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

[sumyungdon]

Thanks as always for the reply Udo - unfortunately replacing C9 did not solve the phantom power issue, although it did change some things.  The underlying buzz is gone - the line and dynamic mic signal is totally clean now (was getting a buzz and 60hZ hum before).  Also, I should have mentioned this before, but before I replaced C9, I would lose the LED activity of the MIC, Polarity, and PAD switches when the 48V switch was engaged.  They would light up their respective LED's when pressed in, but when the 48V button was pressed, those LED's would not work any more.  After reading this entire thread I thought that was normal, as I read something about those LED's being in series with each other - but now that I have swapped out C9 for a new cap, all of those LED's work when their switches are pressed, when the 48V switch is pressed.  So changing C9 solved two issues - the slight buzz I was hearing as well as the LED activity of the other switches.

However, my 48V still does not make it past R20 and R21.  I tried a lot of things.  I wicked and re-soldered all of the joints close to those two resistors.  I took both R20 and R21 out of the board completely, checked them for resistance, measured a cool 6.8K on both, and switched their positions on the board (former R20 became R21 and vice-versa), and carefully re-soldered them in.  I have spent a few hours now scratching my head and checking solder joints.  When the 48 V switch is engaged, the voltage makes it to the front end of both R20 and R21, but does not make it across them - they see less than 200mV on the other side - it still seems to be a wall where the 48V stops.  Without a schematic, I am at a loss as to what to look for next - I'm now thinking there is some other errant component which originally damaged my first C9, or maybe the board is damaged in some way that I cannot see, but my theoretical understanding of this circuit so far ends about here.

Any insight or further ideas are greatly appreciated!  Perhaps someone has access to a schematic for this project?  Is Jeff out there to chime in at all?

Again, I really appreciate all the help.  And discord, I hope you are getting close to solving your issues - sorry if my replies are throwing you off when you receive notification.

Andrew

 

[kante1603]

Hi Andrew,


so C9 was one culprit.Not too bad,because this is a huge improvement.
The phantom going to both R20 & R21 on one side but not to the other clearly indicates you have a short after them.Something is loading down the phantom here.
Since there won't be many parts from them to the card edge in between (I would even guess none!) you must have a short condition somewhere.Could be caused by a solder blob,a tiny cut-off piece of wire or parts-leg(!)..... maybe inside one of the card edge receptacles?.....or a non-balanced mic cable with one of either pin 2 or 3 on the xlrs shorted to the shield.
O.K.,please measure both sides from the two resistors to the modules'  corresponding card edge (the "gold fingers").If they are identical (2 times around some kilo ohms,two times zero ohms) we can expect the culprit to be outside the module.You can also measure these points refering to audio ground.


I hope we don't have a transformer issue here.


To check that please measure a known line level sine wave fed to the input,mic switch depressed,gain set to unity,fader to unity,no phamtom and measure the level (dmm set to vac) between the pins 2 & 3 on the output of the rack,nothing else connected please.E.g. if you feed a sine with 0dBu (or something like -18dBFS from your daw) will give you roughly 0,775 vac on the xlr.Don't use a high frequency,best to use something lower than 1kHz
because your dmm might not be able to read this properly.Try 400 Hz or so.What do you get?


About schematics,only Jeff has them,they are not public for some good reason,I'm sure you understand that.
But he'll definetely help you once he's back.


I'll be back home in one or two weeks and might be able to get hands on one of my own modules if the problem is not solved until then.


To not confuse Jeff too much with too many long long emails all he must know for now is that the phantom doesn't make it across those two resistors.Just as a "summary" if all fails.


Best,


Udo.

 

[sumyungdon]

Hi Udo -

so I am measuring some funky resistance between the far ends of R20 and R21, and the gold fingers for slots 8 and 10 on the rack.  R20 is measuring 28.5 ohms to (gold finger) slot 10, and 0.000 to slot 8.  R21 is measuring 29.7 ohms to slot 8 and 0.000 to slot 10.  the traces run through the MIC switch.  R20 hits R19, R19 hits the PAD switch, and that trace runs to the MIC switch.  R21 hits R17, and then that hits the MIC switch.  I followed the resistance up to the MIC switch, and it seems like theres about 28 or 29 ohms resistance between pins that the voltage should travel through (if you look at the switch vertically, the 5th pins down from the button on the left and right have that resistance between them - same with the 6th row, 8th row, and 9th row pins).  The pads that have a direct trace shot to the gold fingers for slots 8 and 10 have 0 ohms resistance, these are down towards the end of the MIC switch.  I'm currently trying to unseat the MIC switch (which is quite a task) to check for a short underneath where I cannot see.  I will keep posted.  Expecting not to find much, but who knows - if I do not solve the issue by checking the switch, maybe there is some component linked to this group of traces that is causing some resistance to happen there? 

Thanks again!!!  Your support is really helping me learn a lot, I really appreciate it!

