[BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread

[jsteiger]

It is most likely a bad solder joint on the toggle or on the HPF PCB. Also reflow the solder on the 5-pin 90 degree header. The audio enters and leaves the HPF PCB twice. Once for -6 and again for the -12 position.

 

[Potato Cakes]

How bout that. Apparently one of the switches on the HPF frequency selector switch had one of the pins not completely soldered to the HPF PCB. Usually stuff doesn't work because either my bad reading skills or intermittent dyslexia causes me to misplace components. Nothing like a bad solder joint to ruin your day.

Thanks for your help!

Side note, I did a live recording using an Earthworks SR40V on the vocal running through your VP26 with Red Dot DOA and it sounded fantastic. I was very pleased at how it could sit back in the mix without hardly any Eq or compression but still remain perfectly intelligible. I wished I had been building my own gear all these years and saved myself a ton of cash.

 

[novisplit]

OK. So I checked all the test points and checks out good and getting the same readings from a good unit and a bad unit. The only thing that is different on the bad unit is that C20 negative wire doesn't seem to be connecting to the red and orange wires on t3 like they do on the good unit. I flowed some solder over to connect the negative lead and the orange wire but still no sound. Any ideas?

 

[jsteiger]

Desolder all of the leads for T3 and measure DCR.

Set your DMM to read less than 100 ohms and check
red to orange
brown to black
green to blue

 

[novisplit]

Looks like the board wasn't connecting the negative lead of c20 and the red wire from the transformer. Totally fixed now.

 

[jsteiger]

The trace was missing? Can you send me a pic. I don't recall ever having that happen before.

 

[burn4ever]

Thanks a lot for this extremly well documented manual 

Are there any advices what I should check with a multimeter before I setup the preamp for the first time?

 

[jsteiger]

Just make sure you have no direct shorts from either of the main voltage rails to ground and then follow the tips in the VP28 Assembly Guide from the first post of this thread.

 

[burn4ever]

Hey Jeff,
thanks for the info. One last question.
Is there a circuit diagram or any info about how many volts the DOA sockets must have?

 

[jsteiger]

Typical for 500 series racks, the sockets should have appx +/-16V on them respectively.

 

[Unit7]

Building a couple more of these, and me likes sorting sheets

 

[markphilpot]

So I just finished my first pre and I, unfortunately, am dead in the water so far. I get a Signal light, intermittent LEDs from the other 4, and no actual signal when I turn up the gain…the polarity, 48V, and pad will make a popping noise, but that's it.

Being my first build, not sure where to start on trouble shooting this guy. Any thoughts ? Any help is appreciated.

MP

 

[Potato Cakes]

Did you build the DOAs or buy them pre built? If you built the DOAs, do you have any know working ones to do some testing? Also, double check all of your solder joints, even if you are experienced at soldering. I recently had a bad connection even though it appeared to be soldered correctly. And lastly, there is the tedious task of checking component placement. Meter all of your resistors and compare them with the BOM and PCB overlay. I've built a number of Jeff's preamps and every time I had an issue it was one of these things. These are some of the best kits available and double checking your component values should yield a fantastic sounding mic preamp.

 

[jsteiger]

Did you build the DOAs or buy them pre built? If you built the DOAs, do you have any know working ones to do some testing? Also, double check all of your solder joints, even if you are experienced at soldering......

+1, exactly what I was about to say!

Also be sure the opamps are fully seated. I have said this numerous times in all of my support threads but it gets missed often for some reason.  http://classicapi.com/catalog/DOA_Install.php

 

[markphilpot]

thanks…I've been reading through the thread…The transistor switching on the op amp was an issue, but I've fixed those…Not sure if this has any bearing,  but my LED for signal is always lit up, and the other 4 now don't light up. Still no actual signal coming through.

Checked the seating on the DOA, seems ok.  Don't have any other DOA's to use, though I'll probably by some assembled ones from the store to check.

I'll start running through component checks with the DMM and see if i can figure anything out there. At risk of displaying my noob soldering - here's a pic



MP

 

[Potato Cakes]

I've had ye 'ole transistor switched before and found it easier and more reliable to just build a new kit. The very tight spacing of the components leaves very little margin for error and unsoldering and resoldering significantly increases the chance of ruining the component you're swapping or even the board itself. Having a pair of pre built DOAs to do testing I believe is a must if this is your first time. Also, don't forget that LED's are directional (have polarity).

On another note, the VP28, as it says on the CAPI website, is a more challenging of a build. I actually had built several of the VP25's and VP26's before attempting the VP28. I'd recommend building a variable version of a VP25, 26, or 312 to get a better feel for these designs and hone your soldering skills. No, I don't get paid to promote CAPI's  products. But for myself being someone who not too long ago got into the DIY arena, this is probably the best advice I can give someone else who is recently new to all of this.

 

[markphilpot]

thanks PC -
  I went back and checked the polarity on all LED's - they're correct - must be something with my soldering…resistors seem to check out. I knew I was biting off a lot for being a first build but thought I'd plow through and see - this may be where the rubber meets the road, eh.

Thanks - I may be out an expensive lesson, but I'm going to keep trouble shooting and see what I can find. Thanks again - and I'll likely invest in a 312 or 25/26 very soon!

MP

 

[markphilpot]

I've ordereed 4 DOA's from Gary and those should be in later this week…I built another 2520 to test b/c one was smoking (literally).  Here's an update:

I'm getting the signal lighting up when I clap/talk etc, phantom power, all LED's work, but signal is very low and distorted. I found the signal PDF referenced, but don't know how to actually test that…so I'll ask - Could any of you vets point me to a link/site that shows how to test signal? I've got a DMM and that's it.

Learning a ton - thanks for the help!
MP

 

[jsteiger]

You can inject a sine wave and use a DMM to measure the AC voltage. That is how the test points doc is written. The freq should be low like 100Hz or 400Hz.

The sig present circuit is at the output of the first or preamp stage so that could be OK, from your description.

 

[michaelwinston]

Hi guys,

I'm building my first VP28 and am populating the board currently.  I've done SCA projects as well as a few pedals so it's not my first build by any means.

I have either misplaced or was not sent the TI RC4558ip and the socket.  I had a socket laying around and I also have a couple of different 4558's laying around from pedals... one is a JRC4558D and the other a TI RC4558P.  I'm assuming these are suitable replacements... but wanted to ask someone who might know for sure.

Thanks!
-Michael