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

[Hillie]

I've been slow in getting my FC526s built, but I bought 4 kits and got them pretty much put together about a month ago. Started measuring TP9 and my 20 year old Fluke 87 reported -10.03 on all 4. As the deviation was the same across all 4, I assume my DMM was the issue and it's at Fluke undergoing calibration (I've had it for 20+ years and it was never calibrated, Fluke service folks said "we can calibrate, but we're long out of replacement parts") so I'll have it back soon. In the mean time I got myself a Keysight U1242C as well, and its calibration numbers for DCV are spot-on. Just measured all 4 on TP9 and I saw: -10.034, -10.026, -10.029 and -10.032 volt.

What should I look at? I know "bad solder joints", but I've made sure to keep a good eye on that as I was putting the boards together and I've made sure to properly flux and clean. So I have no clue. Is -10.03 (ish) too far off? I guess so.

Another question: That MTR 11 point at the start of the calibration document, which joint is it? It's close to 3 solder joints. None of them are within spec anyway on all my builds.

 

[jsteiger]

I've been slow in getting my FC526s built, but I bought 4 kits and got them pretty much put together about a month ago. Started measuring TP9 and my 20 year old Fluke 87 reported -10.03 on all 4. As the deviation was the same across all 4, I assume my DMM was the issue and it's at Fluke undergoing calibration (I've had it for 20+ years and it was never calibrated, Fluke service folks said "we can calibrate, but we're long out of replacement parts") so I'll have it back soon. In the mean time I got myself a Keysight U1242C as well, and its calibration numbers for DCV are spot-on. Just measured all 4 on TP9 and I saw: -10.034, -10.026, -10.029 and -10.032 volt.

What should I look at? I know "bad solder joints", but I've made sure to keep a good eye on that as I was putting the boards together and I've made sure to properly flux and clean. So I have no clue. Is -10.03 (ish) too far off? I guess so.

Another question: That MTR 11 point at the start of the calibration document, which joint is it? It's close to 3 solder joints. None of them are within spec anyway on all my builds.

That is fine and nothing to worry about.

 

[RandyBeast]

Hello guys, I need a little help here, I just finished building 2 and one was perfect and the other is giving me problems. I'm using a known good opamp and here is what I get at my test points. I'm using my daw to send a signal generator 1K sin at -30 dbu or .024V to a capi rack using a capi Jig. it seems the signal is being boosted by a crazy sum. I have checked all solder joints and everything look's fine there, I have also used a different opamp and got the same results. I'm wondering if I put a resistor in the wrong spot, but I'm not seeing it. any help here?

TP1: 11.2mV / -36.8dBu
TP2: 975mV / 2 dBu
TP3: 6.94V / 19.05 dBu
TP9: -10.03 DC

 

[RandyBeast]

Hello guys, I need a little help here, I just finished building 2 and one was perfect and the other is giving me problems. I'm using a known good opamp and here is what I get at my test points. I'm using my daw to send a signal generator 1K sin at -30 dbu or .024V to a capi rack using a capi Jig. it seems the signal is being boosted by a crazy sum. I have checked all solder joints and everything look's fine there, I have also used a different opamp and got the same results. I'm wondering if I put a resistor in the wrong spot, but I'm not seeing it. any help here?

TP1: 11.2mV / -36.8dBu
TP2: 975mV / 2 dBu
TP3: 6.94V / 19.05 dBu
TP9: -10.03 DC



View attachment 98751

I might have found it, a capacitor is in backwards. I'll fix that and update the situation

 

[jsteiger]

Hello guys, I need a little help here, I just finished building 2 and one was perfect and the other is giving me problems. I'm using a known good opamp and here is what I get at my test points. I'm using my daw to send a signal generator 1K sin at -30 dbu or .024V to a capi rack using a capi Jig. it seems the signal is being boosted by a crazy sum. I have checked all solder joints and everything look's fine there, I have also used a different opamp and got the same results. I'm wondering if I put a resistor in the wrong spot, but I'm not seeing it. any help here?

TP1: 11.2mV / -36.8dBu
TP2: 975mV / 2 dBu
TP3: 6.94V / 19.05 dBu
TP9: -10.03 DC

It won't be a backwards cap. It'll likely be a wrong resistor value which means 2 wrong resistors if 2 got swapped. Dbl check R31, R69 and R70.

 

[RandyBeast]

It won't be a backwards cap. It'll likely be a wrong resistor value which means 2 wrong resistors if 2 got swapped. Dbl check R31, R69 and R70.

you were right, Cap did nothing. how ever I checked those resistors and they were all correct

 

[RandyBeast]

It won't be a backwards cap. It'll likely be a wrong resistor value which means 2 wrong resistors if 2 got swapped. Dbl check R31, R69 and R70.

Hey Jeff,

I was going through every resistor with my Fluke 177 and found R72 at 27ohms instead of 165 ohms, I'm not sure if that might be it, but I thought it was interesting. I've never had to troubleshoot this in-depth so I don't know if the circuit itself could give me wrong ohm readings when checking across resistors.

 

[jsteiger]

Hey Jeff,

I was going through every resistor with my Fluke 177 and found R72 at 27ohms instead of 165 ohms, I'm not sure if that might be it, but I thought it was interesting. I've never had to troubleshoot this in-depth so I don't know if the circuit itself could give me wrong ohm readings when checking across resistors.

