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

[soundsofsirens]

Hi guys,


i recently got a vp28 off someone online but when testing it, the output was super low and i had to crank it to almost full to get anything out of it and it also distorted. the waveform that appeared on my daw looked weird. is the a waveform of any significance to show what went wrong and how i can rectify it?

I'm really new to this but would like to learn more but don't really know where to start.

 

[nicholasdover]

I've built a few of these now for friends and keep a pair myself, but in helping a buddy out of a tight spot have landed myself with one that has an issue. ("Hey, don't worry dude - you've got a session booked in, I've got some spare time, I'll find the problem...") It's basically functioning properly but the whole sound has a low cut going on. When I engage filters I do hear a difference so it's not that they're always on - the whole bottom end is being sucked out somewhere. I read in a post from a while back that transformers are potential cause but my readings for all 3 of those are normal. I don't have an extension jig so haven't done voltage readouts - perhaps that should be next step? I'm in UK so all moves to order gear have to be done carefully as it all costs a lot. Are there any other components which are (a) always in signal path with (b) capability to suck low frequencies away while retaining otherwise normal functionality and pass signal? No capacitors look blown, but to test them I guess I'd need to desolder one by one, right?
Any advice welcome.
PS Have done full solder reflow, DOA swaps, transformer tests (out of circuit). That's where I'm at.
Cheers
Nick

 

[c1bomber]

On a newly built and seemingly totally functional vp28 I'm getting the following results from the VP2xx assembly guide final testing prodecures:

-v to +v = 32-35k at both op amp sockets (way below 200k)
-v to c = 29-32k at both op amp sockets (way below 200k)
+v to c = 4k at both op amp sockets (way below 200k)
o to c = above 40M at both op amp sockets
+v to o = above 40M at preamp and 35-38k at fader booster
-v to o  = 32-39M at both op amp sockets

is this worrisome?

thanks!

 

[Catrunes]

Hey all, just finished up another pair of these guys and im getting some things happening that shouldnt be. one module isnt passing correctly but ill compare to my functioning builds but the other module is weird. when supplied power from 2 diff racks all the leds besides the signal led are lit at all times. any ideas?

 

[nicholasdover]

The last 6 questions don't seem to have had any answers - has this thread gone cold...?
Or maybe the questions are kind of repetitions of old ones?

If anyone could advise which capacitors might be worth replacing on the off chance?

I really want to get this working again as I'm missing it on sessions and as far as I can tell it can now only be a fault with  (a) a capacitor (b) a resistor  (c) PCB trace, but it's so close to working correctly it's driving me mad!

 

[TDJ]

Hey guys! Not sure if this thread is being answered anymore, but I have a problem. I finished a couple of VP28s and I am having an issue with one. It doesn’t pass signal except for the helicopter ground noise. The green signal led is constantly lit. I measured 25mv on test point one. I moved over to see if the op amps were getting signal and then r29 smoked. Not sure if I accentally moved something that made it smoke up? Or if it’s part of my problem. Any help would be appreciated.

 

[Potato Cakes]

Hey guys! Not sure if this thread is being answered anymore, but I have a problem. I finished a couple of VP28s and I am having an issue with one. It doesn’t pass signal except for the helicopter ground noise. The green signal led is constantly lit. I measured 25mv on test point one. I moved over to see if the op amps were getting signal and then r29 smoked. Not sure if I accentally moved something that made it smoke up? Or if it’s part of my problem. Any help would be appreciated.

Did you test voltages without op amps installed? Did you build your DOAs?

Thanks!

Paul

 

[Potato Cakes]

I've built a few of these now for friends and keep a pair myself, but in helping a buddy out of a tight spot have landed myself with one that has an issue. ("Hey, don't worry dude - you've got a session booked in, I've got some spare time, I'll find the problem...") It's basically functioning properly but the whole sound has a low cut going on. When I engage filters I do hear a difference so it's not that they're always on - the whole bottom end is being sucked out somewhere. I read in a post from a while back that transformers are potential cause but my readings for all 3 of those are normal. I don't have an extension jig so haven't done voltage readouts - perhaps that should be next step? I'm in UK so all moves to order gear have to be done carefully as it all costs a lot. Are there any other components which are (a) always in signal path with (b) capability to suck low frequencies away while retaining otherwise normal functionality and pass signal? No capacitors look blown, but to test them I guess I'd need to desolder one by one, right?
Any advice welcome.
PS Have done full solder reflow, DOA swaps, transformer tests (out of circuit). That's where I'm at.
Cheers
Nick

Check to see who over on that side of the world sell extension test jigs, because they are absolutely necessary building modules, as you are finding out. Sahib at Total Audio Control does and he's in Glasgow.

