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

hecjrivera said:

I recently bought the vp28 Rev B.1 and noticed its different from this build. Does anyone know where I can find the right guide?

Click to expand...

It is basically the same as the first Rev but you MUST follow the Rev B BOM.

All VP28 support docs can be found here:
http://capi-gear.com/catalog/support_docs.php#VP28

Hello,

I'm having a little trouble with a vp28. I built a kit with Litz wire transformers and the pre-built Red Dots. 3 main symptoms:

1) Upon first couple power ups, the green signal LED was constantly lit. I found that it would flicker when pressing the 48v switch. After running signal through it for a few seconds, it no longer does this and works as it should. I guess I'll call that solved?

2) Line mode passes a clean signal, but -10.58db lower than whatI send in. I use Pro Tools to generate a 400hz sine wave at 0dbfs and measured through the vp28 at unity,  then with the patch wire straight from the output, patchbay,  to the input of the converter (rosetta 800). Signal without the vp28 in the chain comes back clean at 0dbfs.

3) In Mic mode, I can see that it powers my 414xls and it passes a clean signal for a few seconds, then dies out to a very low signal.  There is a loud, low rumbling sound in the signal. I have a recording of it if needed.

I tried switching positions and re-seating of my op amps and switching to two of my home-built gar 2520's but the symptoms are identical.

I've switched slots of my Lindell audio 10 space rack but no difference.

I've reflowed every solder joint except the switches on the HPF board, since they were hard to reach once assembled. That will be my task tonight, but if anyone has ideas on something else to check as well, please let me know.

Thanks!

Edit: This is the Rev B, green pcb. I followed the proper BOM

I would check the windings on the output transformers to make sure nothing is internally shorted. If you scroll back thru this thread, it should be covered a few different times.

I've done some more testing. I suspect that I may have previously had the preamp gain knob set incorrectly. Now, passing a line level signal at unity returns within 0.33 db.

After desoldering the hpf board and reflowing, the c414xls mic signal is much more consistent, but still problematic. It seemed to be working until I tested the high pass section. I eventually get loss of signal and a low frequency rumble.  Signal then seems to slowly swell and diminish.

Here are the results from the T2 and T3 leads as well as the test points. Thanks so much for the continued help. I can't imagine how frustrating it must be to remotely diagnose the problems of an internet moron like myself! It's truly appreciated.

Test Points:
From Pro Tools: 400hz Sine wave -15.8db, measured 1.271 Vac
TP1: 115.4 mv
TP2: 0.729 v
TP3: 1.29 v
TP4: 315.1 mv
TP5: 315.2 mv
TP6: 315.1 mv
TP7: 0.664 v

T3:
Orange-Violet: 31.6 ohms
orange pink: open
orange - grey: open
orange-redpen
orange-black: open

T2:
Orange-Violet: 31.1 ohms
orange pink: open
orange - grey: open
orange-redpen
orange-black: open

Also have a look at the gold fingers at the card edge connectors. I had one new Rev B pcb here at my place where a small residue of copper has shorted out some of the contacts when the module was plugged in.
I had similar symptoms (green led always on, signal fading out....) but was able to fix it with some gentle scratching along both angled edges of the connector.

Just wanted to leave a quick comment to say I just finished my third VP28 (my first Rev B, the other two I've had for a bit are Rev As). Sounding great! I had an issue where the phantom power light wouldn't come on, and when the button was depressed it cut out the Mic light. I reflowed the solder for the phantom power switch, and all seems good! I've got two more waiting to build, and I'm sure I'll eventually make it 6, if not more. Love these, thanks Jeff!

I believe I found the cause of my issue. My c414 mic is dead.  I thought it would be a great mic to test with because it has LED's to show that phantom power was working. 

I suppose it's possible that I had an issue with my vp28 build that killed the mic, which I later fixed after reflowing the solder joints. Otherwise it's just a coincidence that a known working mic suddenly stopped working upon the first test of this build.

Don't take any part of the scientific approach for granted!

Derrils said:

....I suppose it's possible that I had an issue with my vp28 build that killed the mic, which I later fixed after reflowing the solder joints. Otherwise it's just a coincidence that a known working mic suddenly stopped working upon the first test of this build.

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Highly unlikely. I imagine it was a coincidence.

