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

[ddld]

Hi there. I built two VP28s, with two sets of opamps. One of the VP28s is working (and sounding glorious), the other is not passing any signal. I have verified that the opamps are good by swapping them around. I am somewhat new to analog electronics. After having built five kits of various vendors, this is the first time I'm running into a problem, so I could use a little help debugging the issue.

I'll try to be very specific about what I did so far:

- Set up a function generator to generate a sine wave at 400 Hz, 150 Ohm output impedance. At that load, I need to set it to 2.613 Vpp to measure 1.228 VRMS on the DMM.

- I hooked up the function generator output to pins 8 and 10 of my 500 series jig. With the DMM, I can verify that the voltage, when connected, is just a little bit below 1.228v.

I measure 41 mV at TP1, a maybe 1 mV at everything else. The signal indicator LED of the unit doesn't light up when I send in a signal, but it does when I measure TP1.

UPDATE: I meticulously reflowed the input transformer contacts, and now I measure a solid 112 mV at TP1. The other points are still completely zero though.

I've attached an overview of the unit. I've checked and reflowed all connections. I'm fairly confident that the soldering is of decent quality. I've also checked all the resistors against the other VP28 that works. I've built and populated them simultaneously, so it's unlikely (but not impossible) that this is a stupid component mixup.

I'd appreciate any suggestions on going forward. It would be particulary helpful for me to know what the test points mean, since there is no schematic as far as I can tell. I assumed TP1 was behind the input transformer, but I don't know where the others are in the path. This makes it hard to guess where to look.

Thanks in advance!

 

[jsteiger]

Hi there. I built two VP28s, with two sets of opamps. One of the VP28s is working (and sounding glorious), the other is not passing any signal. I have verified that the opamps are good by swapping them around. I am somewhat new to analog electronics. After having built five kits of various vendors, this is the first time I'm running into a problem, so I could use a little help debugging the issue.

I'll try to be very specific about what I did so far:

- Set up a function generator to generate a sine wave at 400 Hz, 150 Ohm output impedance. At that load, I need to set it to 2.613 Vpp to measure 1.228 VRMS on the DMM.

- I hooked up the function generator output to pins 8 and 10 of my 500 series jig. With the DMM, I can verify that the voltage, when connected, is just a little bit below 1.228v.

I measure 41 mV at TP1, a maybe 1 mV at everything else. The signal indicator LED of the unit doesn't light up when I send in a signal, but it does when I measure TP1.

UPDATE: I meticulously reflowed the input transformer contacts, and now I measure a solid 112 mV at TP1. The other points are still completely zero though.

I've attached an overview of the unit. I've checked and reflowed all connections. I'm fairly confident that the soldering is of decent quality. I've also checked all the resistors against the other VP28 that works. I've built and populated them simultaneously, so it's unlikely (but not impossible) that this is a stupid component mixup.

I'd appreciate any suggestions on going forward. It would be particulary helpful for me to know what the test points mean, since there is no schematic as far as I can tell. I assumed TP1 was behind the input transformer, but I don't know where the others are in the path. This makes it hard to guess where to look.

Thanks in advance!

TP2 is immediately after the A1 2520 opamp. It's either a problem with A1 or the surrounding components just left and below it. If reflowing solder joints on the input transformer helped maybe there are other cold solder joints somewhere? I do see a few joints where it looks like a pad could be shorted to the groundplane. Might be photo perspective.

 

[ddld]

TP2 is immediately after the A1 2520 opamp. It's either a problem with A1 or the surrounding components just left and below it. If reflowing solder joints on the input transformer helped maybe there are other cold solder joints somewhere? I do see a few joints where it looks like a pad could be shorted to the groundplane. Might be photo perspective.

Thanks! I will reflow those connections.

I’m trying to learn, so please educate me… how exactly can a pad be shorted to the ground plane? Isn’t the ground plane isolated from the surface? Where exactly do you see suspicious points?

