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

[santiagoclement]

Hi Jeff,

I did apply 1kHz sine wave -30dBu with the fc526 bypass, but i received -60dBu on the output level.  (It seems there is something wrong with the bypass), as it does not let out what goes in) When I flip the Bypass toggle to ACT, I have on the output monitor -22.6dBu, what it is only +8.6dBu. (I did as you said, I monitor the output of the fc526 going back into the DAW)
There is something else. Before I jumped RV7 (I forgot to do it so),  I had 2.498V DC on point #11, but after ding the jumping in RV7, and power in up the module, I find to have 2.616 V on point #11.
Do you know what could be the issue? I also have since the first time I powered it up, all the LED illuminated except the last "20" red LED.
Thabk you.

Regards,

Santiago

 

[santiagoclement]

This is another picture from the meter PCB

 

[santiagoclement]

Hi,

Testing the audio path, with 1 kHz sine wave -30dBu, I have:
-TP1=6.8mV
-TP2=118.9mV
-TP3=OL
What could be the issue not to have signal in TP3?

With the FC526 in hard bypass I get very very little signal, nothing to do with what goes in.

I have not seen any issue in the bypass relay. How could I check it out?

Thank you.

Regards,

Santiago

 

[Bowener]

Thanks for that tip Jeff.  I appreciate that you want to steer this kind of info to the support page so that other may benefit!    I'll quote your email response here for context:

"I would first verify that's what his problem is. One of the TP's is the VVR FET's gate. Depending on your DMM it can skew the reading but I would almost guess that you could check the good one and then check the bad one to see if they are close. I ship matched VVR FET's when someone orders a pair so they should only be off by no more than .005 volts."

So according to the docs:
"TP11 Gate lead of F4 the VVRFET"

So those readings are a good place to start...  I'll check back in when I'm back on the bench.  Thanks for the great service!

Well, I managed to swap the FETs, and the result is still pretty much the same.  After biasing and establishing 7db of gain reduction,  when GR is engaged, I can never get a positive voltage at TP 10, and ALL the LEDs are on.  Aside from that, it sounds great!  When I disassembled it,  I noticed that it wasn't trimmed and cleaned quite as well as I would have liked, but the build is otherwise decent.  What to check next, Obi Wan?

 

[santiagoclement]

Hi,

Finally I have got TP3. I´ve made a mistake reading it with my DMM.
So testing the audio path, with 1 kHz sine wave -30dBu, the results I have are:
-TP1=6.8mV
-TP2=118.9mV
-TP3=964mV
And output is aprox +7.75dBu

I then continued with the calibration guide, but when I got to "Gain Reduction Meter Calibration", I found that the output pot in Step #11 is in 1:00 instead of 10:00 to get +7dBu. And on TP10 I have DMM DCV = 1.416V. Far away form the +2.487V I should reading.
At the same time, GR switch does not seem to be working.

I then returned to test points guide to test "Sidechain Audio path", I am not able to even start it, as I not able to get the -10dBu Gain reduction. I don´t have any output difference while I engage GR switch. Doesn´t seem to be working or responding at all. So I am not able to measure TP4 TP5 and TP6.

Any tip????

Thank you.

 

[santiagoclement]

Forgot to say in my last post that
TP9: -9.96V
Point #11=2.566V 

 

[Bowener]

Well, I managed to swap the FETs, and the result is still pretty much the same.  After biasing and establishing 7db of gain reduction,  when GR is engaged, I can never get a positive voltage at TP 10, and ALL the LEDs are on.  Aside from that, it sounds great!  When I disassembled it,  I noticed that it wasn't trimmed and cleaned quite as well as I would have liked, but the build is otherwise decent.  What to check next, Obi Wan?

I dove a little deeper into this by doing a side by side comparison of all the resistor values in the "good calibrated" unit and the unit that turns on all the LEDs when gain reduction is engaged.  testing the values in place, I noticed that on the bad unit,  R16 and R17 are reading almost double the values recorded on the good unit, despite their color codes looking identical.  I tested all the resistors in place but probing either side of each resistor, so I'm not sure what if any interference is happening in the circuit.  Does that help narrow it down?

