[BUILD] CAPI VP2X~500 Series~Preamp Kit~Official Support Thread

"When trouble shooting like this, the module needs to be flat on a bench or table, laying flat in front of you. It cannot safely, effectively or scientifically be carried out while installed in a rack."

>> how do i get power to the module if it isn't in the rack?  you said it needs to be under power?  could i find some other type of power generator? 

Not sure what "cap" you are referring to.

>> oops, i meant resistors.

An audio mic level signal needs to be applied to the typical input for the module. This needs to be something like a 1k sine wave from a signal generator of some sort.

>> that old EL instruments circuit design system / signal generator i got doesn't power on...  after i learn how to fix the preamp, i'll try to fix the instrument that could have been useful in helping to fix the preamp (or itself).  i guess i need to buy some kind of signal generator in the meantime then, yeah?  would you recommend buying a scope?  would it make sense to invest in these things if i plan to continue building equipment from these pages?  Also, is a signal listening probe something you could build or is it something you'd have to pick up?

in this description (which sounds safer to me, to have the unit not powered) --

1) a signal source (signal generator, ipod etc) with a cable that ends in alligator clips or some other type of clips
2) a signal probe to listen to the signal source
with the opamp removed and the preamp NOT powered, sitting on your bench, check the input up to the opamp non-inverting input:
A) connect signal source, to "In Hi" & "In Lo" (pin 10 & 8 on the edge connector, or the side of the 750R resistors that doesn't connect to the 160R resistor)
B) connect the 0v/ground of the probe to a 0v point on the preamp.
C) probe the non-inverting input pin on the opamp - you should hear the signal source.
D) if you don't trace the signal from the input, following the schematic - see where it stops.

my questions on this -- is HI the 6th gold finger down from the top of the unit?  what point is a 0v point on the preamp?  also, which is the non-inverting input pin?

sorry for the young questions.  thank you to the electronic gurus (especially jeff) -- thanks for teaching.

i set my dmm to continuity.  i followed every point in the schematic.  every point beeped. 

anything that even looked like a slightly cold, non-shining solder i re-heated.

have been working on this project every day for a month now.  at a loss.

Continuity is not the exact sort of test to do be doing here. You will need to apply a balanced, mic level audio signal to the input. A line level sine wave can be used if you create a U-pad with the resistors I described earlier. From there, you will need to follow that signal thru each and every component to see where the signal stops. Use the schematic as your guide. Most likely, whatever is just before the point where the signal stops is the problem.

alright -- did more homework.

first off, for anyone wanting to build a signal / listening probe, i followed this guide -- http://www.seymourduncan.com/tonefiend/diy/three-ridiculously-useful-easy-cheap-diy-tools/

successfully built that guy/gal, but instead of soldering cables to the ends, i soldered alligator clips.  connected the jack, rca, into a speaker.  then took crappy headphones, clipped them, and soldered alligator clips to the ends where the earbuds used to be... connected that to an ipod.  thanks haima!

input test -- connected signal generator to pin 8 & 10.  signal probe to ground and then started searching around.

my findings --
on the input side... first off, i was worried my diy test equipment was made wrong at first as i wasn't hearing any signal from pin 8 & 10... then went forward and connected the alligator clip to the side of r11 and r9 (the 750r resistors) that didn't connect to r10 (the 160r resistor)... then i was able to get signal throughout the pre.  when putting the alligator clips back on outside gold finger pins 8 & 10... i was able to get signal, but the hi (pin 10) was very finicky. only if the signal generator (alligator clip) was placed just right on it, would it produce signal through the preamp, which was my first note.

on the input side i got signal almost everywhere... probed through the different points on the schematic, moving along the electronic highway.  however, on the input stage... there was a ton of noise and the signal i got was very quiet almost everywhere with a lot of hum as if something weren't grounded right (even though i connected the ground of my signal probe to a ground point (one of the gold outside rings of the preamp)).  some parts of the signal were much louder than other parts though.  for example, pin number 5 of the input transformer connected to c1 (220pf) were among the loudest and clearest!  i can't wait to get a pin 5 input transformer sound out of this preamp.  and further, for example, that side of the c1 cap was louder than the other side of it connecting to r2 & r2 was much quieter.  the side that connected to r3, was much louder and clearer... and r3 was much louder and clearer.  r3 and pin 5 of input transformer were the clearest of this test.  however, i got signal pretty much everywhere (with these different volume levels)

on the input side -- i heard signal on every op-amp lug.

there are only 2 places i had strange signals.

