[BUILD] CAPI VP2X~500 Series~Preamp Kit~Official Support Thread

Hey guys measured for +48 on pin 15 and 13 and then 15 and 5.  Both positions show 48.5v.  So it looks like its getting power. Im just not sure what would be causing this.  After the phantom power is switched on there is a loud pop when I engage the pad in or out (this is with a dynamic mic, it still does not power a condenser).  Does anyone have any suggestion of what I should look for now?  Thanks everyone!!

Hello,

yes,this is correct.
Next step is to measure the female xlr,so the input on the back of the lunchbox.
Again measure with your dmm set to dc voltage and measure both xlr pin 1 against pin 2 as well as pin 1 to pin 3.You should get two identical readings here.
The loud pop with phantom on while using the pad indicates that there seems to be the dc in the circuit already.
I hope your mic cable is o.k.,you can check the phantom on its' female end too,same way as described before.
Or is the signal running through a patchbay or so?

Good luck,

let us know,

Udo.

Edit:You must have a module plugged in on the slot you measure at of course.

  I'm suspicious about a part I received in my VP26 Rev. B BOM. R14 is listed as a 10 ohm resistor on the RevB BOM sheet. After stuffing the rest of the board I am left with a single resistor which has only a single black color band on it. My DMM reads this resistor as having less than 1 ohm of resistance, and the BOM calls for 10 ohms in the R14 position. Do I need to find an additional 10 ohm resistor to sit in it's place?

Thanks for the help,

-P

Hi,

that's not a normal resistor but a bridge or jumper,same as a piece of wire.
So you get that close to zero reading.

Best,

Udo.

I just wanted to give a quick update.  After messing with the vp26's  a little more this weekend I did find the problem.  It was the mic cable, for what ever reason it would not pass 48+, but I still am having one problem.  I have tested the preamps with several different mics, but for what ever reason it will not power my CAD m179 microphones which is what I use for toms.  Im really not sure what to even think about this (but at the end of the day I could use it as a great reason to pickup some more mics!!!  ;D)

Thanks everyone!

Glad you got it sorted!

Udo.

Yeah I wish I could just figure out why its powering my other condenser mic's and not the CAD m179 though! I would almost suggest that I have an issue with the mic but it doesn't seem likely that all 3 mics would go bad/break at the same time. 

Hello,

you should post this over in the mic build thread.
I'm sure it is mic related.
Just to keep this thread clean.

Best,

Udo.

In a quick google search it looks like the m179's are known for phantom power issues. The 48V circuit on my pre's is about as simple as it gets. If other mics are working with phantom, it could be a rack or PSU issue with a small phantom supply. Did you try one mic at a time or all 3 at once?

  I just finished a Rev B. VP26. My signal level from the mic pre is low and requires the stepped gain grayhill switch to be completely turned up to get any kind of signal. I believe all of my resistor values are correct.

  Some things that I've thought may be the problem are, the resistor at R1. The DIY build does not have a resistor in that position. Also, I neglected to put a piece of double sided tape underneath the input transformer. Could there be a problem there? Also, is it worth checking the output transformer resistor values? Also, the resistor at R14, listed as a 10 ohm resistor on the bill of materials, did not come in the bag of resistors. Instead, I had an extra shorting resister. I put in a 10 ohm resistor at R14, is there a chance that the shorting resistor is supposed to live there instead?

  Thank you for the help.

  -P

pittsburgh said:

Some things that I've thought may be the problem are, the resistor at R1. The DIY build does not have a resistor in that position.

Click to expand...

This is on the BOM and schematic. It's just a R for the phantom LED.

Also, I neglected to put a piece of double sided tape underneath the input transformer. Could there be a problem there?

Click to expand...

No.

Also, is it worth checking the output transformer resistor values?

Click to expand...

Maybe, but not yet.

Also, the resistor at R14, listed as a 10 ohm resistor on the bill of materials, did not come in the bag of resistors. Instead, I had an extra shorting resister. I put in a 10 ohm resistor at R14, is there a chance that the shorting resistor is supposed to live there instead?

Click to expand...

This was a packing error on a handful of resistor packs that went out. 10R is the right value. Sorry about that.

The first and foremost question is always "who built the DOA"? If the DOA and preamp were both fresh builds, it is very hard to figure out where the problem can be unless you have another way to test the DOA or another DOA to test the preamp with.

This is a pre assembled GAR 2520. I have a JH 990C that I can swap out. Should I try that?

If the gar was prebuilt it will not be that. Gary burns them in and tests them before he sends them to me. Is it seated properly? http://classicapi.com/catalog/DOA_Install.php

If it's not that, you must make sure it is nothing outside of the pre. Rack, cabling, converters and so on. Next it will be time to inject signal and see where it stops. There are many posts in this thread about how to do that.

Thanks Jeff, I checked out that link and am now confident in the seating of the op amp. Also, the attenuation pot attenuates at the 0 marking and does not attenuate at the infinity marking. Is this the opposite of what it should be?

Yes, that is very weird. Dbl check your output transformer wiring.

Here's a picture of my output transformer. The brown and green leads were cut a little short but they are in properly.

I'm not getting adequate signal from my VP26. I have noticed two significant issues. The phase switch kills all signal, and the attenuate signal works in reverse where the pot set to 0 on the face plate results in a fully attenuated signal, and 0 passes signal. I have passed all tests listed at the end of the install guide, and am passing signal with both dynamic and condenser microphones, but the signal is distorted and requires the stepped gain as well as the input attenuate pot, (in reverse,) to be set to full. The output transformer is wired to the PCB properly, and I have also swapped out the pre built GAR op amp with a JH990C that I know is in good working order. Here is my PCB. Please let me know if you can help. I probably did something really juvenile here.

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