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

[ddld]

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.

Yes, I tried both opamps in my other VP28. They work. Thanks for this! I’ll try this tomorrow and report back.

 

[marktokach]

Great job on this one, Jeff! Successful build, took a few evenings of an hour here and there.
(only 1 leftover washer, and couldn't seem to find two lock washers, but I probably shouldn't build these things late at night!!)

In the studio now, comparing it to my other pre's... Enjoying it so far.
Thank you thank you thank you for the signal indicator LED!!!

-Chris

EDIT: *love* it on guitar. driving the preamp gain hard results in some happiness.

 

[marktokach]

Yes, I tried both opamps in my other VP28. They work. Thanks for this! I’ll try this tomorrow and report back.

Here are the results from the test points on the CAPI VP28 PREAMP 500 modules.




Test equipment used:

Audio Signal Generator-BK Precision 3301 Audio Generator.

Settings-420Hz (closest setting to 400Hz as recommended by Jeff)

Voltage across pins 2 and 3 of an xlr-1.228vAC from the Audio Generator

The BK did not allow me to set output impedance it is 600 ohm only.

Digital Multimeter used-Fluke 177 set to vAC



CARD 1 passes audio and the signal led works. The readings are as follows:

TP1-103mvAC

TP2-649mvAC

TP3-1.15vAC

TP4-286mvAC

TP5-285.8mvAC

TP6-285.8mvAC

TP7-598.9mvAC



CARD 2 passes audio and the signal led works. The readings are as follows:

TP1-103.2mvAC

TP2-.92vAC

TP3-1.63vAC

TP4-507.4mvAC

TP5-507.3mvAC

TP6-503.2mvAC

TP7-1.068vAC



CARD 3 DOES NOT PASS audio and the signal led DOES NOT work. The readings are as follows:

TP1-102.8mvAC

TP2-26mvAC drops to 1mvAC

TP3-20mvAC drops to 1mvAC

TP4-56mvAC drops to 1mvAC

TP5-50mvAC drops to 1mvAC

TP6-78mvAC drops to 1mvAC

TP7-72mvAC drops to 1mvAC



CARD 4 passes audio and the signal led DOES NOT work. The readings are as follows:

TP1-102.6mvAC

TP2-641.8mvAC

TP3-1.143vAC

TP4-282.6mvAC

TP5-282.6mvAC

TP6-282.6mvAC

TP7-591.4mvAC



CARD 5 passes audio and the signal led works. The readings are as follows:

TP1-102.3mvAC

TP2-643mvAC

TP3-1.14vAC

TP4-282.2mvAC

TP5-282.2mvAC

TP6-282.2mvAC

TP7-591.5mvAC

 

[ddld]

Here are the results from the test points on the CAPI VP28 PREAMP 500 modules.




Test equipment used:

Audio Signal Generator-BK Precision 3301 Audio Generator.

Settings-420Hz (closest setting to 400Hz as recommended by Jeff)

Voltage across pins 2 and 3 of an xlr-1.228vAC from the Audio Generator

The BK did not allow me to set output impedance it is 600 ohm only.

Digital Multimeter used-Fluke 177 set to vAC



CARD 1 passes audio and the signal led works. The readings are as follows:

TP1-103mvAC

TP2-649mvAC

TP3-1.15vAC

TP4-286mvAC

TP5-285.8mvAC

TP6-285.8mvAC

TP7-598.9mvAC



CARD 2 passes audio and the signal led works. The readings are as follows:

TP1-103.2mvAC

TP2-.92vAC

TP3-1.63vAC

TP4-507.4mvAC

TP5-507.3mvAC

TP6-503.2mvAC

TP7-1.068vAC



CARD 3 DOES NOT PASS audio and the signal led DOES NOT work. The readings are as follows:

TP1-102.8mvAC

TP2-26mvAC drops to 1mvAC

TP3-20mvAC drops to 1mvAC

TP4-56mvAC drops to 1mvAC

TP5-50mvAC drops to 1mvAC

TP6-78mvAC drops to 1mvAC

TP7-72mvAC drops to 1mvAC



CARD 4 passes audio and the signal led DOES NOT work. The readings are as follows:

TP1-102.6mvAC

TP2-641.8mvAC

TP3-1.143vAC

TP4-282.6mvAC

TP5-282.6mvAC

TP6-282.6mvAC

TP7-591.4mvAC



CARD 5 passes audio and the signal led works. The readings are as follows:

TP1-102.3mvAC

TP2-643mvAC

TP3-1.14vAC

TP4-282.2mvAC

TP5-282.2mvAC

TP6-282.2mvAC

TP7-591.5mvAC

That’s some very interesting info! Quite some variance there.

