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

[nuts]

Hi Jeff, i bought 2 VP26 kits with Grayhill stepped gain pots.

How am i supposed to mount the Grayhill pot (with dedicated PCB)  on this VP26 rev A.2 PCB ?

I found it can only be mounted upside down: Grayhill's pcb facing the ceiling if the VP 26 PCB is the floor. And  solder 2 wires in pin 1 and 3 from grayhill PCB to pin 1 and 3 of R14 position.

Am I right ?

 

[jsteiger]

Hi Jeff, i bought 2 VP26 kits with Grayhill stepped gain pots.

How am i supposed to mount the Grayhill pot (with dedicated PCB)  on this VP26 rev A.2 PCB ?

I found it can only be mounted upside down: Grayhill's pcb facing the ceiling if the VP 26 PCB is the floor. And  solder 2 wires in pin 1 and 3 from grayhill PCB to pin 1 and 3 of R14 position.

Am I right ?

Indeed you are exactly right nuts. I have pics on my site that show it just as you describe.

 

[nuts]

Sorry i didn't seen it, thanks for your answer.

 

[OOF]

Hi Jeff,
I built a couple of VP26 modules a few weeks ago and one is perfect while the other often gives me a bit of fuzzy distortion along with the signal.  the signal is loud and pretty clear compared to the working module but there is the distortion along with the audio.  i have opened the module twice now and resoldered all points, especially on the stepped attenuator, but still have the problem.  Everything else functions as it should.  It sounds great when the distortion is gone, but then it will appear, so it can't be used in session.  Any thoughts?  i can pop it out and take some pics if that would be helpful.
Thanks very much for such a wonderful sounding pre.  i've sold all of my 512s since yours sound so much better. 
David

btw- i LOVE the verification questions need to post.  you need to know your American history and astronomy!

 

[jsteiger]

David

What opamps are they and who built them?

Are they seated correctly? Take a look here. http://classicapi.com/catalog/DOA_Install.php

If they are pre-builts from me, they were either built by Scott or Gary. They both test and burn them in so probably won't be the opamp if it's a pre-built. If the opamps were kits, that will most likely be the issue. Swap the opamps to see if the problem follows the opamp or stays with the pre.

Cheers, Jeff

 

[andreconde]

I'm sorry if this is not the right place but... is there anyone selling these kits in Europe or do I need to order directly from the USA?

Cheers,
André

 

[OOF]

Hi Jeff, thanks for the reply.  the red dot was pre-built and i did follow the document on installing.  i've already switched the op amps between the 2 pres and  the problem stays with the pre.  i did re-solder the pins for the op amp but no improvement.
I've had this sound before with other gear and generally it is a poor connection- sometimes even just a dirty patch cable at the console- but i have touched up all of the solder joints twice now, even sucking the solder from all of the points on the stepped pot and re-doing them.  Still can't find the issue.  wondering if it could be something internally on one of the 2 pots.  is it possible to bypass them to test.  i remember reading that on an earlier thread, but i think it was for the non-stepped pot bridging points A and C (stay away from B!)
thanks again for your help.  I've got some downtime this weekend and aim to solve it once and for all.
thanks again for the help,
David

 

[jsteiger]

... i remember reading that on an earlier thread, but i think it was for the non-stepped pot bridging points A and C (stay away from B!)

That is correct for bypassing the t-pad. What version of PCB do you have? If the t-pad is mounted directly to the main board, it will most likely be fine. If it has the adapter board, I would try the bypass. This distortion will probably not be a component especially the t-pad or switch.

 

[OOF]

i have  the latest black board which has the pot mounting directly to the board.  i thought perhaps i might have bridged two resistors with solder or if one of the resistors had a poor connection it would degrade the signal, no matter what setting the pot is at.  does the signal pass through the resistors in series, accumulating resistance as you increase the pot?
i don't think i have the letters A or C on my board.  aren't there numbers instead.  will pop it open this weekend and get to the bottom of it.
thanks again,
David

at least now i know who Patrick Henry is.

 

[jsteiger]

You will not have the A, B, C pads on your board with the latest Rev. I don't think it will be around the t-pad. That is the main reason I wanted the footprint of the Bourns version right on the PCB. It really eliminates any errors associated with the t-pad.

The stepped preamp gain R's are a string in series. Bridging 2 will only change the gain where 2 adjacent steps will be the same. It should not cause audible distortion. Never say never.

Now, a poor solder joint directly in the signal path could produce results like what you are saying. A lot of times though a joint will either be good or bad but for sure there is also a point in between.

I assume you have also eliminated the rack slot and all associated cabling?

Cheers, Jeff

 

[J Adams]

Question for Jeff:
So far I have built 2 vp25s and 2 vp312s.  On one of my vp25s, the pad button seems to do some weird things like not 'un-pad' after I push it.  I then have to push it a few times and it eventually work.  Any ideas before I start poking around?  These are my first builds ever, so I am not up to par on troubleshooting yet.  Thanks man!

p.s. really like these new black pcbs and being able to mount the gain pots onto the board (stepped and output)!

 

[jsteiger]

I would try a simple reflow of solder on the pad switch. It is extremely rare that the switch will be bad. I usually work each switch a dozen or so times when I do a build just for good measure.

