad that my assembly skills were up to the task, but on the other hand, I must genuinely apologize for taking up your precious time.  These things are slamming!  Awesome sound, and well worth the effort!    Now to dive into that patchbay...

 

[jsteiger]

  I did forget to readjust the QBIAS full CW.

Here are the revised readings.  De-soldering the red wire does not change TP 3

TP 1 - 5.7mVAC
TP2 - 126.1 mVAC
TP3 - 1.0089 VAC

OK those readings look more on target. You can monitor the signal unbalanced by leaving the Red output transformer lead desoldered and taking the signal from the Red solder pad. See if that is full range.

 

[Bowener]

OK those readings look more on target. You can monitor the signal unbalanced by leaving the Red output transformer lead desoldered and taking the signal from the Red solder pad. See if that is full range.

  As I mentioned in my modified post above, it turns out that the Comp is fine.  It was a faulty patchbay slot!  I changed slots on a hunch and was rewarded with tones of bottom end and gain.  It's all calibrated and groovy now.  See my profuse apology for taking up your time above!  Thanks again, Jeff.  Your modules are awesome and your support has been fantastic!

 

[jsteiger]

  As I mentioned in my modified post above, it turns out that the Comp is fine.  It was a faulty patchbay slot!  I changed slots on a hunch and was rewarded with tones of bottom end and gain.  It's all calibrated and groovy now.  See my profuse apology for taking up your time above!  Thanks again, Jeff.  Your modules are awesome and your support has been fantastic!

Ah, good news. I typically don't read modified posts only new ones when I get the email notification.

 

[alocaster]

Jeff, I have a recent build that appears correct in components and solder. Alas I have some audio problem giving me 2.5mv at tp 1. On an O-scope it looks more like noise than signal.

thanks
rob

 

[jsteiger]

Jeff, I have a recent build that appears correct in components and solder. Alas I have some audio problem giving me 2.5mv at tp 1. On an O-scope it looks more like noise than signal.

Rob, TP1 is directly after the discrete follower which follows the input transformer. The only thing before that are the bypass relays.

Check the bottom of R71 for input + signal and bottom of R73 for input -. If you don't have audio there, your relays are not switching over properly.

If that is good, check the top of R42 which is immediately following the 2622's secondary.

If that is good then I would look at swapped transistors or something for Q1 and Q2.

 

[alocaster]

Jeff following your guidance i traced the signal to q1 and q2. This is where it dies. The orientation of the transistors appears correct. I have made it no further yet. Do you have q1 and q2 voltages. Thank you.

 

[jsteiger]

Jeff following your guidance i traced the signal to q1 and q2. This is where it dies. The orientation of the transistors appears correct. I have made it no further yet. Do you have q1 and q2 voltages. Thank you.

Sorry I missed this.

Q1, BC560C:
C -15.43
B  -.588
E .046

Q2, BC550C:
C 15.14
B 0
E -.589

 

[alocaster]

After installing the new pots post drop test. I re checked the audio path beginning with the transistors. The values you provided were close to my build. I then went back to the primary side of the transformer and found the signal not what it should be it appeared as if the transformer was loading the ckt. I reflowed the secondary side where there was almost no signal. No improvement. I then removed the transformer after which pin one had the correct amplitude. I hoped to check the transformer statically from the CA2622 schematic. I found 68 and 70 ohms on each of the primaries and 1.59K on the secondary. Should these values be representative of the listed 150/600 and 10Kohm printed. Thanks for the help.