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

[Buffstuffbuddha]

Haha I think I'll take your advice...now just to play devil's advocate. Is your mic/line switch circuit different than, say, an API 512 circuit? Just wondering what they do differently because there is only a very tiny pop when engaging the switch on a 512...I promise I will never engage that switch with phantom on again

 

[jsteiger]

Haha I think I'll take your advice...now just to play devil's advocate. Is your mic/line switch circuit different than, say, an API 512 circuit? Just wondering what they do differently because there is only a very tiny pop when engaging the switch on a 512...I promise I will never engage that switch with phantom on again

I honestly don't know what is happening with the 512C mic/line. I have never really looked. If its like the 1608, the line input goes straight into the 2520 as a balanced receiver so AFTER the input transformer. My line in goes into the 2622. Basically same path as a mic in except thru a u-pad. My mic/line switch is only switching in a 3 resistor u-pad.

 

[digitalblack]

I'm measuring 11R across RF3 instead of 36.5R (it's the proper resister, measured when installing). Is it possible this is causing my low output issue?

 

[tttbag75]

Hello !
I've just built my pair of VP28. And I have a problem with one of them. The first is OK, but with the second I have 444mV at TP3.
I don't know where does the problem come from.
Do you have an idea ?
Thank you for the answers !
Thomas

 

[tttbag75]

... I forgot...

- VP28 rev.B
- gar1731 v4.1
- gar2520 v4.1
- Litz transformers

Thomas

 

[jsteiger]

... I forgot...

- VP28 rev.B
- gar1731 v4.1
- gar2520 v4.1
- Litz transformers

Thomas

Everything is OK at TP2?

If so, double check your wiring for T2 (since it is Litz). If that is good, desolder T2's lead from the blue pad and measure the test voltage at that loose lead.

 

[tttbag75]

Yes, everything is OK at TP1 and TP2.

My wiring for T2 are OK.

I have desolder the blue pad of T2, and now I have 462mV.

 

[tttbag75]

... I have 694mV at the red pad of T2.

Thomas

 

[jsteiger]

You should next desolder all leads for T2 and check the windings for shorts. Make sure no leads are touching each other. Set your DMM for less than 200Ω.

You should only have resistance between the following leads:
Black to Red
Gray to Pink
Violet to Orange

Don't just check between the above sets but between all leads.

 

[tttbag75]

After desolder :

- black to red : 15,7R
- grey to pink : 31,1R
- violet to orange : 31R

... And I have resistance between this leads :

- grey to orange : 31,1R
- grey to violet : 61,7R
- pink to violet : 31R
- pink to orange : 0,4R

My transformer is dead ?

 

[applespace]

I would highly recommend not engaging the Mic/Line switch while phantom is on. Never. Not even to compare when you get a 2nd unit.  ;D

Could this blow PR1-4 on the main board? Just built 2 VP28s and when I plugged in one I accidentally had phantom and mic depressed. It fried those 4 resisitors. Any other ideas where to look if not?

Cheers

EDIT: I get the sarcasm now (idiot). Any ideas though? Bad OpAmp in the pre-amp section?

 

[geardude]

Hey guys after tackling lots of vp 26's I dived into a 28. Fired her up and everything works like a charm! I do have one question however as when I go to shut off my snackbox/lunchbox I hear a loud squeal/laser beam noise.

Turn it on and everything operates fine- and again shutting it off I hear this again.

Doesn't happen with my other vp builds. Is this a known thing or is something a miss here?

 

[geardude]

Alright that's comforting to hear  : 8)

 

[achase4u]

Curious, did you guys get flat washers with your kits for the X-formers?  I did not.  I am wondering if they have since been omitted as unnecessary?

 

[jsteiger]

Curious, did you guys get flat washers with your kits for the X-formers?  I did not.  I am wondering if they have since been omitted as unnecessary?

The info is all posted on the support docs page with detail drawings 
http://www.capi-gear.com/catalog/images/gallery/VP28/Build-docs/VP28-New-Style-Hardware-BOM.pdf

 

[pachi2007]

Hi.
I had a perfectly working VP28 that now is giving me some trouble.
The  thing  now is I have noise ( even signal light shows it and it´s almost constant but it comes and goes  ) but it disappears when I switch the phantom on.
This is with a dynamic/condenser plugged or with nothing connected at all. Both mikes work great and there´s no noise with phantom but the noise comes back when I switch it off.
I can see a constant healthy +48 at the output though.

