ddld
Member
Yes, I tried both opamps in my other VP28. They work. Thanks for this! I’ll try this tomorrow and report back.
[BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
- Thread starter jsteiger
- Start date
Help Support GroupDIY Audio Forum:
Yes, I tried both opamps in my other VP28. They work. Thanks for this! I’ll try this tomorrow and report back.
marktokach
Well-known member
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
Well-known member
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
Member
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
Well-known member
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
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
Well-known member
Any luck?
ddld
Member
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.
Attachments
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ddld
Member
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.
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.
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ddld
Member
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.
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.
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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.
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
New member
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
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
New member
Thanks Jeff..had another stab and sorted it. Even managed to reuse R29. I'd clipped the legs unfortunately.
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