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

[jsteiger]

ok, I measured DCR 0 to ground. no connection,
Next I measured 101mV  between C20+ and ground, so that seems ok. (btw, I have removed the leaking cap before measuring)

OK this sounds normal. For a quick test, you can jumper over C20 to see if your output is normal. I would check the DC offset of your other opamps. Maybe one of the other ones was the problem and caused the cap to leak? I know you mentioned that you were moving them around. I would somehow mark the ones with low DC offset so you keep them straight.

 

[Zamoht]

OK this sounds normal. For a quick test, you can jumper over C20 to see if your output is normal. I would check the DC offset of your other opamps. Maybe one of the other ones was the problem and caused the cap to leak? I know you mentioned that you were moving them around. I would somehow mark the ones with low DC offset so you keep them straight.

tried to jumper C20, but still no output.
I measured all opamps in both preamps on the A3 location. The soundskulptor sk25's measured 95mV and 101 mV and the  1731's measured -15mV (which seems a bit odd? they do work in the working preamp....)

 

[jsteiger]

tried to jumper C20, but still no output.
I measured all opamps in both preamps on the A3 location. The soundskulptor sk25's measured 95mV and 101 mV and the  1731's measured -15mV (which seems a bit odd? they do work in the working preamp....)

It is a different opamp design so not that surprising.

That said, its possible that the trouble is before the output of A3. I would suggest using the test points doc linked above to see where your signal stops. Time to be analytical about it. Start with the good unit and notate the signal at each point and then move to the bad one.

 

[nckptmn]

just wanted to follow up my opamp woes with some good news! i just installed a pair of pre-assembled gar2520s and my vp28 is up and running! i still have to do a line-level comparison but so far everything seems to be working perfectly! going to begin building my next vp28 soon and i'm gonna try my hand at the 1731 kits and hopefully put one of each op amp in my 2 pres. Thanks Jeff!

 

[darkus]

Hi everybody. I just built two VP28´s and having problems with one of them.  Signal is very low, no matter how much gain applied, it looks like its clipping bad (and surprisingly sounds like that too) until i flip the phase switch and the preamp works like it should.

Is my switch bad or is there something else? Ive switched opamps between the units but that didnt help anything as the preamp works fine after the phase being switched.

 

[jsteiger]

I would first guess its a soldering issue. The likelihood of it being a bad switch is very slim. Like 1 in 10,000.

 

[darkus]

I would first guess its a soldering issue. The likelihood of it being a bad switch is very slim. Like 1 in 10,000.

I checked the soldering in that switch and reflow some solder. All looks good, same problem still occurs. Where else should i be looking at? The signal level im getting with phase switch in normal position is somewhere around -60dB. once i flip the phase everything works like should

Oh and the signal LED wont light up either until i flip the phase switch.

 

[choskins]

Built two VP28s.  First is absolutely superb in all respects.  Second works but has very low output.  I've emailed Jeff, but he's on vacation - I don't expect to hear from him any time soon.  I have no other test equipment except for a nice multimeter.  I've ruled out some things: not the opamps (used same pair in both units), not the microphone, not the rack or the slot in the rack, resistor position, diode position and orientation, cap position and orientation of polarized caps, input transformer position, Litz transformer leads identical to working unit, no solder bridges, no apparent inadequate solder connections.  Are there any measurements I can make that might narrow things down a bit?  Thanks in advance for any help, Carlo.

Update: Forgive me, but I do not know how to use this message board - I could not find a reply icon to respond directly to Potato Cakes from Nashville.  Thank you very much for your response.  Since I have no way to measure those voltages while it is powered up, I am going to contact Jeff today to get one of those extension test jigs to start running this down better.  But get this.  With the problematic unit in the rack, I can lightly rap on the face of it with my knuckle, and suddenly it works perfectly.  Then over the next minute or two it begins fading out all on its own - this is very annoying to say the least because it sounds **sooo luscious** on a guitar cabinet, acoustic guitar, and kick drum with my Royer 121.  Then it fades out.  Anyway, I rap on the face again, and it works again.  I began suspecting the DOA sockets, but all I could do to check was to swap the two to see if THAT made a difference - it did not - same exact problem.  Mystified here.

Final update - problem resolved.  I got to thinking about it some more and knew that the problem was not an assembly issue since it would function identically to the other momentarily.  I decided to touch up every single solder joint that didn't look perfect, and now the unit works perfectly.  Taught me a lesson: don't move to the next solder joint until the one you're working on is as perfect as you can make it.  Thanks again to the fellow who responded - I really appreciate it.