 

[jsteiger]

I dove a little deeper into this by doing a side by side comparison of all the resistor values in the "good calibrated" unit and the unit that turns on all the LEDs when gain reduction is engaged.  testing the values in place, I noticed that on the bad unit,  R16 and R17 are reading almost double the values recorded on the good unit, despite their color codes looking identical.  I tested all the resistors in place but probing either side of each resistor, so I'm not sure what if any interference is happening in the circuit.  Does that help narrow it down?

R16 and R17 on the Meter PCB or Main PCB?

 

[Bowener]

R16 and R17 on the Meter PCB or Main PCB?

Main PCB.    I exchanged meter boards  between units, and they both function perfectly.  Sorry, should have specified that!

 

[jsteiger]

Main PCB.    I exchanged meter boards  between units, and they both function perfectly.  Sorry, should have specified that!

On the Main PCB, the right end of R16 is connected to TP9 so should equal the -10V reference voltage. The left end goes to Pin #3 of the Q_BIAS trimmer RV2. The wiper of RV2 is TP8. I would compare TP8's between the 2 units and make sure the Release pot RV3 is set to the same position.

The voltage at the left end of R16 should be pretty similar since the FETs are matched when someone buys a pair. They would likely not be identical but should not be way off.

My money is still on misplaced resistors. I do my best but I have not personally experienced all of the problems that folks have had with these. It likely comes down to builder error sorting resistors.

 

[Homestudio]

Hi Jeff, if I change out the op amps do I need to recalibrate?

 

[jsteiger]

Hi Jeff, if I change out the op amps do I need to recalibrate?

No.

 

[toga]

Dug this "non-functioning" unit out of a dusty box. I had been experiencing the same "hi pass filter effect" as detailed at #265. Flipped the switch on the test jig. BOOM! Beef brought. Thing sounds great.  Thanks Jeff.

 

[emrr]

Yep, still sounding great. 

But.

One of my V1's has some intermittent rustly background noise that is cured temporarily by exercising the BYP/ACT switch.  I presume that would be a funky relay most likely? 

 

[jsteiger]

Yep, still sounding great. 

But.

One of my V1's has some intermittent rustly background noise that is cured temporarily by exercising the BYP/ACT switch.  I presume that would be a funky relay most likely?

Hmmm. Its possible but since those relays are sealed I'm not sure if they would ever "get dirty". Could it be something else in the audio path that gets a wakeup when the flow of electrons starts/stops?

 

[emrr]

Hmmm. Its possible but since those relays are sealed I'm not sure if they would ever "get dirty". Could it be something else in the audio path that gets a wakeup when the flow of electrons starts/stops?

I assume it's creating a strong transient in the audio path when toggled. 

Almost always comes up noisy, flip of switch cures it for awhile.  It creeps back in over time. 

 

[jsteiger]

I assume it's creating a strong transient in the audio path when toggled. 

Almost always comes up noisy, flip of switch cures it for awhile.  It creeps back in over time.

You could monitor the balanced audio just after the relays to see if the noise is there at that point in the circuit.

 

[XAXAU]

Hi Jeff!

Was wondering if there is a possibility of linking the the sidechain of 2 units?

Also, is it possible to put in a larger value attack pot to get longer attack like in the original 1176 AE (unsure how your circuit works) or will it mess with calibration etc? Think they switch in a 200k in series with the 25k pot, but in diy peeps use a 250.500k pot instead

Cheers

 

[boji]

So...total rookie mistake:  Misidentified BC550C and BC560C transistors (should have used magnifying glass!) and reversed their placements at Q1 and Q2, respectively.

Powered it up, turned my back for a second and...that lovely burnt pcb smell.  PR7 and PR8 resistors turned a crispy brown/black before scrambling for the PSU switch.

My question is, how much damage have I likely caused? 

After fixing the mistake with fresh BC's, everything powers on,  but the audio is thin; highpass-sounding, like up to 600Hz.  I'll repost tomorrow with test point voltages, but does anyone recommend what to first check? 

Thank you so much for your help!
-Boji

 

[Thunbo]

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?