Any advice to get me started? Any help much appreciated.
Thx

 

[BennyS]

Hi There
Thanks for a nice build forum!

I recently bought a VP28.Rev B.1 Stepped Gain entire bundle. The VP28 works fine for dynamic mics with a large gain and clear sound. For Phantom power, however, there is no signal at the output.

I have measured 200mV between pins 1 and 2 on the Input XLR with Phantom enabled and that should be ok - It is anyway identical to the VP312 I also acquired which works perfectly! The dynamic and condenser mics are both working fine on the VP312.

So the phantom power is there on the In-XLR of the VP28 but the signal is zero - again, the dynamic mic is working fine on the VP28.

While not having the schemo at my disposal, I would appreciate it a lot if anyone of you could give me a hint to where to start looking?

Ps. I saw a similar thread in this forum a while ago, posted by Andrew, but that thread ended abruptly - solution found or still open? Ds.

Best regards
Benny

 

[BennyS]

Hi

Short: Problem solved!
Long: After posting my question on the forum, I (again) looked over all solder points on the VP28. Did not find anything. Then I decided to resolder all points - and that was it - a bad solder point (somewhere).

I am happy that it now works fine !

Thank you!

Hi There
Thanks for a nice build forum!

I recently bought a VP28.Rev B.1 Stepped Gain entire bundle. The VP28 works fine for dynamic mics with a large gain and clear sound. For Phantom power, however, there is no signal at the output.

I have measured 200mV between pins 1 and 2 on the Input XLR with Phantom enabled and that should be ok - It is anyway identical to the VP312 I also acquired which works perfectly! The dynamic and condenser mics are both working fine on the VP312.

So the phantom power is there on the In-XLR of the VP28 but the signal is zero - again, the dynamic mic is working fine on the VP28.

While not having the schemo at my disposal, I would appreciate it a lot if anyone of you could give me a hint to where to start looking?

Ps. I saw a similar thread in this forum a while ago, posted by Andrew, but that thread ended abruptly - solution found or still open? Ds.

Best regards
Benny

 

[Commander Fluffypants]

Hi there, folks,

It's been a while since I last attempted to fix my last (of eight) VP 28 build. Most of the others worked just fine right away. A couple had minor issues. I still don't know what to do about this one and I have some time in the coming weeks to put into it.

This thing seems really hot. Like loud. It's also noisy, like hiss. At a certain point, it squeals. For instance with the fader at +2, when the preamp gain gets all the way up, it squeals. I have attached an mp3 of exactly this sound. Couple of notes: I did this test with known working OpAmps from one of my other working VP28's. Also, if the fader is higher than +2, the preamp gain would squeal at a lower volume.

TURN YOUR VOLUME DOWN BEFORE HITTING THIS LINK:

https://dl.dropboxusercontent.com/u/1643668/Squeal.mp3

As always, thanks for the help.

 

[Commander Fluffypants]

In reference to my post above, this is where I left off the last time I tried to work on this last VP28. It is very possible that I'm not testing it correctly, assuming that testing is the next step. I am confused by the fact that when in the jig, the gold fingers are almost completely covered by the jig. It is also possible that I'm not understanding the description of where to get ground.  These things are paint by numbers to me. If I am meticulous, everything is great. If I make a mistake, I have no idea what this dang thing is, nor where a mistake might be, etc.

Thanks again.

OK... If I tested this right, and I'm not sure if I did, there is no DC present at gold fingers 8 or 10, either with phantom on or off.

So in order to get access to the gold fingers while the unit is in the jig, I had to take the whole thing apart. I am attaching a picture of the back of the unit with the jig on and just the smallest bits of gold fingers sticking out of the jig. Hopefully this is what I was supposed to do. I put the ground lead right on that little black pad on finger five. There are no fingers six or seven, so the next one down is eight. No nine so the next after eight is ten.  Holding the one lead on ground, I poked at gold fingers eight and ten, and even tried different ranges on the DMM (yes, checking DC.) The DMM never showed any reading at all. Not even zero. It showed where the decimal point was as I moved the range around. I was just using it to measure resistors as I built the other 1731, so I believe it is working. If anything is wrong, it's operator error.

Oh... I did this test with no DOA's in. Perhaps that's the problem? I'm going to take my time and not proceed until I have more of an idea of what I should be doing. What's my next step?

Thanks again.