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

[jsteiger]

My 10M (R5) and 3.9M (R20) are measuring below 2.8M and readings never seem to settle on a number when measuring with multimeter (Fluke 179).

I'm tempted to think the problem is the Fluke 179 approaching its max because it seems unlikely that the 2 highest resistors are the only ones not reading accurately, but I checked online and it said that the 179 goes up to 50M.

Should I buy a new one of each, or what?
I don't have any spare resistors to use in their places.

The parts we supplied are correct. Maybe its your DMM's battery?

 

[Deadly Mix]

The parts we supplied are correct. Maybe its your DMM's battery?

I haven't encountered that before. Are you saying multimeters have trouble measuring high resistance as their batteries wear?
They are the correct resistors, according to the stripes. It's just the measurements that are out of the margin of error.

 

[jsteiger]

I haven't encountered that before. Are you saying multimeters have trouble measuring high resistance as their batteries wear?
They are the correct resistors, according to the stripes. It's just the measurements that are out of the margin of error.

Like I said, I know the parts are good. I would suggest a fresh battery and see what happens. I just measured a bunch of parts and they are will within spec. Both of these are on reels so I suspect from the same batch.

 

[mcrowe]

OK, time to cast my lot in here. I'm attempting to go through the calibration process. Last night I was having a hard time getting readings consistently, so I figured it was cold solder joints somewhere. Touched that up, and now during the Q-bias setting process, I'm getting a little more output than expected (1.97 vAC/8.11 dbU). I'm also getting a quiet but audible hum when no test tone is applied, and I wonder whether that raised noise floor is contributing to my readings. These are my first three test point readings:

TP1: 6mV (nominal)
TP2: 121mV (a hair high)
TP3: 984mV (also a bit high)

Also, my TP9 is bang on at -10V, and I have the expected +/-15V on A3. I tested that 0252 in another module and it sounds fine there.

Is this device particularly sensitive to flux residue? I've cleaned it, but I don't feel like the flux remover I have is great. Tends to leave things kinda sticky. If not that, any other ideas where a hum might be coming from?

Also, for the benefit of posterity: I made a big mistake initially in this build, which was that I didn't notice that RV1 and RV2 aren't interchangeable, despite the fact that they have the same value. I flipped them, so neither trimmer was accessible. With difficulty, I managed to extract them and swap them. By turning RV1 as far clockwise as it will go, I can stop the meter lights from being illuminated, which I'll take as a sign that the part still works (I also measured them both before putting them back in, seemed fine). Anyway...don't be like me and do that if you're looking to build one of these.

 

[mcrowe]

Here's my line of thinking at the moment: when the compressor is active but GR is off, the audio path is something along the lines of input transformer > A2 > A3 > output transformer (kinda educated guessing here). Since I know my A3 to be good, the question is whether I can detect the noise at TP1 or TP2. If I detect it at TP2 but not TP1, it suggests DTO5 as the likely trouble spot? I won't have time to play with this until later today or this weekend, but that's where my head is at right now.

 

[mcrowe]

Maybe I'm off base in thinking there's a problem here at all. I forgot that the input and output are both cranked to the gills. I put my Hairball FET/500 rev A in the rack and cranked its ins and outs all the way, and I'm getting about the same level of self-noise. Does a noise floor of about -47dBFS seem about right? Anyway, I guess I'll continue with the calibration procedure and see where I end up.

Edit: yeah, I think I was just psyching myself out about my measurements being a little higher than the guide says. I got it calibrated with no extra trouble and it's sounding great. Especially that ABI mode.

 

[museric]

Maybe I'm off base in thinking there's a problem here at all. I forgot that the input and output are both cranked to the gills. I put my Hairball FET/500 rev A in the rack and cranked its ins and outs all the way, and I'm getting about the same level of self-noise. Does a noise floor of about -47dBFS seem about right? Anyway, I guess I'll continue with the calibration procedure and see where I end up.

Edit: yeah, I think I was just psyching myself out about my measurements being a little higher than the guide says. I got it calibrated with no extra trouble and it's sounding great. Especially that ABI mode.

