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

[jsteiger]

Hey guys!

I'm building two FC526 and both are measuring really well on the different test points, compared to Test Points Guide Rev_2 and Calibration Guide Rev_1 until ** Gain Reduction Meter Calibration ** where I have reached. However, I do measure -10.05 on TP9 which is a bit more than the guide prescribes (between -9.98V DC and -10.02V) Is that ok?

And a bummer.. After I finisted building these two, I saw that there was this visual guide, at the beginning of this thread, and discover that there is a certain polarity side to the diodes!
..I got those in there 100% random on those meter boards. doh..

Is there a way I can test which side of a diode is what, or is it easiest to just order 2 new batches of diodes?

-10.05 is OK

The arrow on the PCB silkscreen points to the end of the diode with the line around it. The part #'s are also written on the side of the diode's body. Pics at this link will help you identify the parts properly.
https://capi-gear.com/catalog/Build_LC25-40_Rev_A.php#Sec2A

 

[Thunbo]

The arrow on the PCB silkscreen points to the end of the diode with the line around it...

Hey Jeff!

Sorry! I meant the LEDs.
I was so focused on having them sitting nice on the front panel, I just went total autopilot - mounted them randomly.
I read somewhere that if one in the string sits wrong, the chain of light stops. The first device lights up as far as PK, 0, .05 and 1. The other does not light at all.

 

[jsteiger]

Yes they are in a string so the a backward LED will stop the flow to the rest that follow it.

 

[jsteiger]

So, an interesting story that I thought I would share. A customer bought 2 kits and had a "tech" build them. One worked fine and the other he could never get the meter to calibrate. I guess the "tech" messed with it for a while charging the original buyer but never fixed it.

The 2nd "tech" had it and said it was hard to bias the VVR FET. He got another pair of FET's and still never got the meter calibrated.

The 3rd "tech" noticed that the 2.5V reference voltage on the meter PCB was around 2.78V. He was the first to mentioned this issue. I sent him a new 2V5 precision reference IC as well as three new LM339's. He said he could not replace the comparators because his hot air rework tool was melting the LED's.

All of this was happening and the original buyer swore up and down there was a "bad part" supplied by me on the build. For the first time ever I said "send it to me".

30 seconds into looking at the build I noticed that the original builder apparently swapped the 2k2 bias trimmer with the 100k meter range trimmer. Once we righted that wrong...everything was A OK.

So folks, don't forget to check ALL of your parts if you are having trouble with this build.

 

[dudeitsree]

I’m stuck on step 6 of the GR meter cal, I have a +7 injected into the input, when I switch on the GR I measure 267.0 mv (-9.25 dBu) that doesn’t seem right compared to your example in step 8, also  step 8 confuses me for some reason, maybe it because it’s such a different result but I’d love some help understanding.  Am I doing something wrong?  So far all my TP measurements are on point too, any insight? 

 

[jsteiger]

I’m stuck on step 6 of the GR meter cal, I have a +7 injected into the input, when I switch on the GR I measure 267.0 mv (-9.25 dBu) that doesn’t seem right compared to your example in step 8, also  step 8 confuses me for some reason, maybe it because it’s such a different result but I’d love some help understanding.  Am I doing something wrong?  So far all my TP measurements are on point too, any insight?

If you're at step 6 of the GR meter cal section, don't worry about measuring until you've set your comp's input/output controls to have 7dB of gain reduction. In that situation with +7dBu going in, you will have an output of 0dBu when the GR toggle is active.

 

[dudeitsree]

Okay, so I was able to get both of my builds working.  One acts like I would expect with drums or vocals.  The other seems to have issues, the attack and release don’t seem to be changing how it reacts much if not at all.  Also between the two build they seem to have very different input gain.  Any ideas on where to look for the issues? 

 

[jsteiger]

Well, know you now need to follow the Test Points guide and focus on the audio path. Luckily you have a good one to use as a benchmark.

 

[dudeitsree]

I did look at the test points, and they seem in order.  Now I discovered that the unit with the issues doesn’t show gain reduction in ABI.  What the heck did I do wrong!

 

[dudeitsree]

To add even more to my confusion, the unit that is acting more normal C14 is backwards.  Is C14 in the audio path? 

 

[jsteiger]

To add even more to my confusion, the unit that is acting more normal C14 is backwards.  Is C14 in the audio path?

C14 is a ceramic cap. Nondirectional.

 

[dudeitsree]

Sorry C16

 

[jsteiger]

Sorry C16

C16 is an AC coupling cap but typically has a very small amount of DC across it. Easy enough to measure the DC.

 

[critterkllr]

Hey Jeff,

When I get to step 12 of the calibration guide for my FC526-XFMR, I only get .098 vac. I’m using a DMM to measure.

1k sine wave measures .025 vac at input.

TP1 = .007 vac
TP2 = .010 vac
TP3 = .065 vac

Output opamp V+ = 14.80 vdc
Output opamp V- i= -14.26 vdc
TP9 = -10 vdc
MTR #11 = 2.49 vdc
PSU voltage is +15.57v/-15v


Resistance from V+ And V- socket to ground is around 3k.

I’m using a known working red dot 2520.

 

[jsteiger]

Sounds like an audio path problem. Have you gone thru this PDF?
https://capi-gear.com/catalog/images/build/FC526/CAPI-FC526-XFMR-Test-Points-Guide-Rev_1.pdf

 

[critterkllr]

Sounds like an audio path problem. Have you gone thru this PDF?
https://capi-gear.com/catalog/images/build/FC526/CAPI-FC526-XFMR-Test-Points-Guide-Rev_1.pdf

Yes, it seems like something wrong between TP2 and TP3? Not sure how to trace the circuit without a schematic.

I get the same measurement from TP2 as I do from the opamp’s IN, and the same from TP3 and the opamp’s OUT.

 

[critterkllr]

Some pics of the build

 

[critterkllr]

Meter board

 

[jsteiger]

TP2 is post A2 (DTO5) and before A3. TP3 is after A3. Check at the wiper of RV5 output level pot. That is the first thing after TP2. Double check solder joints and values for R30, 31, 33, 69 and 70. C9 is a series cap right after RV5. Make sure C36 has good solder joints.

 

[critterkllr]

TP2 is post A2 (DTO5) and before A3. TP3 is after A3. Check at the wiper of RV5 output level pot. That is the first thing after TP2. Double check solder joints and values for R30, 31, 33, 69 and 70. C9 is a series cap right after RV5. Make sure C36 has good solder joints.

I’ve reflowed all of these solder joints and have verified the values. The problem persists.

RV5 goes from .007 vac when CCW, to .010 vac when CW.

I believe there was an issue with my conversions from dbu to mv to volts. TP2 seems to be very low. I’m getting a reading of .01 vac, when I believe it should actually be .11 vac?

I’ve verified all component values in A2 and reflowed all solder joints. Still have the same issue.

Edit: RV6 seems a little loose. Might have a broken pin. I’m going to order a replacement.