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

[mysticmerlin]

....I have obviously checked to the best of my ability and there  is a weird 10.2 Ohm between C and V+ which seems quite abnormal except I can't find where that bridge is coming from.....

Yes this is very odd. It must be a short or solder bridge. There are only a few parts that connect top the +V rail. I would investigate these areas.

~Socket for each DOA (obviously), make sure to remove both amps to rule them out.
~A2 IC, check orientation and maybe remove it? Check IC socket
~C8
~Right lead of C2
~Right lead of R2
~Right lead of R3

Thanks so much , there was indeed a tiny solder leftover between C2 and R3 that was hard too see. I should probably lose the habit of soldering resistors on both sides of the pcb. It just makes it harder to spot small mistakes such as these .
Thanks Jeff, I was worried I would be stuck without a schematic to follow.
Great Sound I have to say.  I have expensive tube pres , neumann, 1073's, shadow hills, gml's  and the VP28 is the mainstay for our old fender basses , real clean solid low end , just special, (this is my 3rd build).  But I guess all the people in this thread already know this.
Cheers

 

[Commander Fluffypants]

OK...

I'm checking Jeff's instructions from last May (yes, this thing has been languishing for 9 months...)

Can anyone help me make sure I test it correctly?

It seems I am to have the VP28 in the jig, powered up and I am to put one lead from my DMM on gold finger 8 and the  other on gold finger 10 simultaneously. Both with phantom off and phantom on. As opposed to having one lead on gold finger 8 and the other on some mysterious ground or something and then testing gold finger ten the same way...?

Thanks, as always.

Are the input conditions identical when you are checking this? What is the source? Maybe there is excessive DC at the input of the module for some reason?

Popping sounds when switching usually means there is DC present on the switch contacts somewhere. Maybe there is a solder bridge or short somewhere with phantom or something? If it where me, I would measure for DC at gold fingers #8 and #10 with phantom off. There should be none. Then check again with phantom on.

 

[jsteiger]

OK...

I'm checking Jeff's instructions from last May (yes, this thing has been languishing for 9 months...)

Can anyone help me make sure I test it correctly?

It seems I am to have the VP28 in the jig, powered up and I am to put one lead from my DMM on gold finger 8 and the  other on gold finger 10 simultaneously. Both with phantom off and phantom on. As opposed to having one lead on gold finger 8 and the other on some mysterious ground or something and then testing gold finger ten the same way...?

To check for DC at the card edge inputs, connect the module the jig and under power, DMM set to DCV, red probe to gold finger #8 and black probe to ground. The gold pad adjacent to gold finger #5 is the easiest place to get a ground reference. Next check gold finger #10 to ground.

 

[Commander Fluffypants]

Thanks! That's what I needed. Built one 1731 tonight and reflowed the solder on the phantom switch on the VP28 that's working (not the noisy one.) Didn't have a chance to test it, but will do that soon and check the DC on the noisy one.

Thanks again.

 

[Golgoth]

My channel fader knob is a bit loose, it's coming from the grayhill shaft and that's not really pleasant to use. Is there a solution to "fix" it without replacing the entire switch itself?

 

[Commander Fluffypants]

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.

 

[Commander Fluffypants]

I was just working on my other troublesome VP28 that had a tiny bit of distortion that I had traced to a bad 1731. It also had a malfunctioning 48V LED. Reflowing the solder on the switch solved that problem. The noise that I was getting is gone... but I seem to have another noise. I don't know if it was present with the old 1731. When I tested this thing tonight, I switched through every step of each fader knob. When both are up really loud, suddenly noise kicks in. I recorded the sound it makes and twiddled the knobs a bit so you can hear how the noise changes. Both faders need to be on the highest two or three clicks to get this wonderful sound. None of my other six working VP28's do this. It's a lot of hiss that comes on suddenly, plus a little squeal that modulates at one particular setting.

https://dl.dropboxusercontent.com/content_link/SMTSvz4A3ExeQzbhW6dO6X9SUjeOzipgi0vPSJ8KweDDsOlKGUbvhxXoKNXjigIV/file

 

[Commander Fluffypants]

Hi there,

Hoping to make some progress on these tonight. Anyone have any suggestions?

Thanks.

 

[digitalblack]

Hello, helpful internet folk.