 

[Potato Cakes]

Built two VP28s.  First is absolutely superb in all respects.  Second works but has very low output.  I've emailed Jeff, but he's on vacation - I don't expect to hear from him any time soon.  I have no other test equipment except for a nice multimeter.  I've ruled out some things: not the opamps (used same pair in both units), not the microphone, not the rack or the slot in the rack, resistor position, diode position and orientation, cap position and orientation of polarized caps, input transformer position, Litz transformer leads identical to working unit, no solder bridges, no apparent inadequate solder connections.  Are there any measurements I can make that might narrow things down a bit?  Thanks in advance for any help, Carlo.

Do you have the correct voltages at the each of the DOA sockets? Are any of the audio connections shorting to ground? Is it low output regardless of setting (i.e. gain up/down, fader up/down, HPF in/out/-6/-12, polarity in/out, etc)? On one of these I built, I had low output with the HPF engaged, and though it seemed that the soldering looked fine, I indeed had a less than stellar solder connection. Other times I have had a misplaced a component even after I had spent days going over it and thinking everything was in order, even if I had just successfully completed building the same thing. If you do feel that it's definitely not the construction, the only thing I think anyone here can suggest is to break out the schematic, send a 1k tone through both units at equal level, compare signal between the two at each component in line until you find where they significantly differ. I would do the same comparing the voltages downstream of the +/-16V rails. It's annoying and can be very time consuming, but without much more information, this is kinda the only way to find your problem with just a multimeter if you can't visually find any errors.

I'm interested to see if you indeed have a bad component as opposed to a seemingly good yet faulty solder joint.

Good hunting!

Paul

 

[coyotefacerecording]

Hey guys!  First post

Just built 6 of these CAPI kits.  Love em already.

I have one VP28 that all the features works just fine on, except the Fader knob doesn't pass signal (or very little) unless it's wide open (aka turned all the way to the right).  Any suggestions on what the issue might be?  I scanned the PCB a few times with a magnifying glass to make sure everything was on there, with correct polarity and I don't see any glaring mistakes.  Could it be a solder joint?  Bad switch?

Thanks!

*UPDATE*
Tested all joints again.  Made sure all cups were sufficiently filled.  Double checked all resistor locations with magnifying glass.  Double checked leads on Litz transformers.  Made sure op amps were properly seated.  I'm kind of at a loss at what it could be.  Any advice would be helpful.

*UPDATE 2**
Swapped out op amps with a previously working unit.  Still no luck. 

 

[Potato Cakes]

You can try to bypass the switch for the problematic resistors values on the fader to see if you do indeed have a bad switch. It's rare a component is actually bad, but still possible.


Paul

 

[coyotefacerecording]

Any thoughts Jeff?

 

[jsteiger]

Any thoughts Jeff?

Did you find and use the test points jig? You will have to inject signal and trace it thru to see where it actually stops. It was posted a little ways back.

 

[darkus]

I would first guess its a soldering issue. The likelihood of it being a bad switch is very slim. Like 1 in 10,000.

I checked the soldering in that switch and reflow some solder. All looks good, same problem still occurs. Where else should i be looking at? The signal level im getting with phase switch in normal position is somewhere around -60dB. once i flip the phase everything works like should

Oh and the signal LED wont light up either until i flip the phase switch.

I still have this problem, is anyone able to help me out?

 

[jsteiger]

I would first guess its a soldering issue. The likelihood of it being a bad switch is very slim. Like 1 in 10,000.

I checked the soldering in that switch and reflow some solder. All looks good, same problem still occurs. Where else should i be looking at? The signal level im getting with phase switch in normal position is somewhere around -60dB. once i flip the phase everything works like should

Oh and the signal LED wont light up either until i flip the phase switch.

I still have this problem, is anyone able to help me out?

I would desolder the red and orange primary leads from the T2 output transformer and take a few DCR measurements. There is no need to connect the module to power or run signal into it. Use the pad near gold finger #5 as your ground reference for your black probe. With the polarity switch out, the orange pad should be connected to ground. The red pad should have some higher resistance to ground but not a short or low DCR. The red pad should be directly coupled to the negative end of C16. With the polarity switch in, the red and orange pads should swap their responsibilities. So, the negative end of C16 should never have a low DCR to ground no matter how the polarity switch is set.

 

[Commander Fluffypants]

Hi there,

Once again after a long time of not messing around with my VP28's I still have two out of eight that need some love. I've found the sonic problem with the one that had some low level distortion; it needs a new 1731. This one also has a non functioning red LED. I swear it did work when I first built it, but stopped shortly thereafter. The pre does get phantom power, but the LED does not work. It is in correctly and I reflowed the solder just in case. Is it just the LED itself? If so, easy fix.

Next up is THE ONE.  My only lemon.  This one pops REALLY loud when you hit the pad button. It pops less loud, but still loud when you hit the mic button. It makes a very tiny noise when you hit the phase button. I put some OpAmps that I knew were working in it and tested it. You can listen to this mp3 of me blabbing into the thing and you'll hear about 53 seconds of noise before I hit the pad button. I dropped the pop by 15db so your ears and headphones don't explode, but it's still loud. After that, I engage the mic switch and you can (unfortunately) hear the dulcet tones of my voice babbling, describing what I'm doing.