Meanwhile, I’ve been downed by the flu this week, but I did actually make some progress before that. After jsteiger pointed me to the relevant resistors I looked at the traces and figured out that the Grayhill switches and the resistors next to them just form one big variable resistor series to ground. The switch taps in at the desired point.

If I measure the resistance from ground to any point in the series, I see that the channel fader is working as expected, but the preamp gain loses connection circa the fourth resistor. Reflowing (again, and again…) did not fix it.

I suspect that the solder might have been contaminated, even though I kept my work environment clean. Maybe I waited too long to put on a fresh tip, flux cropped up, who knows.

I plan to remove all the resistors in that section, clean the pads, and give it another go.

 

[marktokach]

Here are the results from the test points on the CAPI VP28 PREAMP 500 modules i built. I believe Card 1 works properly. Card 2 is hotter but works, Card 3 does not pass any audio and Card 4 passes audio but the signal led doesn't work. Any help is appreciated. I will post some pics of the solder side soon. The cards are at my friends studio a few hours away for the testing.



Test equipment used:

Audio Signal Generator-BK Precision 3301 Audio Generator.

Settings-420Hz (closest setting to 400Hz as recommended by Jeff)

Voltage across pins 2 and 3 of an xlr-1.228vAC from the Audio Generator

The BK did not allow me to set output impedance it is 600 ohm only.

Digital Multimeter used-Fluke 177 set to VAC



CARD 1 passes audio and the signal led works. The readings are as follows:

TP1-103mvAC

TP2-649mvAC

TP3-1.15vAC

TP4-286mvAC

TP5-285.8mvAC

TP6-285.8mvAC

TP7-598.9mvAC



CARD 2 passes audio and the signal led works. The readings are as follows:

TP1-103.2mvAC

TP2-.92vAC

TP3-1.63vAC

TP4-507.4mvAC

TP5-507.3mvAC

TP6-503.2mvAC

TP7-1.068vAC



CARD 3 DOES NOT PASS audio and the signal led DOES NOT work. The readings are as follows:

TP1-102.8mvAC

TP2-26mvAC drops to 1mvAC

TP3-20mvAC drops to 1mvAC

TP4-56mvAC drops to 1mvAC

TP5-50mvAC drops to 1mvAC

TP6-78mvAC drops to 1mvAC

TP7-72mvAC drops to 1mvAC



CARD 4 passes audio and the signal led DOES NOT work. The readings are as follows:

TP1-102.6mvAC

TP2-641.8mvAC

TP3-1.143vAC

TP4-282.6mvAC

TP5-282.6mvAC

TP6-282.6mvAC

TP7-591.4mvAC

 

[marktokach]

That’s some very interesting info! Quite some variance there.

Meanwhile, I’ve been downed by the flu this week, but I did actually make some progress before that. After jsteiger pointed me to the relevant resistors I looked at the traces and figured out that the Grayhill switches and the resistors next to them just form one big variable resistor series to ground. The switch taps in at the desired point.

If I measure the resistance from ground to any point in the series, I see that the channel fader is working as expected, but the preamp gain loses connection circa the fourth resistor. Reflowing (again, and again…) did not fix it.

I suspect that the solder might have been contaminated, even though I kept my work environment clean. Maybe I waited too long to put on a fresh tip, flux cropped up, who knows.

I plan to remove all the resistors in that section, clean the pads, and give it another go.

Any luck?

 

[ddld]

Any luck?

Inching closer, yes. Progress has been a bit slow because I was/am waiting for additional tools, and I was struck bad by the flu.

I isolated the (?) problem to one single resistor in the RG sequence using the DMM. I took it out cleanly using a desoldering gun, and then I noticed a small mechanical obstruction in the PCB hole.

The obstruction prevented me from properly pushing the resistor lead through. I vaguely remember this from the initial assembly, and I recall thinking it might not be a problem because a good portion of the lead went in. Apparently, that was a bad idea.

I have some miniature drill bits coming in by mail. I hope to use them (hand-held) to remove the obstruction, solder up, and try again.

Update: I added a picture of the obstruction. It is in the RG5 hole. This is not leftover tin. It was there when I started out.