Cheers, Jeff

 

[Seditionary]

i've fixed up lots of gear... usually just doing basic things like replacing caps that are obviously blown out... or resistors that aren't reading correctly... or reversing polarity on channels, changing things from 3 to 2 pin hot...  i guess i'm still a beginner though.

thought i did a really good job building the preamp.  all my connections seem solid (don't think i made any cold connections).  soldering all the resistors in for the stepped gain was pretty insane though... never worked on anything that close together before !  i don't see any solder bridges there or anything though...

i have the preamp all put together... but i'm not getting any sound at all out of it, even when the gain is cranked.  the rack seems to be okay.  tried it on different slots (all with no sound)... after figuring out how to use my ddm for voltage (thanks jeff !)  getting 15.98 v dc when the red is on +V and the black is on C... and getting exactly -16 dc volts when the red is on -V and the black is on C.

any suggestions?  would love to hear this preamp going   i have one of scott's red dots installed.  do you think the output transformer could be bad like in the other case?  how do i test it?  thanks!

 

[Seditionary]

just did the test that michael did... tried running the preamp into another a preamp... i'm in worse shape than he was though... as i still don't get anything...

when the second preamp is cranked (not the capi one)... i hear a low electrical feedback noise... the pad, mute, etc. on the capi preamp does not affect this sound.  however !  the output attenuator does.  if i back the output attenuator all the way down, the second preamp sounds pretty quiet.  however, the louder i bring up the output attenuator, the more of a wahhhhhh, low buzz i hear... the low buzz does not increase or decrease with the preamp gain higher or lower... only the output attenuator. 

interestingly... the power supply for this osa rack makes the same frequency buzz when turned on.  when the power suppy for the rack is turned on or off, still getting the same amount of hum... but i think maybe it is interesting that the power supply is humming the same tone as i'm hearing through the preamp? 

maybe this is insignificant, but as this hum is the only sound i'm getting from the pre, reamped, i guess it is significant to me

 

[jsteiger]

Here are some general tips that I like to send out when guys are having trouble. It's a good place to start.

First, what DOA is it? A pre-built or a DIY? If it's a kit, swap the opamps and see if the problem follows. Also, have you seen the DOA install page DOA install I highly recommend never testing a new preamp build with a new opamp build. There are too many variables and you may never know what the issue is. If it's a pre-built opamp from me, chances are, it is not bad. Gary and Scott both run them in for many hours before sending them to me.

Have you completed the pre-start tests at the end of the old VP2x Assembly Guide? If you are building a VP312, the C to O DCR value will be about 8.5 ohms. Make sure all voltage readings are correct.

Double check all component values and make sure caps are facing the right way. Resistors cannot be measured once both ends are soldered to the PCB. You will get many odd readings. Either use the color codes to verify or one end must be desoldered to get an accurate DCR reading.

Double check the lead colors/pads for the output transformer. Take your time and be sure. Countless times I have heard back from folks who checked and checked and then a few days later realized that some of the leads were placed incorrectly.

Next, reflow the solder on the 3 C&K pushbutton switches.

If it's the Grayhill stepped gain option, do you have 25k when measuring between the to outside pins of the switch? If not, check with a magnifying glass as there probably is a short between some solder pads. These can be checked end to end by probing between adjacent pins. You should get the resistance of each of the R's on the pdf as you work from one end to the other.

Scan the board and check all solder pads with a magnifying glass of some kind. The biggest issue, besides what I mentioned above are cold solder joints. Sometimes they can look fine but not be good.

 

[bigevil]

So,
I built a 2nd and was troubleshooting some errors I was having (I posted previously about the Opamp problem I solved [it was my fault, I switched two transformers]). I've put in a new Opamp that I've been able to confirm has no problems by trying it in my known working VP25 and luckily last night got lucky and figured out that the problem I'm having (very low intermittent audio) seems to be centered around the output knob. If i mess with it, push on it etc.... sometimes the audio will return to perfect levels etc..... usually it just stays at the low crackle output so I figured I need to start there, remove it and resolder?

Just wondering if there are any thoughts before I dive in.... maybe I did something wrong in that part.

Pictures below.

Thanks as always for the help!

 

[jsteiger]

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.

 

[bigevil]

ha, amazing.
ok off to the bench for some detective work.

 

[OOF!]

hi Jeff,
i posted a couple of weeks ago about getting a distorted signal on one of my vp26 pres.  i have resoldered every point twice and checked with a magnifying glass for messy solder points, but nothing worked.  this morning i turned the output pot fully off and yelled loudly into the mic (frustrated?)  now it sounds great.  what could cause this?  a very loud signal clearing up the distortion.  i've got a session with vibes today and will try it out.  until today this pre has been just sitting idle because i fear it's going to fail mid-session.
- also i have poked around many times while speaking into the mic, and wiggling everything on the board and nothing changes, so it doesn't seem like a solder connection.  pressing the 3 buttons many times doesn't change anything either.
thanks very much for the help,
David

 

[jsteiger]

Hi David

Hard to say exactly what that could be. It still sounds like a solder joint thing to me. I saw above about the red dot so you should be fine there. Have you ruled out the rack slot and potential issues with external cabling? I myself have not run across this issue with a build. I'm sure other guys have but it is hard to say if the problem is identical to yours or not.

If the issue comes back, you could always try bypassing the t-pad to rule that out. In all the thousands of t-pads we have sold, I have only sent out one replacement and that was one of the few 388 blue ones we had as hold me overs. Never a Bourns or the 5/8" mod pot style. I doubt that it is the pot be never say never.

Best, Jeff