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

[JeromeMason]

First off, it would be helpful to know if the opamps are new builds and/or if they have been tested in something else. Starting up with 3 freshly built opamps in a freshly built FC526 is a definite no-no! So please, anyone posting for help, let us know those details first.

Having an issue here, when the comp is engaged getting a very thin sound, almost like a HPF as been rolled up to 400hz. Could this be in the Q-bias setup? I've always turned my trim pots CCW before I adjust, did you have this preset or anything?

None of the trim pots should be turned before it is specified in the calibration doc. They are all in their factory set position which is somewhere near 50%. Turning them all CCW is for sure not a good way to start especially if you look at step 14 of the calibration doc which states that the Q-Bias trimmer should be turned CW. I also have it bold and underlined in the doc. 

I would recommend to turn GR off, monitor the output level, turn the Q-Bias trimmer CW until the level stops increasing. This will take the VVR FET out of its conductive range. At this point, measure the freq response to see if it is high passed still. If so, you have a problem with the audio path somewhere. We'll go down that path if and when we need to.

PSA, when adjusting the Q-Bias trimmer, turning it CW will let the signal level increase. Turning it CCW will essentially turn the signal level down. Kinda reacts like turning a volume knob, in a logical fashion. Hope that makes sense. It's opposite of an 1176 so keep that in mind, or just follow my calibration doc to the letter and you'll be fine.

This is good info to have, some builds require that you zero out trimmers, yours did not State that that's my fault, but at least it's on record and No one else will make that mistake. As for the problem I was having I don't believe it was anything associated for The actual compressor, I believe my problem had to do with the routing in my interfaces D a W. The issue right now is the calibration. The compressor seems a bit off, the transients are not as transparent as I remember your kits Jeff. It also seems a little spongy and I'm certain it's in the calibration. The compressor still does have the effect of making the source seem larger and fuller. Once I've done the proper calibration with a reliable scope I will report back as I'm sure that's the issue.

Thanks for the help and response Jeff.

Jerome

 

[jsteiger]

Adding the 10R resister didn't work 

Can you post a pic of the relay area of the build?

 

[jsteiger]

.....The issue right now is the calibration. The compressor seems a bit off, the transients are not as transparent as I remember your kits Jeff. It also seems a little spongy and I'm certain it's in the calibration.....

Well, "calibration" does encompass a few steps but to me the term (and the steps in the guide) mainly refer to the meter alignment.

There are 3 basic steps to the FC526 calibration and only the simplest two to carry out will effect the audio path and "sound" of the unit.

1. Q_Bias of the FET. Pretty simple and straight forward, Just make sure your FET is biased to be reducing the audio path by 0.5dB.

2. THD Null. Will also effect the audio path and sound of the module. Fairly easy to do.

3. Meter. This is the hardest and most finicky thing to adjust or calibrate but will not effect the sound or audio path of the module whatsoever.

 

[rabe]

Adding the 10R resister didn't work 

Can you post a pic of the relay area of the build?

Here is a pic of the relay area.
Your support is greatly appreciated.

 

[jsteiger]

Adding the 10R resister didn't work 

Can you post a pic of the relay area of the build?

Here is a pic of the relay area.
Your support is greatly appreciated.

The transistor by the relays is in backwards.

 

[rabe]

Adding the 10R resister didn't work 

Can you post a pic of the relay area of the build?

Here is a pic of the relay area.
Your support is greatly appreciated.

The transistor by the relays is in backwards.

Thanks!! need to get my eye glass prescription checked

 

[dandeurloo]

This may help a few guys, maybe not.  Be sure to check you voltage at TP9.  It should be very close(.01%) to -10vdc if not exactly.  If it is off then you have a problem with the voltage and it will make it impossible to calibrate your comps.  It took me quite awhile to figure out (with the help of Jeff) that my rails on my 51x  PSU had been sagging to low causing the voltage into the modules to be to low to properly get my -10VDC causing meter calibration issues.  So, just be sure you have -10VDC there and save yourselves a lot of headache in case your PSU is drifting or sagging like mine was.

 

[emrr]

I'm gonna bet this is inline with Dan's problem above:

Two FC526's with same status.
Meter alignment step 5) should be nothing but the green LED lit, I have -2 (3 yellow) lit, true for all but ABI ratio which is none lit.
TP9 -9.85VDC, op amps +/-15.2VDC,  connector +/-16VDC.
15 modules powered.
Remove 11 modules from load (3A PSU), no obvious VDC changes anywhere but meter then lights to -7 (all but 2 LED’s). 

Late, gotta stop for the night, assume PSU needs to be trimmed higher.  I thought +/-16VDC was the standard at the connector, but the first power check here looks for +/-16VDC at the op amp pins. 