Good to know! I have a pair of these in the box awaiting the soldering iron. I foresee much sonic tastiness in the results when assembled.

 

[mcrowe]

Good to know! I have a pair of these in the box awaiting the soldering iron. I foresee much sonic tastiness in the results when assembled.

Watch those 2K trimmer pots. Put the front-facing one in the right spot, or you'll be cursing yourself. Also don't forget the DTO5...I got confused initially because the available build guide is for the old version that used three DOAs, and I also ordered a 553F at the same time and assumed it belonged to that, so I was confused why I had zero output until I looked at photos of some other builds and saw I was missing a crucial part. And obviously, triple-check resistor values and orientation for polarized components, yada yada.

The 526 is much more transparent sounding than my FET/500 rev A, which has sort of a crunchy vibe as part of the deal. Should be useful when I don't want to hear the compressor working.

 

[museric]

Watch those 2K trimmer pots. Put the front-facing one in the right spot, or you'll be cursing yourself. Also don't forget the DTO5...I got confused initially because the available build guide is for the old version that used three DOAs, and I also ordered a 553F at the same time and assumed it belonged to that, so I was confused why I had zero output until I looked at photos of some other builds and saw I was missing a crucial part. And obviously, triple-check resistor values and orientation for polarized components, yada yada.

The 526 is much more transparent sounding than my FET/500 rev A, which has sort of a crunchy vibe as part of the deal. Should be useful when I don't want to hear the compressor working.

Got it - thank you for the tips. I’ve a pretty strict setup method I use to verify parts (esp for complex kits), so I’ll make sure to stick to that, and pay close attention to said items.

 

[Toral]

Hello friends, I hope you are doing well. I'm new here, but I can see this is a great community.

A couple of years ago, I bought 2 FC526-XFMR for my dad to assemble. He did so, but before calibrating them, he passed away due to COVID-19. I don't want to talk about the sadness of this... I was left with two uncalibrated compressors, as I don't know much about electronics. I'm a musician, I've been studying a lot and have already achieved some things, but there are two issues I can't resolve. I know they're silly for those who know, and I apologize, but I can't find this information anywhere...

In the review sheet, in the "Gain Reduction Meter Calibration" section, point 4 says "Turn the input pot up to approximately 1:00". So, What is 1:00? Let's say if I put the potentiometer fully CCW and move it CW to the first painted dot, is that what it refers to as 1:00? If I do this and move the GR Switch, I don't have any gain reduction. But if I put the potentiometer, let's say, past the middle of the potentiometer, where on a wrist clock it would be 1:00 PM, I do have gain reduction when activating the switch. I'm adding a photo where I left the input and output potentiometers where I more or less get the desired reduction (-7dB). Could you tell me if this is correct?

Also, in the "Pre Calibration Information" section, point 2 says "2. Before proceeding forward with the meter calibration steps below, make sure you have between +2.498V DC and +2.502V DC at the "top" of the meter resistor string for the LM339 comparators, which is MTR point #11 in the pic. See the Test Points Guide for more info."

To check #11, do I put the multimeter on one side to ground (as when checking test points) and the other on #11 as in the second photo I added? This way, I'm getting 2.46 volts. Is this okay, or is something wrong?

I greatly appreciate your time. This was long; I wanted to be as clear as possible. Greetings and have a good weekend!

 

[Toral]

Hi there! I hope this time, someone can help me.

I'm having problems calibrating a pair of fc526-xfmr. My dad built them a couple of years ago and unfortunately, my father passed away from COVID and never calibrated them as I mentioned on my last post ( never get a reply...)

They were never used and now I'm trying to do calibrate.