Hi all,

i recently noticed a crackling noise and slight (DC?) hum on one of my two VP28s that i built in december.  This happens also on lower gain settings, equally in mic and line mode. The other VP28 is just fine. Im running them in a Lindell 510 Rack without any issues so far, they are the only two modules in there. Does anybody have an idea where that could come from?

best regards,

marius

haselnusse said:

I recently noticed a crackling noise and slight (DC?) hum on one of my two VP28s that i built in december.  This happens also on lower gain settings, equally in mic and line mode. The other VP28 is just fine. Im running them in a Lindell 510 Rack without any issues so far, they are the only two modules in there. Does anybody have an idea where that could come from?

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I would try swapping the modules in the rack to rule out outside or rack issues. Next swap opamps around. Besides that I would look for bad/cold solder joints.

Thanks Jeff, i will try that this week!

I quickly put this in my API 500 rack and powered up. The rack was attached to a patch bay. I forgot that this portion of the patch bay was full normal as it was reserved for EQs. Everything seemed fine and powered up but when I engaged phantom power these power resistors had a meltdown. Is this the result of the normalled patch bay (which i believe fed the 48v back into the VP28 based on my understanding of normal) or is it something else. I suspect I did this to myself and have an order for some new resistors but just want to make sure.

Carl Cress said:

I quickly put this in my API 500 rack and powered up. The rack was attached to a patch bay. I forgot that this portion of the patch bay was full normal as it was reserved for EQs. Everything seemed fine and powered up but when I engaged phantom power these power resistors had a meltdown. Is this the result of the normalled patch bay (which i believe fed the 48v back into the VP28 based on my understanding of normal) or is it something else. I suspect I did this to myself and have an order for some new resistors but just want to make sure.

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This indicates a problem on both opamp builds.

Thank you Jeff. I guess you've seen it all.... 

Darn it! I was sure they were the least of my worries. With that said I ordered 2 new assembled DOAs as well as the replacement resistors. While building the Op amps I was thinking next time I would order assembled and tested just due to the tediousness of building them. Live and learn.

Carl Cress said:

I quickly put this in my API 500 rack and powered up. The rack was attached to a patch bay. I forgot that this portion of the patch bay was full normal as it was reserved for EQs. Everything seemed fine and powered up but when I engaged phantom power these power resistors had a meltdown. Is this the result of the normalled patch bay (which i believe fed the 48v back into the VP28 based on my understanding of normal) or is it something else. I suspect I did this to myself and have an order for some new resistors but just want to make sure.

Click to expand...

I just had this issue minutes ago. It was working fine with the opamps in different positions but then when I swapped them they burned up.

It looks as though the +V connection point for the opamp was the instigator.

Does there appear to be anything wrong with the opamps?

Here is the 2520

Here is the 1731.

I was sure that my Opamps were cleanly built but I'll defer to Jeff's experience on these matters. I ordered a new 1731 and 2520  opamps prebuilt and tested with 4 x 10 Ohm resistors thrown in for the repair. They should arrive this Monday or Tuesday and I will let you know if it is the fix for me. I had the 2520 in the preamp position and the 1731 in the fader/booster position and took out the opposing  (lower) resistors.

Carl Cress said:

I was sure that my Opamps were cleanly built but I'll defer to Jeff's experience on these matters. I ordered a new 1731 and 2520  opamps prebuilt and tested with 4 x 10 Ohm resistors thrown in for the repair. They should arrive this Monday or Tuesday and I will let you know if it is the fix for me. I had the 2520 in the preamp position and the 1731 in the fader/booster position and took out the opposing  (lower) resistors.

Click to expand...

Thanks - I'd appreciate that. Currently working on the 312s. Hopefully these go smoothly!

If it is not already abundantly clear from my post, I should provide the following disclaimer: I have zero experience working with electronic components. The CAPI builds are my first venture into the field. I have wanted to do this for years and finally found the opportunity to. Please be patient with me.

I have a few questions for anyone that is kind enough to help out:

1) Is some sort of pad or double sided sticky tape required for the underside of the EA2622 input transformer? What happens if it is not used?

2) The circular gold ground plane of the +V socket of my preamp opamp slot has partially lifted from the PCB surface - Is a solder connection necessary between the pin and that gold ground plane or is it sufficient to have a soldered connection through the hole?

3) Any general or specific advice with regard to building the opamps? I thought I had them perfectly soldered yet one caused the PR2 and PR4 resistors to fry. The Q7 and Q8 transisters (black blocks with the holes) overheat on another.

I was so excited about building a pair of these vp28s and a pair of the vp312s.