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

[jsteiger]

Hey,
I've got a problem with one of my Gar2520s in a VP28. The thing started giving off a wicked smell the first time I put it in my lunchbox, and it seemed to be coming from the op-amp. I tested that same op amp in my other VP28 that has been working perfectly, and same thing happened. This time around, however, I noticed the R9 and R13  56R resistors had been totally fried black :/ And may have singed the neighboring caps.  I double checked all of my resistor placement, transistor placement, solder bridges, and I'm not coming up with any reason.

Any ideas? Thanks in advance!

Sounds like this should be posted on the gar2520 support thread.

 

[thisisntjohnv]

If you follow the steps in the Test Point guide, all measurements shown are done in Line mode, not Mic mode. What is your reading at TP1 when following the proper procedure?

Just wanted to follow up on this. I upgraded my multimeter to get a more accurate reading. Here are my test results:

TP1: 110mv
TP2: 194mv
TP3: 194mv
TP4: 47mv
TP5: 47mv
TP6: 47mv
TP7: 47mv
Output: 35mv

Just so you guys don't have to read through the thread: my preamp gain switch isn't affecting the signal and I'm getting an extremely low level. I have litz transformers and I also have two pre built red dot amps. I went back and checked for cold solders and reflowed anything that looked like it might be an issue. Above are the current readings of my preamp.

 

[jsteiger]

Just wanted to follow up on this. I upgraded my multimeter to get a more accurate reading. Here are my test results:

TP1: 110mv
TP2: 194mv
TP3: 194mv
TP4: 47mv
TP5: 47mv
TP6: 47mv
TP7: 47mv
Output: 35mv

Just so you guys don't have to read through the thread: my preamp gain switch isn't affecting the signal and I'm getting an extremely low level. I have litz transformers and I also have two pre built red dot amps. I went back and checked for cold solders and reflowed anything that looked like it might be an issue. Above are the current readings of my preamp.

Its very hard to tell in the pic but R8 does not look like a 20k?? Your problem starts with TP2 which indicates A1 or the support circuitry around it. That would be R5-R8 as well as R1, C1 and then the gain-switch R's.

Just to rule things out, I would swap the red dot positions and see if anything changes. I have never received a bad opamp from Scott though so I doubt this will change anything.

 

[jbmacdo3]

TP6 is post the fader booster amp so I would look at that.

I just wanted to let you know I got the issue traced and sorted.  Thanks for the help!  Funny enough it was the 6/12db switch that was somehow bridged to ground. The output of the HPF board was getting continuity to ground and not passing audio.  Once I got rid of that the VAC readings went back to normal and I get to keep my hair and sanity 

Thanks for the great kits and amazing support. 

Cheers,

Jeff

 

[CaseyI]

I've just completed my VP28 and it has some issues.  The Signal LED is lit when my 500 rack is powered OFF, and it turns off when I power the rack on.  I can't do any audio tests as I've also managed to fry R14 and R15 on my Gar1731.  Any help is appreciated.

Thank you,
Casey

 

[jsteiger]

I've just completed my VP28 and it has some issues.  The Signal LED is lit when my 500 rack is powered OFF, and it turns off when I power the rack on.  I can't do any audio tests as I've also managed to fry R14 and R15 on my Gar1731.  Any help is appreciated.

Thank you,
Casey

I honestly have no idea how this can be happening. It makes zero sense to me.

 

[CaseyI]

So I figured out the issue with my 1731 which was I transposed the Q7 and Q8 which led to frying R14 and R15.  Now that I've got that sorted out I've discovered that I've also fried PR1 and PR3 and the board itself.  I don't have spare resistors on hand but at least I have some direction on getting this thing up and running.

