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

jsteiger said:

Hmm, could possibly be something with the first stage output trafo. I would desolder all the leads from T2 and check DCR from one lead to all the others.

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Ok, not exactly sure how to check that or if the reading would be accurate with my DMM but... I checked all against the green lead and every one gives me 0 except the blue lead with 24ohms... also measurable resistance between black and brown leads (around 23.2)

Pliplo said:

Ok, not exactly sure how to check that or if the reading would be accurate with my DMM but... I checked all against the green lead and every one gives me 0 except the blue lead with 24ohms... also measurable resistance between black and brown leads (around 23.2)

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Hello,

Here's a pdf from Jeff's homepage:
http://www.classicapi.com/catalog/images/gallery/2623/2623-1-specs.pdf
Look at the pairs and measure them,your secondaries seem to be correct.
Primary is red & orange.Should give a reading too.
After that measure connections between the windings,there should be no connection,the dmm will go out of range so infinite resistance.
I think your dmm showing "0" was meant as no readings possible,right?
If it shows a steady 0 or 0,xx ohms you will have a short,please refer to your dmm manual.
Just hold the probe tips against each other,this is how a short looks like.

Hope to have helped,

Udo.

Thanks, it sure helped! I have a Fluke 115, trying to locate the manual right now... No resistance between red and orange, shouldn't i get a reading from the primaries?

Pliplo said:

Thanks, it sure helped! I have a Fluke 115, trying to locate the manual right now... No resistance between red and orange, shouldn't i get a reading from the primaries?

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Yes!

Pliplo said:

Thanks, it sure helped! I have a Fluke 115, trying to locate the manual right now... No resistance between red and orange, shouldn't i get a reading from the primaries?

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Pliplo, send me an email and we'll get this taken care of.

Hi I just finished Building the vp 28 last week and love it! However, last night I was trying to use my condenser mic and could get a signal. The 48v red LED comes on, and I tested the mic on another unit and it was fine. Any help would be appreciated!

Eric

The LED is not powered from the 48V rail so it can illuminate even if phantom power is not being applied properly.

Like with everything, eliminate the rack slot and all associated cabling first. The return path from the mic is the shield of the mic cable which is pin 1 of the XLR. If that is floating somewhere for some reason, your phantom powered mic will not work.

Thanks for the reply Jeff, sorry it took me so long to get back. I tested different card slots,cords etc and the 48v appears to not be working. When I tested on another unit I have the mic/cord combo works. Everything else, as far as dynamic mic recording, works great.

Please post the VP26 issue in the VP2x build/support thread. Things get to confusing otherwise....although you are missing R1.

As for the VP28 build, do you have a way to power and test a module outside of the rack? If not that will be the first step. I have Extension Jigs at the store for this purpose.

I dont have a way to test it but I'll look into the test jig, thanks for the reply!

hey there, i'm new to the DIY builds, wondering if anyone can offer advice..I've built 2 VP28 kits, and i'm stuck on a problem:  when i rack them without opamps they appear functional(led's light correctly), correct voltage, but when DOA's are inserted the green LED is on constantly and a low hum is passed as the signal, the other led's do not illuminate when the respective button is pushed.  any suggestions on where to start?  would this be a DOA problem?  or am i perhaps measuring the voltage incorrectly and there's a short on the board..i assumed the voltage measurement would be taken from the DOA socket labeled (+) and (-).... thanks in advance! 

Sounds like an opamp(s) problem to me. First, are they seated properly? http://classicapi.com/catalog/DOA_Install.php
Next, who built them? If you scroll backwards in this thread you will see similar situations. Maybe looking at those will help you as well.

So after reading through a few pages I could not find a problem like mine.
This was my first ever soldering project, I knew it was a lot but read this build guide at least 100 times before submitting my order of 4pres. The only problem I ran into was using the multimeter and sorting all the pieces.
I built 2 within a couple days, first one was kinda weird but everything "looked"good (definitely placed a couple of the capacitors in the wrong spot). The second build felt great, after getting my technique right I flew through this one.

Then I received my lunchbox......
First test I ran a synth into each pre, got signal so that was a sigh of relief... but quickly turned into a different kind of sigh. I noticed the signal was extremely weak, when pushed to its max the volume was very low. Then tested the second one and it had the exact same problem!
Then came an actual frown when I plugged a sm57 in. No signal..... Then tried a tube mic with its own power source...no signal

Now here I am, wanting to throw my soldering station I'm the trash and having someone build my other 2 modules and fix the first 2.

Any advice guys?
Cheers

I see you have red dots so no need to worry about those. Rule out any outside possibility like cabling, converters and so on. This happens more than you can guess. I would then skim back thru this thread for a PDF that shows some test points. There will be some tips there about injecting signal and probing to see where you signal stops.

dherdesouth said:

Any advice guys?
Cheers

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Check the polarity on your caps.  Looks like c16 is backwards from the photo.
Also,  hard to tell from the photo but looks like you may have some cold solder joints.

I know this seems like a bonehead comment, but be sure the mic/line switch is positioned properly.  Also, looking at mine, that cap is backwards (The big blue guy between the green and red WIMA's)
Patrick

I see R's that are placed incorrectly. The 6k8's and 4k99's are definitely in the wrong locations.

Except of wrong parts in wrong places I think gunpoint recording is on track too,nearly all solder joints I can see seem cold or bad,at least on the filter boards.Guessing joints are from the same quality on the mainboards I would at least do a good reflow job (after all parts are in their correct positions of course).Maybe you can show us some clear pics from the solder side(s) too?

Best regards and don't give up-these preamps are fantastic (I have eight! ),

Udo.

dherdesouth said:

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Hi,

Your C16 and C20 were soldered in reverse direction.

BTW, I've just finished my first 02 VP28s, both at the same time, same stage from 10AM to 4AM and mis-positioned the Q7 and Q8 on 2 GAR2520 and 1 GAR1731 because I thought they are the same >.< I then had to de-solder and pulled them half way out, then trimmed the legs, swapped back to correct positions and soldered on the up-front pads. I tried to work as fast as I can, and both VP28s are working good now, but I still don't know if there's any heat affect to the transistors' operation.

I also had a reverse Phase LED which makes the other LEDs off but I figured it out.

Big thumps Up to Jeff, very excellent designs and very big, nice, detailed sound with these VP28s !

P/S: I have a VP312, too, not as big but sharper sound than VP28

Regards,

Linh.

Hi!

Does anybody here know if it would be possible to put NTP M100 DOA in the VP28? I have a couple of NTP 100 and it would be interesting to try them in the VP28, but there are a lot more pins on the ntp 100. Does anyone know the pins to use and if I have to do anything more (short some pins etc.). Also I think the NTP 100 are spec. to +/-15V, and maybe that´s also a problem?