 

[ddld]

One problem solved, another isolated! It looks like I have a Grayhill switch that doesn't do anything.

So, my microdrill bits arrived, and I removed the obstruction/manufacturing defect in the PCB hole of RG5. The RG series of resistors now properly measures around 25k for the total, so that problem seems solved.

Next, I tried to measure the entire assembly of the Grayhill switch with the RG registors. That did not work, to my surprise. I couldn't measure any signal to pass through the switch, so I desoldered the Grayhill entirely (yay, desoldering gun!). The switch does not make any connection in any position. To be sure I wasn't misunderstanding how it is supposed to work, I popped out the channel fader switch and did the same measurements for comparison. It behaves as I would expect.

@jsteiger: is there a way to apply for replacement parts? Your website is down/blank at the moment.

Addendum: I lifted the red and orange leads of T2 as you suggested. That didn't change much. I used the working Grayhill (from the channel fader) in the preamp gain position now, but that doesn't solve the problem of having no signal at TP2.

 

[ddld]

SUCCESS!!

While looking for the original API schematics I found that the Capi VP26 does in fact have a published schematic. Since the first half of the VP28 is largely the same, I could finally check a whole lot more than just the test points, and I could reason about what the measurements mean.

It turned out that the opamps were not getting a correct negative rail voltage. The voltage was not being passed through/over the CR1 diode. Once again, reflowing didn’t help. I removed it, slightly enlarged the left hole (the leads are very thick, and the holes aren’t always smooth on the inside), put it back in, and now I am finally seeing 700 mV on TP2. Whoop!

So, to summarize, I had four problems in total: 1. a bad solder joint at the input transformer, 2. a mechanical blockage/manufacturing defect in a resistor hole, 3. a dead Grayhill switch, 4. another hole/lead fitting issue with a diode.

It’s been quite an adventure, this one. I’ll try to get the Grayhill replaced, and I hope for smooth sailing in stage 2.

Update: @jsteiger sent me a replacement Grayhill. I just put it in, now I'm seeing all the correct test point values! I'm moving it it to the studio and putting it in the rack later to test the sound.

 

[arsmusic]

HI, I have a vp28 rev c to build. I cannot find any build guides for this. I see the rev 1 on page 1 of this forum. And the BOM for rev c on the capi site. Do you have to just use that to make it? I have built a few 11 space 500 capi racks and they had a easy build guide. I guess not for this one though. Any one have any good resources for building one unit? Other than the BOM page?

 

[jsteiger]

HI, I have a vp28 rev c to build. I cannot find any build guides for this. I see the rev 1 on page 1 of this forum. And the BOM for rev c on the capi site. Do you have to just use that to make it? I have built a few 11 space 500 capi racks and they had a easy build guide. I guess not for this one though. Any one have any good resources for building one unit? Other than the BOM page?

We don't yet have a full on pictorial build guide for the VP28. Its been on my list to do for years but never have enough time to dedicate to it. Everything we currently have is posted here https://capi-gear.com/catalog/support_docs.php#VP28

There is an assembly aid posted there. That and this support thread and the BOM is plenty for most everyone to cross the finish line.

 

[arsmusic]

Will this work without this switch here for the 80hz LCF? One of the switches is broken. I want to finish the build and test it before I get the replacement ordered. Will it work just without the 80hz cut working? Or will the entire pre just not work yet?

 

[jsteiger]

Will this work without this switch here for the 80hz LCF? One of the switches is broken. I want to finish the build and test it before I get the replacement ordered. Will it work just without the 80hz cut working? Or will the entire pre just not work yet?

Signal runs thru that switch so it'll need to be in there.

 

[ozzyphil]

Hi all..
New builder here on my third vp28. First 2 went swimmingly apart from some led leg shenanigans
For this build i have stupidly soldered R29 (10R 0.5 watt) into RP1.
I've had success in the past desoldering with wick but after a couple of attempts i'm getting nervous and making some marks
Can i just leave it in place and continue with PR2 3 and 4 and replace RP29 with locally sourced 10R?
Or should i persevere with desoldering?
Thanks for any advice
Beautiful kits and very satisfying to build

Phil

 

[jsteiger]

Hi all..
New builder here on my third vp28. First 2 went swimmingly apart from some led leg shenanigans
For this build i have stupidly soldered R29 (10R 0.5 watt) into RP1.
I've had success in the past desoldering with wick but after a couple of attempts i'm getting nervous and making some marks
Can i just leave it in place and continue with PR2 3 and 4 and replace RP29 with locally sourced 10R?
Or should i persevere with desoldering?
Thanks for any advice
Beautiful kits and very satisfying to build

Phil

I suppose you could leave it. I myself would change it out so its correct.