1) all 4 connectors, which solder in the output attenuator have no signal... unlike the connectors soldering in the input stepped gain, which all have signal...  (however the actual connectors underneath the output attenuator all passed signal... just the 4 outside ones didn't)

2) cr1 and cr2 -- the sides connected to pin 12 and 14... i mostly hear buzzing on both of them (and only a tiny bit of signal).  on the other sides (connecting to c6 & c5), i hear some better signal though.

for the output test -- connected the signal generator to O on the op-amp.  the other side of the signal generator to ground (gold circle on side).  connected the ground of the probe to the same gold circle... and used the other side to probe around.

on the output side of things -- wow!  testing the output... the signal is MUCH louder... it has NO ground buzz... super clear all the way through the circuit.  get really loud, clear signal throughout the whole preamp.

again though, the 4 solder points that hold the output attenuator in... have no signal on the output test.  (and again, the actual connectors underneath the output attenuator all passed signal... just the 4 outside ones didn't)

also, cr1 and cr2 connected to pin 12 and 14 send signal, but extremely, extremely quietly compared to all the other components.

looking forward to your thoughts!  have signal everywhere (of varying volumes), except for those outer 4 on the output attenuator and much reduced signal from cr1 and cr2 connected to pin 12 & 14.

THANKS!

ps -- if these results happen to be normal -- how do i test the rack?

Matt

Sounds like you made a little headway. You are on the right track now.

A few things to point out. The only place you will want to connect your signal generator are the main inputs of the card, gold finger 8 and 10. There is no need to ever connect it anywhere else, IMHO.

Out of the 4 larger mounting holes, only the top right (near the output transformer) connects to the ground plane. The rest make no connection by design.

I see no reason to ever connect your listening device to the power rails like JP1-12, 14 or anywhere on CR1 or CR2. You should only be concerned with the audio signal path.

Pin 5 of the 2622 is it's output which boosts the signal at the input by 17dB.

From there, check at the opamp input or the left side of R4. The signal should be nearly the same as pin 5 of the 2622.

Next, check the + side of C10. This is the output of the opamp. The gain pot (or switch) should effect the level of the signal probed here.

From there, I would check the cards output at card edge 2 and 4. If you have good, strong signal here, I would start looking at the rack & cabling. There you can use your DMM set to beep continuity.

The 4 outer pins of the t-pad are only the bracket. They are there for mounting only and carry no signal.

Cheers, Jeff

thanks jeff!

troubleshooting has been an awesome lesson so far -- thank you so much again for your time.  sorry for listening on the power rails... i followed to about the op amp along the schematic highway and then just started poking around to hit every component.  ha.  thanks so much, i will check those readings... but first...

just checked the rack.  on every channel, at the xlr on the back, pin 1 beeps out at 1 (input / output)... but i'm not getting a continuity beep on any channel 2 with 2 or 3 with 3 at the xlr...  is this correct?  i can get 2 and 3 (on both input and output) to beep when touching different points on the card slot though... so maybe this is normal.

thanks again jeff!

with the new grounding... my results are very different.  actually, i have no signal at r4 at all.

there is also no signal at other components i.e. r6 (pin 5 of the input transformer still sounds great, but now more than half of the components carry no signal)

the problem starts (at the beginning of tracing things in).  the side of r8 that connects to r7... continuity between them is good, but i have no signal either on r8 or r7 (the two sides that connect)... strangely, the other side of r7 has signal.  but i guess it could be picking up signal through the pad? 

well it seems like there's one place to resolder at least, yeah?  the solder point is good and shiny though.

Not all the components are series components. They do not all have audio passing thru them. Forget about R6, R7 and R8 right now. Those are the phantom power resistors. I never said to probe for signal there. I did say to probe the left end of R4 though. Did you do that?

If pin 5 on the 2622 sounds great, that is good. You need to follow the schematic from there. Where does the signal go from pin 5 of the 2622? Hint, it is going in the direction of the + or non-inverting side of the opamp. That is why I told you to probe at the left end of R4. I never said anything about R6, R7 or R8. There are only two series components between the 2622 and the + input of the opamp. Probe all ends of those and report back. Also, tell me what their labeled names are so I know you are following me.

I also never heard about you verifying the rack. That is a must do right away. I am sure you don't want to chase your tail thru a preamp to possibly find out the problem is not preamp related.

Hey Jeff -- thanks man !  sorry, please see my responses above the last one I made -- confusing as I responded a few times above as I got results (r6, etc was an after-thought post).  I had checked r4 and said there is no signal on either end of it (reply #366).  Also wrote a bit about the rack in the post above that one (please see that post (reply #365) to see my results trying to verify the rack).

R3 has good signal.  C2, c3, and r4 have no signal.  I resoldered all of their points again.  All still shiny (not cold solders), but no change in their passing signal.  Thanks again man!

Only the signal path side of R4 will have signal. They other end is to gnd so no signal will be there.

If R3 has signal at both ends that is good.

The next thing in the signal path is C2. You say no signal is there but which end? At both? If you have no signal at the - end of C2, you will have to look closely because there is only a small trace between R3 and C2. Really no good reason unless that end of R3 is a cold joint.

One step at a time since you never specified if one or both ends of C3 had no signal.

As for the rack thing, you need to be specific.

...beep when touching different points on the card slot...

Click to expand...

does not really help.

The input XLR pin 2 should go to card edge 10.
Input XLR pin 3 goes to card edge 8.
Output XLR pin 2 goes to card edge 2.
Output XLR pin 3 goes to card edge 4.

That is info that you can get from my schematic or any 500 series pin-out legend.