First, I connected it and moved the Q-bias CW to leave it open, put a signal in the input (-30dBu 1khz sine wave = 24.50 mV) and it gave me 7.7 dBu (1.8800 volts). Everything seems correct. The test points seem correct:
TP1: 5.92 mV
TP2: 119.7 mV
TP3: 0.950 V
TP9: -10.01 V
2.508 V at top of the meter resistor (11)

Then, I increase my generator output (DAW) until it reaches 8dBu (1.9456 V), I start lowering the q-bias CCW and leave it at -7dBu (1.7341 volts) but I notice that after a while, this value slowly decreases.... I readjust it and continue with the LED calibration, everything seems correct, when activating the GR it drops 7 decibels (in the DAW I put it in an input of my interface and it gave -1.2 db and when triggering the GR switch it gives me -8.2) everything seems perfect.... TP7 almost matches what's written on the FET bag: TP7: -1.48
FET bag: -1.18

read in the fc526 build forum that you mention this should be between -1.4 and -1.7 so everything seems correct.

I decided to leave it connected with the signal and this is where the problem is with both - the voltage keeps dropping. I left it connected for 8 hours, thought it was a matter of stabilizing but no. I kept testing for 4 or 5 days leaving them connected for more than 11 hours, one with signal and one without signal but turned on. Two days ago I recalibrated it after leaving it on for 9 hours and I've taken measurements for two consecutive days and it's more or less the same always on all days (the output voltage drops more or less the same). This is what happens, on one side it shows the volts it's giving me at the fc526 output, injecting 1khz @ -35.6 db (this is what gave me 8dBu at the fc526 output when I calibrated it and set the q-bias to 7dbu, that is, at 1.7341 volts) and this is the result by hours of the day, two days after doing the second calibration:

Volts: Time:
1.8010 - 10:00 am
1.7535 - 12:00 pm
1.7483 - 12:30 pm
1.7464 - 1:00 pm
1.7440 - 2:00 pm
1.7400 - 2:30 pm
1.7380 - 3:00 pm
1.7365 - 4:00 pm
1.7342 - 7:00 pm
1.7371 - 8:00 pm

This is yesterday's result:

Volts Time:
1.7940 - 9:30 am
1.7610 - 10:00 am
1.7490 - 12:00 pm
1.7390 - 2:00 pm
1.7310 - 3:00 pm
1.7260 - 5:00 pm
1.7264 - 6:00 pm
1.7280 - 8:00 pm
1.7310 - 9:00pm
1.7350 - 11:00 pm


It's worth mentioning that the measurement moves slightly in small circles cycles, first slowly increasing and then slowly decreasing. I tried connecting the rack directly to power and also through a power conditioner, and the results are similar.

What I'm using is an empty capi rack 511 (only the two modules connected and tested in different slots) with original CAPI power supply, my beloved Unit-T 117c multimeter capable of measuring 1khz with 60,000 counts.

Is this behavior normal? If it's correct, when do I do the calibration? Because when I leave it fine after 8 hours of being on, it's perfect, but the next day when turning it on again, no LED lights up and the difference is quite large. Am I doing something wrong?

My measurements are sending the sound wave from the DAW, from there it goes out of my interface and I measure it by connecting pins 2 and 3 to my multimeter, then I connect it to the fc526, and from the fc526 output I measure with the multimeter again (in my two fc526s, when putting them in bypass, it's exactly the same volts as those from the interface output). And I always check the volts from the interface output before measuring the fc526 output volts to make sure I'm always injecting the same level.

I hope anyone can help me, I'm desperate already, it took me a long time to figure out how to do the calibration and even longer testing, kind regards.

 

[jsteiger]

I would guess it just normal heat and FET drift especially since you say it happens with both units. The front panel hole to adjust the meter zero is there to compensate for the meter drift. That part of the circuit is very 1176-ish. Those meters rarely sit solid on zero. I would imagine that if the unit is left on longterm it would stay around the 1.73 you mention.

 

[Toral]

I would guess it just normal heat and FET drift especially since you say it happens with both units. The front panel hole to adjust the meter zero is there to compensate for the meter drift. That part of the circuit is very 1176-ish. Those meters rarely sit solid on zero. I would imagine that if the unit is left on longterm it would stay around the 1.73 you mention.

Hello Jeff! Thank you so much for your response; I really appreciate your time. So, is this normal? It makes me very happy to know that they are working correctly . I’d like my dad to know this too. Thank you very much! Big hug and have a great week.