 

[ddld]

Ok, so. I reflowed everything around A1, and now I'm seeing signal at TP2, and I have a working signal-present LED. Yay!

The test point measurements are still funky, so I'm not quite there yet. Test points are measuring as follows:

TP1: 110 mV
TP2: 110 mV
TP3: 199 mV
TP4: 49 mV
TP5: 49 mV
TP6: 49 mV
TP7: 103 mV

Any suggestions/possible explanations?

By the way, is it necessary to choose specific gain settings to perform these tests? It isn't mentioned in the test point guide, but I would assume these readings must change when you change the gains (which I can't easily try without putting the knobs back on)?

 

[Tronfxdx]

I know this a support forum but I just want CAPI to know that I love this thing. Plenty of gain for my Ribbon mic's. It sounds like Magic. I am just sorry I only built one. Running the gain a bit high with my Mellow u87 clone, it really punches my vocals through. Great kit. Building the CAPI Rack next.

 

[jsteiger]

Ok, so. I reflowed everything around A1, and now I'm seeing signal at TP2, and I have a working signal-present LED. Yay!

The test point measurements are still funky, so I'm not quite there yet. Test points are measuring as follows:

TP1: 110 mV
TP2: 110 mV
TP3: 199 mV
TP4: 49 mV
TP5: 49 mV
TP6: 49 mV
TP7: 103 mV

Any suggestions/possible explanations?

By the way, is it necessary to choose specific gain settings to perform these tests? It isn't mentioned in the test point guide, but I would assume these readings must change when you change the gains (which I can't easily try without putting the knobs back on)?

Line mode, unity gain fader at zero.

 

[ddld]

Line mode, unity gain fader at zero.

Hi Jeff, thanks! It might be a good idea to add this to the test point guide. Seems pretty essential information to me.

Meanwhile, I’m away from the unit at the moment, but I am fairly sure I left both gain controls at 12 o clock. That means the preamp gain was a bit too high when I measured, but doesn’t that make it extra strange that TP2 measures 110mV, the same as TP1?

I’m very new to electronics, but as I understand it the gain of an opamp is controlled using a resistor in the negative feedback path. A short somewhere along the path could explain the fact that the opamp is only following the input, correct?

In an earlier post you mentioned that you thought you could see a possible ground plane short in one of my photos. Could you elaborate on what that looks like, and how it might happen? I haven’t been able to find a lot of info on this.

Thanks in advance!

 

[jsteiger]

Hi Jeff, thanks! It might be a good idea to add this to the test point guide. Seems pretty essential information to me.

Meanwhile, I’m away from the unit at the moment, but I am fairly sure I left both gain controls at 12 o clock. That means the preamp gain was a bit too high when I measured, but doesn’t that make it extra strange that TP2 measures 110mV, the same as TP1?

I’m very new to electronics, but as I understand it the gain of an opamp is controlled using a resistor in the negative feedback path. A short somewhere along the path could explain the fact that the opamp is only following the input, correct?

In an earlier post you mentioned that you thought you could see a possible ground plane short in one of my photos. Could you elaborate on what that looks like, and how it might happen? I haven’t been able to find a lot of info on this.

Thanks in advance!

It does say line mode in the guide and I was just ASSuming that people would know if you put 1.228V AC at the input and are expecting nearly the same level at the output (unity gain) then the controls would be set for unity gain. May bad.

 

[ddld]

Could you answer the two questions I asked, please?

 

[jsteiger]

Could you answer the two questions I asked, please?

Look at the photo of the bottom of your PCB and you'll see a few solder joints that look like they have way too much solder and could be splayed out onto the PCB's groundplane. Pretty self explanatory IMHO.

Not sure exactly of the other question but it seems as every time you mention touching up solder joints the levels change. That seems like a clue to me.

Besides misplaced components, bad solder joints are the #1 issue in DIY builds.

 

[ddld]

Look at the photo of the bottom of your PCB and you'll see a few solder joints that look like they have way too much solder and could be splayed out onto the PCB's groundplane. Pretty self explanatory IMHO.