 

[Homestudio]

Hey Jeff, I've been tracking with the vp28 for a while now and it's been sounding great, I mean great. But last night I was tracking and halfway through the volume just cut completely almost to nothing. Fired it up this afternoon and again the same thing happened it sounded great and then completely cut out after about 10to 15 minutes. Any ideas?
Thanks

 

[dmp]

Hey Jeff, I've been tracking with the vp28 for a while now and it's been sounding great, I mean great. But last night I was tracking and halfway through the volume just cut completely almost to nothing. Fired it up this afternoon and again the same thing happened it sounded great and then completely cut out after about 10to 15 minutes. Any ideas?
Thanks

Sounds like something in heating up and failing. Check that the power supply is not the problem. Thermal fuses sometimes kick off.
If it is a problem on the vp28, after it cuts out examine it for any hot components.

 

[Mole-man]

Hello gang,

Proud owner of a self built vp26 and vp28. The vp26 works as it should but the vp28 has issues with its phantom power. It works fine with dynamic mics but does not work with capacitor mics. It outputs no signal with P48 but when I engage the pad I get a hum and a highly distorted low level signal (the pad works fine with dynamic mics). I've gone over most of the joints and the two 6k8 resistors are in the correct place. I've measured the inputs pins 1 to 2 and 1 to 3 and I'm getting 47.5v on both.. Which is confusing for me. Is this a grounding issue? I'm very confused .
Any light you guys could shed on this would be most appreciated.

 

[Potato Cakes]

Pins 2 and 3 on the card are output. You should not be getting any phantom power on those. If you refer to any API 500 series pinout chart you will see that pin 8 and pin 10 are your inputs, which is where you should be seeing phantom power. This is also shown on the schematic for VP25, 26, and 312. Jeff doesn't have a schematic posted for the VP28, but the phantom power scheme should be the same or close to it. What you are describing sounds like there is little to no VDC being sent to the condenser mic or that the mic being tested is faulty. Double check for phantom on pins 8 and 10 and then go from there.

Thanks!

Paul

 

[jsteiger]

Hey Jeff, I've been tracking with the vp28 for a while now and it's been sounding great, I mean great. But last night I was tracking and halfway through the volume just cut completely almost to nothing. Fired it up this afternoon and again the same thing happened it sounded great and then completely cut out after about 10to 15 minutes. Any ideas?
Thanks

Classic symptoms of a cold solder joint. The most common issue on DIY builds.

 

[Mole-man]

Pins 2 and 3 on the card are output. You should not be getting any phantom power on those. If you refer to any API 500 series pinout chart you will see that pin 8 and pin 10 are your inputs, which is where you should be seeing phantom power. This is also shown on the schematic for VP25, 26, and 312. Jeff doesn't have a schematic posted for the VP28, but the phantom power scheme should be the same or close to it. What you are describing sounds like there is little to no VDC being sent to the condenser mic or that the mic being tested is faulty. Double check for phantom on pins 8 and 10 and then go from there.

Thanks!

Paul

Thank you for the reply.

I meant pins 1&3 + 1&2 of the xlr when connected to the input! I can see how that was confusing..

If the mic is seeing enough juice I don't understand what's happening further down the circuit..

 

[Potato Cakes]

Thank you for the reply.

I meant pins 1&3 + 1&2 of the xlr when connected to the input! I can see how that was confusing..

If the mic is seeing enough juice I don't understand what's happening further down the circuit..

Do you still have +48V on XLR Pin 2 and 3 in relation to pin 1 (card pins Pin 8 and 10) with the microphone connected? This check will require the test jig. If the preamp is working fine as you say with a dynamic mic, then the issue you are describing has to be either phantom power getting to the condenser or the mic itself. Switching on +48V doesn't change the signal path of the preamp, as you know, so I don't see how it can electronically can be anything else unless you have solder bridge somewhere that is bleeding 48VDC to the opamps affecting the voltage your DOAs are receiving. You'll have to trace down the path that phantom takes from the card to audio +/- input. If it's not there then it seems you are about to embark on the magically frustrating journey of quadruple checking components and reflowing any suspicious solder joints and then quadruple checking everything again until you find the problem which turns out to be something you were certain was not the problem.

