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

[Mrosso]

DMP,
Thanks for the help. I dialed in the power supply voltages per Classicapi's documentation. The power voltages test good using Fluke 179 DMM. I have four other 500 modules all working fine in rack too. I have a couple of Hardy 990 DOAs I can swap from my Lola, although a bit worried about damaging them. Good advice all around - I'm trying to follow traces, etc., but I'm a real newbie at troubleshooting and electronics in general -- that's why I really prefer building the kit builds (SCA, hairball 1176, etc.). I'm going to replace the cap as soon as I can dig one up (probably have to order from mouser), but I'm not sure how it fits into the circuit.

Thanks again!

 

[jsteiger]

Like DMP mentioned, that is the DC blocking cap before the gain switch. Hard to believe there would be enough V on that cap to cause that. Improper seating could cause some issues but don't think this would be a result.

Swap out the opamps with other known good ones just to rule them out. Gary burns them in and tests them before shipping to me so they should be fine but best to be positive.

You can desolder the blue and black leads from T2 and use those as a balanced output point to test the preamp section. That will remove the filter and 2nd stage from the equation. This will help narrow down where the issue lies.

 

[dmp]

The orientation of the cap looks correct, based on the other pics by Chunger and the PCB pic on classic API.

First thing to do before swapping in other DOAs is measure the voltage at the +V and -V socket with respect to the C socket (ground).
If those are +16v and -16v your DOA should be safe. You can check the + - inputs and the O output also if you want to be REALLY safe.

As you are following traces it will help to know what the building blocks are. If you look at the API 312 schematic on Jeff's site, theVP28 is going to basically have something very similar as the first stage. Input at the card edge connector goes through the pad and into the primary of the 2622 input transformer. Then from the transformer secondary to the DOA gain stage and out to the first output transformer.

After that I believe it goes through the high pass filter and the fader, followed by the second DOA gain stage and out to the second output transformer. It looks like a lot of components but once you start breaking it down into different blocks, it will make a lot more sense.

If you can make up a test probe, I could suggest some places to probe the circuit to see where you are losing the signal.

 

[Mrosso]

Quick update - measured DC volts between on each DOA socket +V to C and - V to C, all ok - both sockets give me between +16 and -16 on my DMM. Swapped John Hardy 990 DOAs and still nothing. Rechecked rack voltages again for sanity sake, still all correct, incl +/- 24v rails. Will try to trace per dmp's suggestion. When you say test probe, I'm assuming that is something to trace the AC audio signal, no? How do I make one of these? Starting to get above my level of competency.

 

[mulletchuck]

test probe means you run some AC signal (like a -8dbFS sine wave from your DAW) into the unit, and measure with your volt meter along all the PCB traces to see where the signal stops or disappears. 

 

[dmp]

You can also make a test probe that hooks up to a little amp/speaker. That way you can hear the signal (or not)
Basically you use a capacitor to probe different areas in the circuit with music hooked up to the vp28 input.
There's a picture here:
http://www.diystompboxes.com/pedals/debug.html

 

[clemfuture]

Hello everybody !

I just started diy (i only did cables before!!) and I just finished two vp26 with good results they sound incredible ! Thanks Jeff !!!
I also ordered two vp28's and I just finished the first one. Takes much long than the 26... But it was fun.
I had some issues with it and maybe someone could help me to fix it.
I explain you my woes:
Everything was quiet all right unil the sw6 switch, it was very late in the night and I made wrong with it, when I tried to placed it, i broke a pin, then i continued my work without taking care of it. I finished the all thing as chunger did on his thread and finally tested it with a mic.
And unfortunatly, no sound came up !!! I noticed at that point that I made something wrong and discovered the issue.
So I'm wondering before going back on it if it's possible that I blast a component plugging it in the lunchbox with the broken sw6 ?
Then I moved away the broken switch and before putting a new one instead, do you think I damaged my vp28?
For information the green led never switch on too ! The overs have no issues.
Hope someone had the same trouble than me and will could help me with that, it's so frustrating
Thanks
Clement

 

[Mrosso]

Update 2 - dmp, man thanks for the tip on the signal probe, that was a huge help. I have one of those ribbon-cable extender tab things from JLM, so I can have the module connected to the rack but outside of the rack case. I probed the circuit using the 312 schemo and a lot of guessing - basically flipping between vp28 bom, pcb, and 312 schematic. I was able to trace the signal bit by bit from input leads on the card-edge connector all the way to output leads. I got signal at pin 1 of the input transformer, O pins of the two opamps, green & brown leads on the rear output transformer, and finally the outs of the card-edge connector. I was excited and a little bummed - looked like I may have jostled something loose that was shorted - preamp may work but bummed I didn't ID a problem that may happen again. Next, re-packaged everything back up in the aluminum frame, attached knobs, etc. plugged into rack and .... NOTHING! Same f-ing problem. This time I got some strange thumping-like noise when I pegged the gain, but nothing else. This is so f-ing frustrating! Anyone else have any ideas?

 

[jsteiger]

It's probably a cold solder joint somewhere on the pre.

I would pull the module out but leave it assembled and use the extender. Maybe you can get lucky and find the culprit.

 

[Mrosso]

Success! Finally! It was that damn C1 330uf cap!