Not sure exactly of the other question but it seems as every time you mention touching up solder joints the levels change. That seems like a clue to me.

Besides misplaced components, bad solder joints are the #1 issue in DIY builds.

If it was self-explanatory, I wouldn’t have asked. Twice.

As I wrote earlier, I was under the impression that a ground plane was always on the inside layers of a PCB. I now realize that it is any of the large areas between the traces.

I’ve built five kits before this that simply worked on first power-up, including a vp28. You need to run into trouble before you can learn. Sorry if my questions seem silly to you.

 

[Djacobs]

Hello, I'm new to this forum and am building a pair of VP28s and 4 opamps to go with them. I am building these for a friend who has neither the time or experience with this kind of thing. I do have the time to do it along with years of through-hole fabrication experience so I have taken on the job of building them. The problem is that I have no 500 series equipment available here with which to test the finished units. I really would like to at least have a fair idea that all parts were placed properly and I didn't burn up a semiconductor in the soldering process. I'm old and my eyes may not work as well as I could wish these days. Mistakes can be made! Is there a way to bench check these units without an official 500 series backplane etc? I do have dual power supplies along with a function generator, a scope and all kinds of DMMs so I might at least do a smoke test and quiescent voltage measurements. MAybe just a resistance check at the testpoints? I dunno. I do have the pinout for the backplane so can most likely wire it without blowing it up. would that be wise or not advised? I am trying to avoid having him try to use it only to find I made a boneheaded mistake. Hes on tour a lot and has lot of session work so we don't have a much time to spend troubleshooting at his place in Chicago. I'm in the far west suburbs. Anyway, Thanks in advance!
Dave

edit: I have finished one VP28 and 3 opamps so far. The build is going very well for me with minimal issues. Thanks to the designer for what looks like very nice preamps! I have heard they a top quality and based on the attention to detail, I can believe it!

 

[qmp audio]

Hello, I'm new to this forum and am building a pair of VP28s and 4 opamps to go with them. I am building these for a friend who has neither the time or experience with this kind of thing. I do have the time to do it along with years of through-hole fabrication experience so I have taken on the job of building them. The problem is that I have no 500 series equipment available here with which to test the finished units. I really would like to at least have a fair idea that all parts were placed properly and I didn't burn up a semiconductor in the soldering process. I'm old and my eyes may not work as well as I could wish these days. Mistakes can be made! Is there a way to bench check these units without an official 500 series backplane etc? I do have dual power supplies along with a function generator, a scope and all kinds of DMMs so I might at least do a smoke test and quiescent voltage measurements. MAybe just a resistance check at the testpoints? I dunno. I do have the pinout for the backplane so can most likely wire it without blowing it up. would that be wise or not advised? I am trying to avoid having him try to use it only to find I made a boneheaded mistake. Hes on tour a lot and has lot of session work so we don't have a much time to spend troubleshooting at his place in Chicago. I'm in the far west suburbs. Anyway, Thanks in advance!
Dave

edit: I have finished one VP28 and 3 opamps so far. The build is going very well for me with minimal issues. Thanks to the designer for what looks like very nice preamps! I have heard they a top quality and based on the attention to detail, I can believe it!

You could get a pcb like this, or get a card edge connector and make a jig to connect to your power supplies and function gen.

https://capi-gear.com/catalog/product_info.php?cPath=87_88&products_id=406

 

[Djacobs]

You could get a pcb like this, or get a card edge connector and make a jig to connect to your power supplies and function gen.

https://capi-gear.com/catalog/product_info.php?cPath=87_88&products_id=406

Yes. Thank You! That jig would do the job I think! Just noticed that I sold my 48v supply so won't be able to check that part of the circuit. Hope that won't mess things up. I've been retired for quite a few years and haven't had much need to use a lot of my old gear!

 

[Ricardus]

You could power it and check inputs and outputs without the edge connector but it could get ugly and you could short something out quite easily. +48, -16, GND, and +16 are all on adjacent pins.