Andrew

edit:  well, the switch is now in shambles, and I am still measuring a strange 25 ish ohm resistance across the pads where the 48V should hit.  R21 by way of R17 hits the 5th pad down from the LED of the MIC switch.  R20 hits the pad directly across that, by way of R19 and the PAD switch.  These two pads, the 5th pair down from the LED of the MIC switch, are measuring about 25 ohms resistance across them.  The traces of the MIC switch lead over to the R27-R32 section, where I measured 25 ohm resistance across R29.  Am I even going in the right direction?  I would assume there should be 0 ohm continuity all the way through for the 48V, but where it hits the pads of the MIC switch, there is resistance.  The 8th pair of pads down from the LED of the MIC switch have O ohm continuity through to the gold fingers for slots 8 and 10 on the rack.  I triple checked the placement of R27-R32, reflowed solder, and am now getting 24 ohms resistance across the 5th pair of pads on the MIC switch. 

I am assuming I need to wait to get a new switch in here before performing the line level test you recommended?  Although it did pass muster on this test when I did it before.  .775 VAC in and .775 VAC out.  I am assuming and hoping it is not a transformer issue in that case.  I feel like I'm going down a rabbit hole and have no idea what could be causing this resistance across the pads where the 48V hits.  It seems to match the resistance across R29.

 

[kante1603]

Hi Andrew,


o.k.,at this point I need to follow your readings on a working module to compare what is good and what is seems off.
The phantom -related resistances look fairly low but I can't proof it for now.They might be lower as their original value measured in circuit due to to other parts,but that low.....hmmmmm.....are you sure you have the correct values for r20&21 soldered in?Should be 6k8 both,you can lift one leg of them and measure them.
Sad to hear the mic switch is broken now,I did not ask you to do so,hahaha.....seriously,they don't like too much heat during assembly same as removing them.The circuitry around this switch is a pad resistor network that is used for line level mode (switch depressed).
The good news is that you say the unity gain check worked out fine,this means all transformers and the rest of the audio circuit are fine.


Please do me a favour and don't disassemble further parts for now,let's wait or Jeff to chime in.


S**t,this must be something totally ridiculous......


We're so close......


I still wonder why your former C9 went bad,what the cause was and which other parts might have gone bad.


Best,


Udo.

 

[sumyungdon]

Hi Udo -

yes, I feel like this is caused by something ridiculous that I have overlooked - but cannot figure out what it could be.  I promise not to disassemble any further parts!  haha I thought I was so cool, about to figure out the issue    I am going to use this time to set the module aside and order a new 6PDT switch, and hopefully clear my mind!

I am absolutely, 1000%, quadruple-checked positive that R20 and R21 are 6k8 ohms. 

I also have been wondering why C9 originally went bad - it was definitely seated in the correct orientation, I checked that more than a number of times and am looking at pictures I took and it's definitely been correctly oriented this whole time.

Thanks again for the help - I will patiently await Jeff's input. 

Andrew 

 

[kante1603]

Patience is the key,heads up Andrew ,


Udo.


P.S.:I'll have no internet access for a while now,maybe a week or so.When back home I'll try to follow your measurings.Meanwhile you might give Jeff a short summary via email?

 

[northamrec]

I finished my first CAPI VP28, installed it into my lunchbox, and powered it up....smoke  :'( :'( :'( I went very slowly to try to get everything right. Pretty bummed at the moment and I'm not sure where to start to try to fix it. Can anyone point me in the right direction? Should I post pictures? Thank you.

 

[jsteiger]

It sounds to me like there is a short somewhere after the 6k8 phantom R's. This could be on the VP28 board or quite possibly after the board (rack, cable, mic, patchbay etc). Can you be more specific as to your test rig setup? It would be best if you have nothing connected to the module when first measuring for 48V on the input gold fingers of the preamp card. I am trying to eliminate any outside issues.

FYIW, the functioning of the status LED's has nothing to do with the 120µF cap. That is only some odd coincidence like a cold solder joint that got touched up or bumped into working mode.

Back to the phantom R's, they connect directly to card edge gold fingers 8 and 10 so there is no circuitry on the PCB that comes after them.

 

[northamrec]

I removed the op amps and tried again-- no smoke. Measured voltages at A1 and A3 with DMM and they're all just below 16V. So does that suggest the problem is the op amps? Especially considering that they're DIY.  Should I just purchase a couple pre-assembled op amps to throw in to the preamp? A top down picture is attached.

 

[jsteiger]

I thought the main issue is no phantom power? Can you measure for phantom at the card edge gold fingers with nothing connected to the card except DC?

 

[northamrec]

I'm sorry Jeff, I think you might be confusing me for the previous poster.

 

[jsteiger]

I'm sorry Jeff, I think you might be confusing me for the previous poster.

Oops. Right you are. I feel like a post was deleted.....