 

[HummingPumpkin]

After a long wait on a chassis, I was finally able to test out my recent VP28 build this evening.  I'd like to say it worked the first time, but that wasn't the case... although very close.  I actually bought two kits, but still had yet to do that last pair of opamps.  Anyhow, I stuck the complete build in the chassis (I chuckled a little at the irony of it going into an actual API lunchbox...) and found that I had signal, but it was crazy distorted.  I took a step back and calmly cursed for a minute, and then decided to test the the other build with opamps to get an idea if it was the preamp or the opamps, potentially.  I popped the first opamp and quickly saw that there as a wire coming off a resistor that I had evidently forgot to clip.  The wire was actually pressing up against one of the pins that plugged into the main board.  Whoops... but actually an easy fix!!  I decided to proceed with the swap and popped the second opamp.  Guess what, I missed the same wire again!!!  It just hid right next to that pin.  Clipped it, put the preamp in and heard clean, wonderful tones!!  It was a quick/lucky find and a very easy fix.  I can see why one of the first recommendations offered up is to pull the opamps when debugging.  I swapped the other build back in and that worked as well.  Just need to finish the other two opamps this weekend and I'll have a stereo pair, can't wait!  The effects of those unclipped wires touching the pins on the mainboard were "fun".  The MIC button pushed in gave you crazy distortion, pushed out the signal was much cleaner, but obviously very quiet.  The PAD and 48 switches worked as expected.  When the signal light went on, all the other lights went off.  The lights were also kind of weak.  Clipped the missed resistor wires and it's all normal.

Even though it's my first post, I'd like to pass along a few comments on the build for any other newbs out there thinking of grabbing a VP28.  It was way more time consuming than I initially thought.  Sure it's just soldering a bunch things on a board, but each step takes so much time.  Sort, map, install, solder, clip, repeat and repeat and repeat...  Chunger has a great set of instructions in this thread.  I found them highly useful when I had any doubts, thank you, sir!  And by the way, Jeff S is super responsive and helpful.  I had one question before I bought and one question after I got them and Jeff answered via email maybe in an hour or less in each instance.  Anyhow, back to the build...  I recommend reading these instructions carefully before you buy (and also making sure you are confident with a soldering iron... if you don't know what a cold solder joint is, learn that first and practice).  I pre-made my resistor/cap/diode sorting sheets (as Chunger displays) which helped me get an idea of what I was getting into.  Once I got the units, I sorted all the parts first and taped (painters tape) them down the sheets to keep everything together and organized (it's amazing how those resistors can fly off the table just by walking by).  By the way, I only put tape on the ends that I knew would get clipped off.  That was day one.  The second night I did a marathon run and put the main board together.  The third night I put the opamps together side by side (gar2520s).  I actually would recommend against the marathon runs.  I see people say they did it in this or that time, and that's great, but if you're new, take your time.  It's so easy to get tired or impatient and make a dumb mistake.  It was definitely easier the second time around.  The opamps were not as bad as I thought they might be.  Between Chunger's pix and the included diagrams I was able to put things together with confidence.  You just gotta be careful to keep the soldering extra clean on those little boards.  It's definitely nice to have a "helping hands" type of holder to hold the boards for you.  I can't wait to spend some more time on them this weekend.  I'm looking forward to trying out Jeff's other products and all the other things out there in the DIY world.  Good stuff!

 

[biosynth]

Hi all,
i've just finished my first VP28, such a beautiful piece!  :-*
faceplate is really high quality, just as grayhill pots (suiss watchmaking) and PCBS. It's very pleasant to work with great material.
Jeff you seems to like good quality, it's very sensible and we are grateful to you.

A little note with knobs, which are too small in my point of view. I (we?) like to have big knobs in your hands, especially if it rocks!
(do you know where to buy big API knobs?)

congratulations Jeff, and thank you.
phil 

 

[Unit7]

In the middle of the build of my first VP28. Very fun and relaxing thing to do before going to bed. Love the kit, and Chungers photo manual is just great.

I'm curious about that double-stick tape for the 2622. Is it necessary? What is it good for?

Cheers
Paul

 

[jsteiger]

Paul, I use it more as a spacer than anything. I don't pull the protective paper from the side that touches the transformer's belly. That makes the 2622 hard to remove if it needs to be done for any reason.

 

[Unit7]

Paul, I use it more as a spacer than anything. I don't pull the protective paper from the side that touches the transformer's belly. That makes the 2622 hard to remove if it needs to be done for any reason.

Thanks Jeff!
Spacer you say - heat related then?

 

[useme2305]

the spacer keeps the transformer housing away from the solder holes to prevent contact and shorting.

 

[Unit7]

the spacer keeps the transformer housing away from the solder holes to prevent contact and shorting.

Brilliant, thanks!

 

[Unit7]

Ready! This was my third kit ever, and after Horvitz's chorus and a pair of eqs this was a breeze. I'm sooo addicted. These kits are so beautifully laid out and fun to do. Just love sitting at the table behind our TV sofa, breathing the soldering fumes, and felt almost disappointed tonight when the VP28 was complete. Such a nice pre-go-to-bed routine. Luckily another pair of TB550s are on the way in, and then LC53As for sure.
Gonna test this baby tomorrow (w GAR 1731/2520) and compare to my AMS/Neve reissues, and then decide if I'll have to order more VP28s... Kudos to Jeff and to Chunger for the photo manual.

 

[Unit7]

Everything seems to work here.  Less noisy, but slightly more hum than my AMS/Neves, but that could be my studio (very old building and unfortunately a small issue with magnetic fields). I took an SM7, used a Y-cable (which perhaps isn't kosher) and fed both pre amps. They sounded very very close which is pretty impressive regarding the price difference. Perhaps the VP28 sounded a tiny bit brighter and lack something in the low mids, but it was so close I believe I would have failed in a blind test. Look forward to try it on drums. The two stage design is just awesome.

Only question: My unit had clearly more hum when engaging the phase shift. Anyone else have this?

 

[qmp audio]

Only question: My unit had clearly more hum when engaging the phase shift. Anyone else have this?

I cannot hear any noticeable hum at all on any of mine