Troubleshooting someone else's work can be tricky, as you have to very carefully go over the BOM and the PCB overlay and make sure everything is in the correct place and oriented correctly, even if you already have you still may have missed something. That has happened to me several times.

Depending on how much roll off there is, it could be a bad solder joint on the input or output transformer or the wires for the output transformer are wired incorrectly. You could also have a very tiny solder bridge that is making an unwanted connection on the filter board that is hard to see. If all the components are good then something is either misplaced or a suspect solder joint is present.


Thanks!

Paul

 

[Potato Cakes]

Hi guys,


i recently got a vp28 off someone online but when testing it, the output was super low and i had to crank it to almost full to get anything out of it and it also distorted. the waveform that appeared on my daw looked weird. is the a waveform of any significance to show what went wrong and how i can rectify it?

I'm really new to this but would like to learn more but don't really know where to start.

Go to the VP28 build guide on CAPI's site and read it step by step and compare what you see actually on the unit you have. It's either the DOA's, transistors, or any of the solder joints connecting them to the board.

The VP28 is not a good project if you are new to this. It's even harder if you are correcting someone else's work.

Thanks!

Paul

 

[Potato Cakes]

On a newly built and seemingly totally functional vp28 I'm getting the following results from the VP2xx assembly guide final testing prodecures:

-v to +v = 32-35k at both op amp sockets (way below 200k)
-v to c = 29-32k at both op amp sockets (way below 200k)
+v to c = 4k at both op amp sockets (way below 200k)
o to c = above 40M at both op amp sockets
+v to o = above 40M at preamp and 35-38k at fader booster
-v to o  = 32-39M at both op amp sockets

is this worrisome?

thanks!

The VP28 is different that the VP2x (VP25 and VP26).

This guy:

http://www.capi-gear.com/catalog/images/gallery/VP28/Build-docs/RevB/CAPI-VP28-Rev-B-Test-Points.pdf

If you did have something actually wrong, it wouldn't work right, would have burnt out some resistors, or both. You're probably fine.

Thanks!

Paul

 

[jsteiger]

Hey guys! Not sure if this thread is being answered anymore, but I have a problem. I finished a couple of VP28s and I am having an issue with one. It doesn’t pass signal except for the helicopter ground noise. The green signal led is constantly lit. I measured 25mv on test point one. I moved over to see if the op amps were getting signal and then r29 smoked. Not sure if I accentally moved something that made it smoke up? Or if it’s part of my problem. Any help would be appreciated.

I am subscribed to this thread but have not received any email updates. Seems to be a glitch in the site here.

Our friend Potato Cakes has posted some very helpful info above, thanks Paul!

R29 does nothing more than connect the chassis ground plane (gold finger Pin-1) to the PSU ground plane (gold fingers 5 and 13). I have never smoked this R nor can I think of a simple reason on the build why this might happen. Not sure what rack you are using but theoretically, these card edge pins should be connected inside the PSU at a star ground point. If there is no connection inside the PSU (error on many racks) some current can be dumped and if the VP28 is the only card in the rack with something connecting them, R29 could smoke depending on the situation.

 

[TDJ]

Did you test voltages without op amps installed? Did you build your DOAs?

Thanks!

Paul

Yes, the op amps were in when that measurement of 25mv was taken at test point 1. And yes, I built them, but they worked inside of a different module. I don't have any 1/2w resistors on hand, so I'll post back once I can get a new R29 in. I'm thinking something must have moved when I was digging around to test around the DOAs. If you guys have any other suggestions of things I could check in the meantime, that would be great. Thanks in advance!

 

[jsteiger]

Yes, the op amps were in when that measurement of 25mv was taken at test point 1. And yes, I built them, but they worked inside of a different module. I don't have any 1/2w resistors on hand, so I'll post back once I can get a new R29 in. I'm thinking something must have moved when I was digging around to test around the DOAs. If you guys have any other suggestions of things I could check in the meantime, that would be great. Thanks in advance!

