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

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Jroq82

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May 18, 2018
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Good day to all,

Just finished building one of the 2 VP28's. Having some issue with the input I think. Seems to be extremely low. When I plug in a mic it won't even pick up the mic. But what I'm wanting these for are for my 2 buss. My setup, Audio interface outputs going to summing mixer, summing mixer going into VP28, vp 28 going into SSL comp., then SSL comp. going back into interface. (this setup work flawlessly with the 312's I built before)

When sending audio out of summing mixer I can hear the signal very faintly. when turning up the input and output knobs on VP28 it brings level up maybe 3db. Im sure its doing more than that but its all I can hear.

I haven't done the test points yet, but it's because I don't fully grasp on how to do them, so any detailed information on how to go about that would be great, I would be using a DAW for signal generation.

The opamps were pre assembled at CAPI so I'm extremely confident that can't be the issue. (2520 for both)

Ive double checked the soldering and I could clean up some spots, so Ill try that and repost an update.

It sounds like it could be a simple solution and I hope to be able to find it. Thank you kindly in advance for any insight and information.
 

JMan

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Oct 26, 2019
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Hey All,

Really enjoyed building this unit!

Quick question regarding DC voltages on the op amp connections. I'm getting the +/-16 without issue, but I have some very small DC voltage readings at all of the other points as well (this is with everything built, but op amps and green LED driver IC not yet installed). I'll list them below. Do I need to go hunting for a trouble spot?

Preamp
C = -7.3 mV
+ = -7.3 mV
0 = 251 mV
- = 251 mV

Fader Booster
C = -7.3 mV
+ = -7.2 mV
0 = 251 mV
- = 251 mV

As you can see, there are obvious relationships between [ C & + ] and [ 0 & - ]. I'm not sure if these are things that will resolve themselves once I install the op amps, but I don't want to do that before I know more.

EDIT: I guess this is unnecessary information since they're connected, but the 0 and - voltages match what I'm reading on the positive side of C18.
EDIT AGAIN: So, I should say, these voltages are all in reference to chassis ground. I'm not sure what I should be looking for, or if I should be measuring between different points instead. Since I can't find any documentation of this part, and I've not built one of these before (nor in fact anything with a discrete op amp), I'm just feeling uncertain.

Thanks for any help you can offer!
 
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JMan

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Well, I think I didn't have anything to be concerned about. I went ahead and plugged everything in. AC voltages were good across all test points, and holy crap, this pre sounds beautiful! I did just a simple test talking into an SM7b -> cloudlifter -> VP28 -> converters and messing around with the knobs, and I was just floored. Like, I usually have the thing where you hate the sound of your own voice on a recording, but I actually liked listening to it here! This is my first outboard preamp (not including my interface's, of course), and I was skeptical that it would make a truly meaningful difference. Consider me humbled. Jeff, thank you for your awesome work with these projects!
 

Dolbah

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Jun 6, 2021
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Greetings
I just finished 2 x VP28s. All well and everything sounds good! But i had a accident while soldering and shorted the circuit plates of LED4 (no power on) and when i tried to remove all the excessive solder i managed to loosen the positive/anode plate on the PCB and it fell off.
So, i then bridged the parts instead, but still i got no light in LED2 - LED4 (yellow ones).
The red one works and of course the green which i not releated imo. All releated buttons also work as they should.

So here is my bridge:
Screenshot 2021-06-12 at 16.39.13.png
I thought this would remedy the issue, but obviously not, and i can't seem to find any schematics so i can only work from the visible curcuit paths. Does anyone have an idea if i am missing a bridge here?
 
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Potato Cakes

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Meter which solder points on the switch of the working LED are connected to the the + and - solder pads of the LED. Then apply to the broken switch. Also, are your LEDs oriented the correct way?

Thanks!

Paul
 

Dolbah

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Meter which solder points on the switch of the working LED are connected to the the + and - solder pads of the LED. Then apply to the broken switch. Also, are your LEDs oriented the correct way?

Thanks!

Paul
Hi Paul
Thanks for the pointers. This is exactley what i've done, and then i ended up bridging the connections as shown in the last picture. By measuring my other VP28 which works 100% fine and comparing, i have now the exact same connections between the switch and the broken LED on the module with issues, but no light.

Yes, my LEDs are installed correctly.
 

Dolbah

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Hi Paul
Thanks for the pointers. This is exactley what i've done, and then i ended up bridging the connections as shown in the last picture. By measuring my other VP28 which works 100% fine and comparing, i have now the exact same connections between the switch and the broken LED on the module with issues, but no light.

Yes, my LEDs are installed correctly.
Update: I am measuring a short between ground and the Cathode on my Red LED1, this measurment only occurs on the unit with isuues, not on the working one. Any hints on usual suspects are much apprechiated as i can not find any visual shorts at all. Ironicly the Red LED works fine
 

jsteiger

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Are your leads too long, hanging off the back of the solder pad and shorting to the ground plane? I've seen people do this many times.
 