Another strange thing about this pre is that I'd had the issues with it right after I built it; the pops and noise, but it was only tonight when I plugged it in that I fried the 1731 (R14), prompting me to test wit with known working DOA's.

Here's the link to the mp3:

https://dl.dropboxusercontent.com/content_link/IlUyVMsK3Q1fBkz1dQWoqWEPC30afPF0bPq0JezTflT5ePzTY2ipSIWuGr2Bm8p4/file

As always, thanks everyone for their help and thank you, Jeff, for not only making these available, but also guiding beginner DIYers through the building/troubleshooting process.

 

[mysticmerlin]

Hi,
I have built two VP28's , one works fine and sounds great the other doesn't work at all
I have obviously checked to the best of my ability and there  is a weird 10.2 Ohm between C and V+ which seems quite abnormal except I can't find where that bridge is coming from. I checked R36 first since it's 10 Ohm but it seems to be properly soldered.
Any help would be most welcome

Thanks

 

[jsteiger]

....I have obviously checked to the best of my ability and there  is a weird 10.2 Ohm between C and V+ which seems quite abnormal except I can't find where that bridge is coming from.....

Yes this is very odd. It must be a short or solder bridge. There are only a few parts that connect top the +V rail. I would investigate these areas.

~Socket for each DOA (obviously), make sure to remove both amps to rule them out.
~A2 IC, check orientation and maybe remove it? Check IC socket
~C8
~Right lead of C2
~Right lead of R2
~Right lead of R3

 

[jsteiger]

.....This one also has a non functioning red LED. I swear it did work when I first built it, but stopped shortly thereafter. The pre does get phantom power, but the LED does not work. It is in correctly and I reflowed the solder just in case. Is it just the LED itself? If so, easy fix.

Its possible that the LED is bad but unlikely. I would also reflow solder to the top 6 phantom switch pins. Basically there is current always flowing to the LED. The switch just shorts out the LED's leads when it is out. When it is engaged, there should be no contact between the LED's leads. This can be checked with a DMM when not connected to power. If you need an LED I can send you one no problem.

Next up is THE ONE.  My only lemon.  This one pops REALLY loud when you hit the pad button. It pops less loud, but still loud when you hit the mic button. It makes a very tiny noise when you hit the phase button. I put some OpAmps that I knew were working in it and tested it. You can listen to this mp3 of me blabbing into the thing and you'll hear about 53 seconds of noise before I hit the pad button. I dropped the pop by 15db so your ears and headphones don't explode, but it's still loud. After that, I engage the mic switch and you can (unfortunately) hear the dulcet tones of my voice babbling, describing what I'm doing.

Another strange thing about this pre is that I'd had the issues with it right after I built it; the pops and noise, but it was only tonight when I plugged it in that I fried the 1731 (R14), prompting me to test wit with known working DOA's....

Well, with phantom engaged, there will always be a pop when engaging the Pad switch. Since there is DC present on the contacts, it will happen. If this happens when phantom is off, I would check for DC at the input pins of the card, gold finger #8 and #10. A loud pop when switching most usually means there is DC on the audio switch. There shouldn't be any there with phantom off. Maybe its from an outside source? Whatever is plugged into the pre's input?

 

[Commander Fluffypants]

[Its possible that the LED is bad but unlikely. I would also reflow solder to the top 6 phantom switch pins. Basically there is current always flowing to the LED. The switch just shorts out the LED's leads when it is out. When it is engaged, there should be no contact between the LED's leads. This can be checked with a DMM when not connected to power. If you need an LED I can send you one no problem.

Thanks! Reflowing the switch makes sense because, I do remember it working when I first built it.

Well, with phantom engaged, there will always be a pop when engaging the Pad switch. Since there is DC present on the contacts, it will happen. If this happens when phantom is off, I would check for DC at the input pins of the card, gold finger #8 and #10. A loud pop when switching most usually means there is DC on the audio switch. There shouldn't be any there with phantom off. Maybe its from an outside source? Whatever is plugged into the pre's input?

Yes, it pops no matter what the source is... although I haven't checked it with a line in, but with, for instance, a plain old 57 and the phantom OFF, it pops. Did you hear the noise also? That noise is there even with the volume all the way down.

And I beg for your patience again as I ask exactly how to check for DC at gold fingers 8 and 10. Do I do this with the unit powered up and in the jig? Seems difficult as the pins are in the jig. Obviously there is basic understanding that I don't have. What setting do I use for my DMM, and exactly where does each probe go and under what conditions (powered on, off, etc.)?

And then if there IS DC there, what does that tell me? Or shall I just cross that bridge when I get to it?

As always, thank you, thank you, thank you.