Well, TP1 is immediately after the 2622 so you should have signal here with no amps installed. Its completely passive up to this point.

As per my email, in theory the pre should function just fine without R29. Its sort of a precaution to make sure those 2 points are connected. What rack are you using? With no modules installed you can easily check if card edge connector pin 1 has continuity to pin 5 and/or pin 13.

 

[TDJ]

Well, TP1 is immediately after the 2622 so you should have signal here with no amps installed. Its completely passive up to this point.

As per my email, in theory the pre should function just fine without R29. Its sort of a precaution to make sure those 2 points are connected. What rack are you using? With no modules installed you can easily check if card edge connector pin 1 has continuity to pin 5 and/or pin 13.

Jeff & I e-mailed back and for for a while last night. For some strange reason, TP1 is now at proper voltage. However, I'm getting 0v at TP2 now. We've eliminated op amps. We popped in some different op amps that we know work (from a different VP28), and have the same symptoms - not passing audio, helicopter noise (grounding issue somewhere?), and green led stuck on. If anyone else has suggestions, I would more than welcome them at this point. I feel like i'm running out of ideas.

 

[Paxkal]

Hello,

I just mounted eight preamps CAPI VP28 and I have three that do not work after a transistor reversal on the 1731 and 2520 cards that burned me 10 ohm resistors. I solved the problem of the 1731 and 2520 cards and now I have three modules with problems. Here are the problems encountered:

-Module 1: low level at the output of the preamp
-Module 2: led signal (green) lit without any audio signal in input, I send signal in input but there is no sound in output
-Module 3: the signal enters the card correctly the led signal (green) works perfectly no sound output

Someone to help me?

thank you very much

 

[kante1603]

Hello,

I just mounted eight preamps CAPI VP28 and I have three that do not work after a transistor reversal on the 1731 and 2520 cards that burned me 10 ohm resistors. I solved the problem of the 1731 and 2520 cards and now I have three modules with problems. Here are the problems encountered:

-Module 1: low level at the output of the preamp
-Module 2: led signal (green) lit without any audio signal in input, I send signal in input but there is no sound in output
-Module 3: the signal enters the card correctly the led signal (green) works perfectly no sound output

Someone to help me?

thank you very much

Welcome to the forum.
3 modules down,that's bad.
O.K.,first make sure the rack containing the modules should be disconnected from everything that belongs to audio.
Check voltages,as described in the build manual.
Depending on the version you have you can follow the test points guide (if you have rev B):


http://www.capi-gear.com/catalog/images/gallery/VP28/Build-docs/RevB/CAPI-VP28-Rev-B-Test-Points.pdf


Please only use DOAs that are definetely known as working!
I also highly recommend the use of extender cables or jigs,Jeff has them in his shop:


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


Depending on your location you can get extender cables from other sellers too.


Please post your findings here so others can participate or chime in,maybe even Jeff Steiger.


Best regards,


Udo.

 

[jsteiger]

Paxkal, do you have any 2520 style opamps that are for sure good working amps? Something you have used and tested in another module? If not, you have broken the cardinal rule of building my (or similar) stuff. That is never try to test a freshly built module with freshly built opamps. If something isn't working right, we really have no way of knowing if its the pre or one or both of the opamps. At this point, I would suggest getting a pre-built pair of opamps somewhere. That way you can see if the pre's are truly not working.

 

[kante1603]

Paxkal, do you have any 2520 style opamps that are for sure good working amps? Something you have used and tested in another module? If not, you have broken the cardinal rule of building my (or similar) stuff. That is never try to test a freshly built module with freshly built opamps. If something isn't working right, we really have no way of knowing if its the pre or one or both of the opamps. At this point, I would suggest getting a pre-built pair of opamps somewhere. That way you can see if the pre's are truly not working.

Hi Jeff,


he wrote he has 8 modules "mounted",so -in case the other five are properly working- he should have a bunch of them…..


Cheers,


Udo.

 

[jsteiger]

he wrote he has 8 modules "mounted",so -in case the other five are properly working- he should have a bunch of them…..

Indeed Udo! I missed that part! Time to start swapping out opamps.

The biggest failure with DIY builds are solder joints.

 

[Paxkal]

Actually the solder joints ....

Thank you all !!!!