Resistors cannot be checked with both ends soldered in circuit. The only way to measure with a DMM is to desolder and lift one end. I would suggest reading the color codes. Its way easier at this point.

 

[RandyBeast]

Resistors cannot be checked with both ends soldered in circuit. The only way to measure with a DMM is to desolder and lift one end. I would suggest reading the color codes. Its way easier at this point.

yep, that's what I did, I had everything correct. what I did find once I dug super deep was there was a very small hair-like piece of solder joining two ends. it was very very small. but once I fixed it, I was back in business!

 

[RandyBeast]

The last thing I learned haha, is I've always sent a Sin wave out my daw and measure 0.024V with my DMM for -30 dBu, however, that Sin wave is coming out my daw as a digital signal (1s & 0s), so I learned that if you re-amp the signal and convert it back to analog, you can get better results when calibrating these. overall, I've been very happy with these even with them being calibrated with the digital signal.

 

[RandyBeast]

Hello Jeff, so I got all my FC526's working beautifully except for the one where I had meter problems. I found I put a transistor in backwards on the meter board. however, I have corrected that and the voltage still oscillates keeping me from calibrating it. I've tried other known meter boards and they have the same issue. so I know it's on the main board. With that said. could it be possible the backward component damaged the MTR Transistor?

TP9 = -10.05

 

[hasanova]

Okay, another afternoon and some measurements later - they are finally matched. I made sure to check TP7 to see if my FET Bias DC Voltage was just like written on my PCB. So I re-calibrated both units. Now they are very very close and I like them so much. That compression is extremely versatile. Man, I'm really happy. Cheers everybody!

I just ordered another diy FC526-XFMR. I already have built one in 2018. Is there a way to get them matched for dual mono purposes?
Do I need to get a match of each of the JFET's in one unit? (F4 VVR and F3 MTR).

@jsteiger I didn't realize this when ordering, just when I looked up my build docs. Do you sell a matched pair of FET's or can I order a pair that is a match to the one I already have? Or isn't this an issue at all for dual mono use?

I'll know when I have built it, I'll use sockets...

 

[eighthsonpro]

Hello everyone. I just finished a build of a pair of compressors last night. I have no shorts at the DOA voltage pins, but when I check Tp9 I have voltages of -12.29VDC and -11.46VDC I'm pretty confident that I have all of my components placed correctly. I sure could use some guidance here. Thanks!

 

[jsteiger]

Hello everyone. I just finished a build of a pair of compressors last night. I have no shorts at the DOA voltage pins, but when I check Tp9 I have voltages of -12.29VDC and -11.46VDC I'm pretty confident that I have all of my components placed correctly. I sure could use some guidance here. Thanks!

Most likely misplaced resistors.

 

[hasanova]

I just ordered another diy FC526-XFMR. I already have built one in 2018. Is there a way to get them matched for dual mono purposes?
Do I need to get a match of each of the JFET's in one unit? (F4 VVR and F3 MTR).

@jsteiger I didn't realize this when ordering, just when I looked up my build docs. Do you sell a matched pair of FET's or can I order a pair that is a match to the one I already have? Or isn't this an issue at all for dual mono use?

I'll know when I have built it, I'll use sockets...

2018 FET's:
F3 MTR = -1.225 V
F4 VVR = -1.509 V

2022 FET's:
F3 MTR = -1.230 V
F4 VVR = -1.635 V

@jsteiger Is this close enough..? For on a Bus (dual mono).

 

[eighthsonpro]

Most likely misplaced resistors.

yup 470R vs. 470K in the wrong spots on one of the comps, but the main fault was the diode at CR1 on both preamps. i didnt have it soldered in. that explains the difference in voltage. i have a perfect -10.00v now. Thank you, @jsteiger

 

[jsteiger]

2018 FET's:
F3 MTR = -1.225 V
F4 VVR = -1.509 V

2022 FET's:
F3 MTR = -1.230 V
F4 VVR = -1.635 V

@jsteiger Is this close enough..? For on a Bus (dual mono).

If cal'ed properly they will track very well except for in ABI. You can try them that way. They will be somewhat close but probably not close enough. YMMV

 

[hooty]

Hi,

I am from Germany and was lucky enough to be able to ordered two FC526 kits in October. Unfortunately, it seems that one of the output potentiometers is not working properly. Is there a place where I can order the spare part directly? It is a 51CAD-D20-D15L which is used to control the output.

I would appreciate any advice.

Best regards,
hooty

 

[jsteiger]

Hi,

I am from Germany and was lucky enough to be able to ordered two FC526 kits in October. Unfortunately, it seems that one of the output potentiometers is not working properly. Is there a place where I can order the spare part directly? It is a 51CAD-D20-D15L which is used to control the output.

I would appreciate any advice.

Best regards,
hooty

Sorry I have been out of the office for a bit. How do you know its the output pot itself? Maybe it is something else in the circuit?

 

[Kevin]

Sorry I have been out of the office for a bit. How do you know its the output pot itself? Maybe it is something else in the circuit?

Hey Jeff
I'm assembling the Unit for hooty. I measured all of your test points and when changing the angle of the output Potentiometer - Nothing changed. so since there is no schematic around i checked all traces going from any other parts to the pot and found nothing problematic. So I unsoldered the whole pot and meausred the all resistance of the pot with a multimeter found out that is has loose connections on one pin. Sorry to hear that also Bourns can fail on their pots.