 

[Brian Roth]

Yes. Thank You! That jig would do the job I think! Just noticed that I sold my 48v supply so won't be able to check that part of the circuit. Hope that won't mess things up. I've been retired for quite a few years and haven't had much need to use a lot of my old gear!

If you're just building/testing a couple of modules, an inexpensive solution is buy one of these:

https://capi-gear.com/catalog/product_info.php?cPath=84_86&products_id=147
(or a similar edge connector from some other source) and solder/"fly" wire leads from the solder pins.

Bri

 

[ddld]

Look at the photo of the bottom of your PCB and you'll see a few solder joints that look like they have way too much solder and could be splayed out onto the PCB's groundplane. Pretty self explanatory IMHO.

Not sure exactly of the other question but it seems as every time you mention touching up solder joints the levels change. That seems like a clue to me.

Besides misplaced components, bad solder joints are the #1 issue in DIY builds.

Ok, now that I understand that the amount of solder is more critical on this type of board I removed a lot of it. I have reworked every single point on the main PCB under a magnifying glass. I have checked, reflowed, an re-checked every connection to the best of my ability, and this time I have set the gains to unity/0 for the measurements. I have re-checked the capacitor values and the resistors to the left of the first opamp.

The TP measurements are still consistently and stably incorrect at the following values:

TP1: 112 mV
TP2: 110 mV (incorrect from here)
TP3: 199 mV
TP4: 49 mV
TP5: 49 mV
TP6: 49 mV
TP7: 103 mV

I have two questions

1. Is there a way to isolate the issue further, zooming in on individual parts? I have a DMM and an oscilloscope available.
2. Are there any suspicious joints left you can see in the image? I find it difficult to judge the quality of the joints.

Thank you in advance.

 

[Djacobs]

If you're just building/testing a couple of modules, an inexpensive solution is buy one of these:

https://capi-gear.com/catalog/product_info.php?cPath=84_86&products_id=147
(or a similar edge connector from some other source) and solder/"fly" wire leads from the solder pins.

Bri

Thanks Brian, These 2 VP28s are most likely going to be the only things I will be building for the foreseeable future unless someone comes up with more stuff to build! I am trying not to spend much money in testing them at this point. I have no plans to build any of my own studio equipment here but my buddy is making quite a name for himself and I want to be sure my own work is solid. Another $30 or so for that jig would not be too much to help him out!
Dave

 

[Djacobs]

You could power it and check inputs and outputs without the edge connector but it could get ugly and you could short something out quite easily. +48, -16, GND, and +16 are all on adjacent pins.

Thanks! Those are good points. I was maybe gonna just alligator clip wires to the card edge pads but that could be problematic! I also migh have a card connector in my stash so I'll check that out too!
Dave

 

[jsteiger]

Ok, now that I understand that the amount of solder is more critical on this type of board I removed a lot of it. I have reworked every single point on the main PCB under a magnifying glass. I have checked, reflowed, an re-checked every connection to the best of my ability, and this time I have set the gains to unity/0 for the measurements. I have re-checked the capacitor values and the resistors to the left of the first opamp.

The TP measurements are still consistently and stably incorrect at the following values:

TP1: 112 mV
TP2: 110 mV (incorrect from here)
TP3: 199 mV
TP4: 49 mV
TP5: 49 mV
TP6: 49 mV
TP7: 103 mV

I have two questions

1. Is there a way to isolate the issue further, zooming in on individual parts? I have a DMM and an oscilloscope available.
2. Are there any suspicious joints left you can see in the image? I find it difficult to judge the quality of the joints.

Thank you in advance.

TP1 is immediately after the input transformer. TP2 is after the preamp opamp A1. Not sure if we covered this but are you sure that opamp is working correctly? R5-R8 are related to A1 gain as well as the RG1-11 R's. Next I would try lifting the primary of T2 which is orange and red. See if that changes your TP2 level.