 

[ozzyphil]

Thanks Jeff..had another stab and sorted it. Even managed to reuse R29. I'd clipped the legs unfortunately.

 

[mkerlin]

Could use a troubleshooting help on my first VP28. This is my seventh CAPI kit. 1 thru 6 work perfectly. Built eight Op amps and they all are fine as well. My soldering is excellent (IMO). Using two of the previously built and tested Opamps I get no signal through the preamp. Following the Test Point guide:

Audio Signal Generator-DAW>Apollo x8
Settings-440Hz and dialed the output to 1.227V AC across pins 2 & 3 and at 8 and 10 on the test card
Fluke 177 set to vAC:
TP1-480mvAC
TP2-674mvAC
TP3-664mvAC
TP4-555mvAC
TP5-615mvAC
TP6-572mvAC
TP7-526mvAC

It looks like I have an issue on the front end but I really stumped. Appreciate any help. Thanks.

ps I also tested DCR T2 and T3

Red>Blk 15.9 & 15.7
Pink>Gr 31.0 & 30.5
Pur>Oran 30.7 & 30.7

 

[mkerlin]

Continued troubleshooting, and solved issue above. Given the location of my issue wasn't getting me by TP1 I suspected the input transformer. I didn't know what else to do except pull a CA2622 from another one of my functioning pre's and replace the one in the VP28. Vola, signal light on and all test points are right on the money.

I don't know how to test the suspect CA2622, and I thought maybe it could have been a bad solder, I popped it in the VP26 I stole the good one from and now that preamp does not function. I can only conclude it was a bad CA2622.

 

[kante1603]

Continued troubleshooting, and solved issue above. Given the location of my issue wasn't getting me by TP1 I suspected the input transformer. I didn't know what else to do except pull a CA2622 from another one of my functioning pre's and replace the one in the VP28. Vola, signal light on and all test points are right on the money.

I don't know how to test the suspect CA2622, and I thought maybe it could have been a bad solder, I popped it in the VP26 I stole the good one from and now that preamp does not function. I can only conclude it was a bad CA2622.

Hello,

there's a pdf on Jeff´s homepage, look here:

https://www.capi-gear.com/catalog/images/gallery/2622/CA2622-specs.pdf

It shows the pinouts of a CA2622.
Compare it with a healthy transformer if possible.
If not measure at least the 3 windings for DCR (multimeter set to ohms).The primaries (pins 1 & 3 and pins 2 & 4) should have identical values, the third is the secondary (pins 5 & 8).If any of them shows close to zero ohms then there is a short, when measuring infinity (multimeter out of range) then you have an open condition.Also none of the windings should connect to anything else.Each of these issues shows a broken transformer.Should be done out of circuit of course.
Another hint:Can the transformers be mounted in reverse? Looks like it's possible in the pdf......

Good luck!

 

[mkerlin]

Hello,

there's a pdf on Jeff´s homepage, look here:

https://www.capi-gear.com/catalog/images/gallery/2622/CA2622-specs.pdf

It shows the pinouts of a CA2622.
Compare it with a healthy transformer if possible.
If not measure at least the 3 windings for DCR (multimeter set to ohms).The primaries (pins 1 & 3 and pins 2 & 4) should have identical values, the third is the secondary (pins 5 & 8).If any of them shows close to zero ohms then there is a short, when measuring infinity (multimeter out of range) then you have an open condition.Also none of the windings should connect to anything else.Each of these issues shows a broken transformer.Should be done out of circuit of course.
Another hint:Can the transformers be mounted in reverse? Looks like it's possible in the pdf......

Good luck!

Thanks for the reply

Readings:
1-3 62.0
2-4 68.9
5-6 1.567
1 to can OL
2 to can OL
3 to can OL
4 to can OL
5 to can OL
6 to can OL
7 to can 0.6
8 to can OL

Jeff says this fully checks out. Transformer orientation was correct. I can’t explain it other than to say it didn’t work in two different preamps but the one I swapped from a working preamp did. I’m an engineer and I don’t believe in coincidences, and for me to solder it bad twice I wouldn't have thought likely, but I can't exclude it.

Jeff was nice enough to get a replacement off to me. I'll send him this one and see how he makes out with it.