There is no signal on either side of c2, c3, or r4.  Neither side pass signal.  When desoldering and resoldering, I noticed that I might have lost that gold pad on the bottom of c2.

Well, somewhere in there lies your problem. Your signal at both ends of C2 should be the same as it is after R3. If not, you will have to solve it one way or the other.

The thru holes are plated on both sides of the board. Loosing the side where the trace that runs to R3 is could be the issue. Without looking, that is most likely on the bottom. At the worst, just solder a short jumper wire in between R3 and C2 then run your signal tests again.

just wanted to give an update --

it did end up being that trace -- soldered in a jumper cable and now it sounds so good.

since this, i built 3 more preamps -- vp26 and 2 vp312... all i have to say is... they SOUND Amazing! ! !

thanks so much to jeff...  you are not only a genius designer, but an amazing person for giving us so much for your time.

next project -- building some gar2520 op amps

jsteiger said:

It's most likely a cold solder joint somewhere. I would start by touching up the 9 interior pads of the t-pad adapter PCB as well as the pads for the 3 short leads. If that doesn't do it, try wiggling around the 3 short leads to see if there is a direct effect on the problem. It could also be the leads for the output transformer.

FYI, I was troubleshooting a build once and had a similar issue when the gain knob was touched. It was the GH on the main PCB stepped version. Naturally, you start looking at the switch and the R's around it. Turned out to be a half iffy solder joint on the 470uF coupling cap, halfway across the PCB.

Click to expand...

So i retouched all the solder joints that appeared even remotely unstable or less than solid but unfortunately I'm still having the problem. Audio passes through completely but its very very quiet. I think i may need to order a new t pad and tpad adapter but in the meantime I didn't know if these pictures would help point out any obvious I may have missed (still very much so a newcomer when it comes to this!)







thanks so so much all!

You can bypass the t-pad to rule it out. Just remove the t-pad by desoldering the 3 leads at the main PCB. Then solder a small jumper wire from pad A to pad C while not touching B.

If the symptoms are the same as before, I would skim backwards thru this thread. I had the last guy inject a sine wave signal into the pre and trace along the audio path to see where it stops.

You can also measure the DCR of the output transformer and test to make sure nothing is internally shorted. That is all explained and very recent in this thread as well.

Good luck and keep us posted.

would definitely recommend the signal generator that jeff is talking about !  you can diy build one -- posted a how to link above.  really worth it to have a signal generator + listening probe (and you can build them for 5 bucks if you already have an ipod or phone or cd player or anything with an audio out, etc)... i used to test old gear with a dmm testing for continuity... but a signal generator and signal probe gets you to the problem much faster... (plus you get to hear some bits of tunes passing through your circuit along the way)

jsteiger said:

You can bypass the t-pad to rule it out. Just remove the t-pad by desoldering the 3 leads at the main PCB. Then solder a small jumper wire from pad A to pad C while not touching B.

If the symptoms are the same as before, I would skim backwards thru this thread. I had the last guy inject a sine wave signal into the pre and trace along the audio path to see where it stops.

You can also measure the DCR of the output transformer and test to make sure nothing is internally shorted. That is all explained and very recent in this thread as well.

Good luck and keep us posted.

Click to expand...

Thanks again for the continued help!!

Removed tpad and soldered jumper from A to C (no b!) but no improvement unfortunately....
Removed  output transformer and measured resistance on the 2503 and all were 8.6 Ohms...

next step seems to be the signal tracing with an audio probe/signal generator along the schematic path and seeing where it stops....

I will keep you posted.
Thanks

Hello everyone,

Newb here who just completed my first VP26 pre. First, let me say WOW! This preamp sounds incredible. I tested using a Neumann TLM 49 which is a great mic but relatively flat sonically. I was instantly amazed at the new character and richness I was hearing!

But, my build is not without faults. The output attenuator does not attenuate. I get maybe 1dB of attenuation about half way towards -infinity and no more after that (it might even get slightly louder again at infinity?). Also, the phase switch in the pushed-in position gives a big boost in the signal; of course reversing phase can alter frequency repsonse and possibly the volume a tad but this is probably around a 6dB boost or so... seems something is wrong.

Other than that, the unit works (and sounds) great. I was hoping someone could point me in the right direction to begin trouble shooting. I've searched this forum to no avail. I'll try reflowing the solder on the switches and following the CAPI DOA install instructions as this seems to have helped others' initial build issues. But beyond that, I don't know where to start.

Here's some pertinent information:

- revB board (I used the correct new documentation, i.e., BOM, etc..)
- gar2520 op amp; built myself; I should mention I have good to great solder skills having built numerous guitar pedals and other electronics over the past decade.
- wired jumper from unused phantom switch to LED lead with arrow pointing to it.
- checked all resistor values with my DMM, all good there.
- when checking resistance at mil max sockets per assembly guide, I would initially get really low values upon contact that would take a few seconds to rise to beyond my DMM's metering ability (and well above the 200k spec) but it was weird that is would start low and rise. Not all configurations would do this.

Many thanks for any help!

Cheers,

Jason