Thoughts?

-----EDIT----

problem with the rack itself:
http://groupdiy.com/index.php?topic=59050.msg756890#msg756890

 

[jsteiger]

TP9 needs to be -10V give or take .05V. The precision shunt regulator will snap right to target when the supply voltage is correct. I have tested here and found that the supply to the card edge needs to be no less than +/-15.7V. Sounds like your findings are a little different?

 

[emrr]

I have +/-16 at the card edge.  I'm 0.1V-0.15V lower than expected range at TP9. 

I don't know if it's a clue of not, but the load change affecting the (incorrect) meter lighting seems to indicate....something.  I ran out of time and did not re-run the meter alignment with lesser load, so can't speak to any further indications there.  Wondering if the PSU is on the verge of current limiting, shouldn't be.  Not sure what total load really is, not sure how PSU derates when turned up to +/-16V when A rating is for +/-15V.  I'll look at the load, and maybe try at +/-16.2ish if all else fails. 

They sound good!

 

[jsteiger]

Well, I may look into changing the current limit R for the -10V rail. I thought I had it set for +/-15V at the card edge but recent tests tell me I must have changed it. That was done last fall so I don't recall. The meter comparator string is based on a 2.5V precision reference shunt so the V at the top of the meter R string should be a solid 2.5V. If that is varying for some odd reason, the meter will be very unpredictable. Both the 2.5V R string for the meter and the -10V rail draw very little current on their own.

 

[djroshi]

I'm having issues with the meter LEDs not illuminating. Only the green one lights up. I've checked the orientation of the LEDs of course.

BTW I'm getting -9.88V at TP9.

 

[jsteiger]

I'm having issues with the meter LEDs not illuminating. Only the green one lights up. I've checked the orientation of the LEDs of course.

BTW I'm getting -9.88V at TP9.

That is too low. What is the supply V at the gold finger side of the 1N4004 protection diodes?

The GR LED's are in a string from the green one down to the orange. If one of them is in backwards, that one and all that come after it will not illuminate.

 

[djroshi]

That is too low. What is the supply V at the gold finger side of the 1N4004 protection diodes?

The GR LED's are in a string from the green one down to the orange. If one of them is in backwards, that one and all that come after it will not illuminate.

15.97V and -15.98V respectively.

The LEDs are installed correctly as far as I can tell, e.g. looking from the top in can see the green led cathode, the first yellow anode, the second yellow cathode, alternating as such all the way down to the last orange cathode. I have also checked with my DMM that e.g the green led anode lead is connected to the first yellow cathode lead, that the first yellow anode lead is connected to the second yellow cathode lead, etc right the way down to rule out any problematic solder joints.

 

[dandeurloo]

Guys, I had + & - 15.9 volts in my PSU.  By the time it got into the unit it was down almost a volt and that threw everything off.  If you are building on a test PSU crank the voltage up to + & - 16.9 and see if that solves everything.  Once I did that I had a perfect -10VDC and everything was happy.  It took a ton of time and head scratching to get that figured out.  Hopefully this helps you guys and we can get Jeff some more info to work with.

BTW, I have been using mine on some vocal tracking session into my LA 2A.  They sound good!

 

[daveriot]

any idea why r50 would smoke on an f526?

 

[emrr]

any idea why r50 would smoke on an f526?

From value and position it would appear to be power isolation/filter to the preamp DOA, so probably a short around that DOA.  Just a guess.

 

[jsteiger]

any idea why r50 would smoke on an f526?

As Doug mentioned, R50 is the damping R for the -V supply to opamp A3. That would indicate a serious problem or short on that opamp. What amp is in there?

 

[emrr]

adjusted PSU to +/-16.14VDC, TP9 -9.93. Not there yet.
adjusted PSU to +/-16.27VDC, TP9 -9.98.  Looks good to go.

TP10 +2.487V, no probe = 2 yellow LED’s lit.
engage GR, TP10 +2.196V, 7dB actual reduction, meters 10dB reduction.

If I then adjust TP10 for +2.245V, disengage GR and remove probe I have only the green LED lit.  TP10 then +2.561V.
Readjust RV1 for +2.487 with no GR, two yellow LED’s again lit. 


Reran entire alignment, same result, only difference being Meter Step 7: TP10 +2.18V.


adjusted PSU to +/-16.37VDC (max), TP9 -9.98 (no change), metering no change.

 

[jsteiger]

Doug, I am in the midst of collecting some benchmark test data from a finished unit here. I still have a bunch of stuff to measure and then put it in a useful format for everyone.

In the meantime, your issue sounds like something is not right with the comparator resistor string. Can you check for 2.5V DC at the precision shunt on the meter card? If that is correct at the shunt, something in the resistor string got swapped around.