So I just built a pair of these. One sounds fantastic and the other has extremely low output. I read in an earlier post that this could be caused by a bad input transformer? Mine is indeed wiggling on the pins just a liiittle bit. Is there any way to test my theory without desoldering the transformer?

Thanks in advance for your expertise.

 

[vitalelectric]

Hey there, not sure if this thread is still monitored, but I have an issue with a VP28 I just finished a build on.

Everything seemingly works fine , however the Preamp Gain knob has no effect on the output level UNTIL you get the the maximum setting, and it overloads the converter like crazy.

The unit passes signal, and sounds like its working ok, but I obviously would like to have some control over this. Could be a resistor thing? I double checked all the solder joints, but have yet to do any testing.

Any help would be greatly appreciated, I wanna get this thing up and healthy! 

 

[collinTHEbrewer]

I've had this built for a while, and it was working to my knowledge though I was unable to do extensive testing at the time. Here's my issue:

My VP28 is acting as if there were a high pass filter on the output around 1K. I put some pink noise through it. See the images below. This happens at all gain setting in either MIC or LINE mode.

Any ideas on where to start troubleshooting? The actual high passes seem to work although the frequency content is so minimal in that range.

 

[collinTHEbrewer]

Any ideas on where to start troubleshooting? The actual high passes seem to work although the frequency content is so minimal in that range.

And I'm dumb. It was something weird going on with my 500-series rack...  :-X

 

[kante1603]

Any ideas on where to start troubleshooting? The actual high passes seem to work although the frequency content is so minimal in that range.

And I'm dumb. It was something weird going on with my 500-series rack...  :-X

Excuse me for being late to the party,but it looks or looked like a balancing/debalancing thing.
Often sounds like you described,thin and no bass.
What was the issue?


Udo.

 

[collinTHEbrewer]

Any ideas on where to start troubleshooting? The actual high passes seem to work although the frequency content is so minimal in that range.

And I'm dumb. It was something weird going on with my 500-series rack...  :-X

Excuse me for being late to the party,but it looks or looked like a balancing/debalancing thing.
Often sounds like you described,thin and no bass.
What was the issue?


Udo.

Wow it's funny you say that. The original issue I had was with the DB25 connector on my 500 series rack being loose. I corrected it and the low end response returned. However, I noticed in some subsequent tests that I was having the same issues whenever I used my DIYRE L2P to patch my line outputs into my mic pres.

I think I just realized the issue. I have been using a Behringer PX2000 patchbay since that's what I had lying around. It turns out the PX2000, unlike the PX3000, is unbalanced. What a dumb thing to just now realize.  :-\

 

[kante1603]

I think I just realized the issue. I have been using a Behringer PX2000 patchbay since that's what I had lying around. It turns out the PX2000, unlike the PX3000, is unbalanced. What a dumb thing to just now realize.  :-\

Better now than never,glad you got it solved!


Have fun,


Udo.

 

[Buffstuffbuddha]

I built a VP28 from CAPI and it works great but I am having one small issue. When phantom is engaged and I press the Line switch I get an audible pop. Not a suddle pop like when engaging a pad, it actually clips my converter. I checked for any bridges or otherwise weird looking spots but everything looks fine. Something has to be leaking DC then right?  A bad cap somewhere? I just really don't even know where to start troubleshooting this thing. Everything works on it other than that pop...I know this has been brought up on multiple forums but I have never seen anyone come up with a resolution other than some pins being bridged. I know that's not the case on mine so I don't really know where to start.

 

[jsteiger]

This is normal thing since there is 48V DC present on the Mic/Line switch if phantom is on. There is only one solution and its simple. DO NOT press the Mic/Line switch when phantom is on. Not sure of the resason anyways. By doing so, you are applying 48V to the output of the device you are feeding the VP28 with. Its sorta like driving down the freeway and switching your transmission into reverse. Sure you can do it but its not a good idea 

 

[kante1603]

Exactly what I wanted to say!


Since there's no button for it atm:


Like!

 

[Buffstuffbuddha]

So it should be a big enough pop to actually clip my converter? We're not talking about the same kind of pop you get from engaging the pad right? I may have to buy another one, you know, just to compare the two 8)

 

[jsteiger]

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