Thanks!

Paul

 

[Mole-man]

Thank you for your reply Paul.

I scrutinised the board many times over and compared to the picture on the capi website and all the components are in the right places. I noticed a tiny wisp of solder under C5 and since removing it the preamp is alot more stable with! I assume a tiny solder bridge...

The first couple of times I used it with capacitor mics it worked 85% of the time but would sometimes stop passing level again and only producing almost no output that's highly distorted.

I then discovered that engaging the pad would appear to pass audio normally when this problem arose.

The last 10+ times I've used it with capacitor mics there have been no problems. I'm concerned that it will act up again though.

I've tested both opamps in a vp26 and they seem to work fine.

The only place there could be a solder bridges is under the the switches. I'm assuming the problem is probably with the pad c&k switch but I don't know how to remove it without damaging the switch and probably the board as well.

Any thoughts?

Thanks in advance.

 

[jsteiger]

I've tested both opamps in a vp26 and they seem to work fine.

The only place there could be a solder bridges is under the the switches. I'm assuming the problem is probably with the pad c&k switch but I don't know how to remove it without damaging the switch and probably the board as well.

Extremely unlikely to have a solder bridge under one of the switches. Essentially impossible. If something is intermittent, its likely a bad solder joint somewhere on the build.

 

[Chrome Heart]

So with a 400HZ test tone being injected at 1.228V, and all settings correct for the test I have the following results.

TP1 - 111.8
TP2 - .998
TP3 - 1.804
TP4 - .450
TP5 - .450
TP6 - .450
TP7 - .940
Output at pin 2 is 1.402

Im not sure why they are higher than the guide, but they look close. Are these acceptable values?

 

[Mylithra]

Been forever since I posted here. Recently built a VP28 and a GAR2520 and a GAR1731. Took my time, examined every solder joint as I went and had zero trouble with it. Powered up and sounded gorgeous the first time. The Support Docs from the site coupled with the first revision build guide here, there were no surprises.  Took me about 8 hours beginning to end. (about 2.5/3 hours on the DOAs, and about 6 hours on the VP28s)

If I'm giving advice to anyone building these, take your time, double check everything prior to soldering and try to make every solder joint perfect.

Just as a note, this isnt my first CAPI kit, I built 4x 312s, 2x with DI, I think the 312 w/DI was harder to build than the VP28

 

[Chrome Heart]

So with a 400HZ test tone being injected at 1.228V, and all settings correct for the test I have the following results.

TP1 - 111.8
TP2 - .998
TP3 - 1.804
TP4 - .450
TP5 - .450
TP6 - .450
TP7 - .940
Output at pin 2 is 1.402

Im not sure why they are higher than the guide, but they look close. Are these acceptable values?

:bump:

 

[JRoulat]

Just chiming in here as a recent CAPI owner and Noob to the DIY world.

Built the 511 rack. All XLRS. It’s great.

Built a vp28 and watched it smoke and burn. That’s what happens when you mistakenly put PR 1-8 where R 1-8 are supposed to go.  Say nothing. I chastised myself enough.  Really stupid.  Likely due to hyper enthusiasm.

Just built my second VP28 and flawless startup.

Also bought the test jig.

My lessons learned:

Like my shop teacher said. Measure twice, cut once. Relax and enjoy the process. Very Zen to build one of these.

Know thine DMM.  I am getting better at mine and also organize your components. (See measure twice above).

Get a super small tip for your solder station to do the DOAs. A big tip is like trying to eat sushi with baseball bats.

Watch out for flying leads when clipping them off. One ended up in my sandwich. I kid you not.

Get a good soldering iron. It is your main tool. Seriously- temp controlled.

Yeah though I walk through the valley of the shadow of death I will have at my side a good solder sucker.

Anything else.....  not really, go slow learn and enjoy!

Jeff-BTW- sounds amazing!!!!  You will be seeing more orders. Rock On!!