Dolbah

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Are your leads too long, hanging off the back of the solder pad and shorting to the ground plane? I've seen people do this many times.
Hi Jeff

No, they are not.
I did however do some measuring yesterday, and traced the issue to the SUB-HPF board. I think there must have been a hairline-shortage or a bad connection.
Sadly i did not have any spare angled pins (or patience 😂 ) , so i heated up my desoldering pump and went to action. I have desoldered bigger rows of pins before with success, so full of condficence in my desoldering skills, i of course wrecked a couple of rings on the the main PCB (well deserved i must say 😅). I Got the HPF board loose, went over it and resoldered it.

And this fixed the first issue, but now my problem is that i got a bad connection between HPF board and mian PCB, My status is as follwing:

- LED 1-4 works if i press the HPF board against the main PCB, this holds for a while, but suddenly looses contact and then all LEDS dissapear. Even the RED one which was not affected before.
- Soundwise everything seem to work 100%. Even the filters which also works even though the LEDs are out and the bad connection is in effect.

So it seems its only the circuit that gives power to LED1-4 that is bad (lucky me!)

My fix is that i should make a bridge between the ast point before the bad connection, and the next point after.
However, i am not sure which points to bridge, and the more i try to trace my way with my DMM, the more confused i get.
Could you nudge me in the right direction based on my description?

The black arrows in the pic points to the points that got the ruffest treatment when desoldering, and are most likely the cause of my current issue.

Screenshot 2021-06-13 at 18.37.41.png
 

jsteiger

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The HPF PCB has nothing at all to do with the LEDs functioning. It sounds like you have a one or more bad solder joints on the main PCB and the physical pressing of the HPF board gets them to work. I would guess its on the Toneluck switches.
 

Dolbah

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The HPF PCB has nothing at all to do with the LEDs functioning
No, you dont't say! Here i finally felt that my tracing and errorchecking was getting me somewhere 😂


Well, i can gladly confirm that we got constant light!
First i reflowed every joint on the PCB for the third time, and even though i have desoldered all LEDs earlier, testet them with my DDM and resoldered them and it was actually the red LED cathode that was the one doing the trick this time.

I still can't get my head around it though, but there must have been a couple sketchy joints in this process, that LED me astray 😉

Thanks for all the great nudges!
 
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Cappy

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Jul 10, 2021
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Hello Friends.
Dumb question. I’ve starting building one of two VP 28’s. There is a small set of ground pads at the north end of the main pcb. I couldn’t find documentation on it. In the side view picture on the VP28 page it looks like the two pads are jumpered. Is this the case? Thank you kindly!
 

jsteiger

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Hello Friends.
Dumb question. I’ve starting building one of two VP 28’s. There is a small set of ground pads at the north end of the main pcb. I couldn’t find documentation on it. In the side view picture on the VP28 page it looks like the two pads are jumpered. Is this the case? Thank you kindly!
Those pads are to make an easy spot to hook a DMM probe onto. Just loop a cutoff lead from a diode or large cap thru them leaving enough room above the PCB to hook your probe on.
 

mrgrooves666

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Feb 1, 2021
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Hello, just finished builds of two VP28 with Litz. Enjoyed the project, the first build went smooth as silk. But with the second, when plugged, the 10 Ohm resistors for the fader booster opamp power rails burned inmediately. Upon closer inspection, I discovered I switched BC161-16 & BC141-16 transistors even If I had read the addendum of changes with B version. So Jeff thanks for thinking ahead of us retards and adding those resistors. I was anxious to finish and try these, I didn't double check the second build as I always do, lesson learned the hard way hah.. I managed to switch both transistors, put the opamp back in and fire it up. Other than the burned resistors, the unit seems to be working fine and no smoke is coming out of it lol. I already ordered replacement resistors, but here's my question. I'm concerned if the opamp got screwed, or if some of the excess heat for removing the transistors could have deteriorated them, should I replace this opamp with a clean build one? As I mention, the unit seems to be working fine, did't powered it for long, but I did do a quick test with dynamic and condenser mics and it appears ok....is it safe to use the preamp as is while I wait for the resistors to arrive or shoud I just wait....really anxious to put these to work lol...
Thanks for your suggestions..
 

Clbraddock

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Why are the 2N5087 transistors painted with 3 lines in the ca-0252 op amp kit? Is there supposed to be a matched pair in there? I didn’t see anything about matched pairs in the bom/instructions.

Edit: never mind. I see this was asked in the ca-0252 thread and the reply was that placement doesn’t matter
 

Clbraddock

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So, finished my build and man this thing sounds awesome!

One prob though (totally of my own doing) when building I got in a rush and solder the 2 way -6 -12 switch on the wrong side. In the process of getting it off I more or less destroyed the pads. As such I ended up jumping it’s feet with wires to the other pads I think it’s supposed to go to. The front two pads I didn’t connect. See pic. The HPF works but if it gets bumped wrong all sound drops out until I bump it again. I must have a bad connection. My question is are my jump wires correct or do I need to jump anywhere else?
Thanks!
 

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jsteiger

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Those are correct. You likely have a bad solder joint somewhere on the build. The 5-pin 90˚ header to the main PCB is a common place of cold solder joints...for some reason.
 

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