Sounds like you have already discovered that yours is an opamp issue.

FWIW, I will be selling "built by Gary gar's" soon. I just need to get the support docs page done first.

 

[northamrec]

I'm sorry Jeff, I think you might be confusing me for the previous poster.

Oops. Right you are. I feel like a post was deleted.....

Sounds like you have already discovered that yours is an opamp issue.

FWIW, I will be selling "built by Gary gar's" soon. I just need to get the support docs page done first.

Are there assembled op amps available for purchase now?

 

[jsteiger]

I'm sorry Jeff, I think you might be confusing me for the previous poster.

Oops. Right you are. I feel like a post was deleted.....

Sounds like you have already discovered that yours is an opamp issue.

FWIW, I will be selling "built by Gary gar's" soon. I just need to get the support docs page done first.

Are there assembled op amps available for purchase now?

 

[sumyungdon]

Hi Jeff,

Andrew here with the phantom power issue.  Thanks for your responses.

I have disconnected everything except DC power to the module.  I took the rack apart to look for any shorts inside, reassembled it and it seems to me that there should be no issues with the rack.  So I am measuring with no input microphone and no output to a patchbay - just straight module.  This is the only module I have, so I have space to carefully measure with my DMM while it is seated in the rack.

For the record on this forum, as per our email exchange, I measured for phantom voltage between XLR pins to the microhpone on the back of the rack - and it seems like pins 1 and 3 are swapped, I get 187 ish mV between pins 1 and 3, and between pins 2 and 3.  I get a zero reading between pins 2 and 1.

On the card itself, I get 48.5 V DC at the bottom gold finger, and it makes its way onto the card.  I can follow the traces all the way to the phantom power switch, which receives the 48 V and gates it through to the 6k8 R's.  It is about 45.8 V at the base of R20 and R21.  I follow the board's traces after the 6k8 R's, which seem to hit the pins of the MIC switch, by way of a junction with one end of R17, and from the MIC switch further down (8th pair) I see the traces go to gold fingers 8 and 10.  I did find something at the MIC switch, right on the pin where the 48V trace hits - it looks like I must have knicked the board with my soldering iron right next to the solder pad.  I have attached a (rather crappy) picture to hopefully show you what I am seeing there - kind of a burnt dimple on the board which touches the solder pad of the 5th pin down on the MIC switch (5th pin, on the right, reading down from the LED3 marking).  It looks like a solder blob in the picture but it's actually the black screening of the board that got burnt a bit.  This is by far the messiest spot of my entire build, also considering I desoldered and replaced that switch (I believe this blemish was there before I replaced the switch).  Looks like I may have damaged the board here - and wondering if this is a fixable issue.  That specific pin measures about a 10ohm resistance to gold finger 1 of the card, which (I think?) tells me that pin is shorting to ground.  When I touch the "burnt dimple" itself with the probe, there is no measurement, but touching the 5th pins themselves or the solder pad itself, there is a low resistance straight to the ground gold finger 1.  Hopefully I'm explaining this well enough.  Wondering if those pins are supposed to have connection to ground, and if not, should I chalk this problem up to my own sloppy error?  Is there any way to fix this, or work around it to verify that this might or might not be the issue?  Other than phantom power, the preamp works wonderfully.

Thanks again for your support.

edit: I took the picture right after I wicked solder from the pad in question, in case you were wondering about the solder joint itself.

 

[jsteiger]

It is impossible for "pins 1 and 3 to be swapped" in the rack.

With no power connected, module inserted in the rack, measure DCR between XLR input pins 1->2 and then pins 1->3. You are looking for a close to direct short so set the range on your DMM accordingly.

 

[sumyungdon]

Jeff,

Here are my resistance measurements, with no power, and the module inserted:

XLR pins 1->2 - 4.97 K Ohms

pins 1->3 - 5 K Ohms

I'm not sure what this means, is this a normal reading?  If so, what should I look for next?

Thank you

Andrew

 

[Sparqee]

I have two VP28s with identical red dot DOA's from Scott L.  The two unit used to perform so identically as to be indistinguishable.  Recently one of the units has lost some gain. 

With both unit set to unity gain and filters disengaged I send a 1.5v test tone to each unit.  One unit outputs a signal of 1.67v while the other outputs a signal of only 1.01v.

Any suggestions on how to go about correcting this would be greatly appreciated. 

- Sparqee

 

[jsteiger]

I would firstly swap opamps around. Also make sure they are seated properly http://capi-gear.com/catalog/DOA_Install.php

Next rule out any outside possibilities like rack, cables, convertor settings yada yada

 

[Sparqee]

I would firstly swap opamps around. Also make sure they are seated properly http://capi-gear.com/catalog/DOA_Install.php

Next rule out any outside possibilities like rack, cables, convertor settings yada yada

Bingo!  The DOAs all seemed to be seated properly but when I swapped them my gain came back.  Thanks for the advice.