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

[brewhouse75]

One of my four VP28s has significantly quieter output than the others. Rechecked solder flows and everything looks good. Any suggestions as to where to go from here?

 

[Potato Cakes]

Are these newly built or a new issue with currently working units? Are the knobs oriented correctly to the shaft position? Did you compare signal traces the signal between a working and a non-working one? Did you swap op amps? Are the transformers wired correctly? Component orientation/placement? Transformer winding DCR?

No to be too abrasive, but just saying you rechecked solder flows and not knowing where to go from there means you haven't read through the thread. There are multiple places where people have low output and it is one of the above mentioned issues, but it does require some reading and usage of the search feature at the top of the page. The guys who are putting out these awesome products are very helpful and fully support their designs and even are generous with their time to help sort out end-user issues, but this is still DIY and it does require you to read through the information that is readily available on each specific thread and throughout the forum.

Thanks!

Paul

 

[Potato Cakes]

Hey Guys,
I have an odd one.
I’ve built over 20 of these preamps and this is the first real issue I’ve had with one.

Here is the setup-
Pro Tools tone generator sending pink noise out into an RND R6 rack. VP28 with the issue is plugged into slot 1.

The main symptom is that the unit passes a very very small signal out (basically nothing at all) in line mode when first turned on. The green LED does not show signal and the pre has to be cranked as well as the output fader to hear any resemblance of sound. I discovered that if I engage the “Mic” switch and immediately disengage it, the Line level input is suddenly working and passing signal at a good level. The odd thing is that when I come back the next day to re-test the same issue is occurring. I have also left it on for somewhere in the ballpark of 15-20 minutes and it suddenly drops out. This could then be fixed by again, engaging and disengaging the “Mic” switch.

Here is the trouble shooting steps I’ve taken.
-re-flow all solder points
-replace the Toneluck switch in the Mic/Line path
-triple check resistors are in the correct position.
-I have placed known working Op Amps in both Fader and Pre positions (Gar as well as SL Red Dots)
-I have also used a known working unit in this chain and it has no issues

Thanks for your help in advance.
-Ben

Have you swapped 500 series slots? Have you measured the VDC at the unit with this happens? If you had a second extender card you could run a signal trace between the two and see where it dies off and compare to the nonworking one. I would also check to make sure those caps behind the push button switch and fader are reading correctly. You can also pull the fader op amp, do the initial chassis power up (when you are having the issue), measure the signal at the IN+/- sockets, toggle the mic/line switch in and out, then measure the signal again at the fader op amp. If the signal is the same then the issue is somewhere downstream from there. It is also possible there is some sort of issue with Q1. From the description of the switching the mic/line setting and then it works sounds similar to latching issues with voltage regulators. Let us know.

Thanks!

Paul

 

[resilient]

Built my VP28 today.  And it is not working. 

I believe it's probably my op amps (also built those).  Or one of them, anyway.  I've reached out to Scott for some red dots.  I've also ordered an extender from CAPI so I can take measurements when they get here.  The reason I believe it's my DOAs is because if they're inserted one way, I get a constant signal LED.  Switch them around and there's no signal led. 

Brandon

 

[jsteiger]

Built my VP28 today.  And it is not working. 

I believe it's probably my op amps (also built those).  Or one of them, anyway.  I've reached out to Scott for some red dots.  I've also ordered an extender from CAPI so I can take measurements when they get here.  The reason I believe it's my DOAs is because if they're inserted one way, I get a constant signal LED.  Switch them around and there's no signal led. 

Brandon

Yeah, I would suggest using only known to be working opamps to test a new build.

 

[resilient]

That makes sense.  And I've got two red dots that should be here Wednesday or so.    I'll pop those in and see what happens.  I'm hopeful that fixes the issue and I can move forward.  I'll be back if not.  Haha!

Do you have a general troubleshooting guide, Jeff?  Specifically what voltages I should see where on the test points provided on the VP28?

Brandon

 

[jsteiger]

That makes sense.  And I've got two red dots that should be here Wednesday or so.    I'll pop those in and see what happens.  I'm hopeful that fixes the issue and I can move forward.  I'll be back if not.  Haha!

Do you have a general troubleshooting guide, Jeff?  Specifically what voltages I should see where on the test points provided on the VP28?

Brandon

There's a Test Points guide posted in the VP28 section of the Support Docs page at the store.

 

[resilient]

Awesome.  I'll check it out.  Thanks, Jeff.

Brandon

 

[resilient]

Well, I'm quite embarrassed.  I have made some progress.  Decided to try a known good 990 op amp.  No dice.  Then I realized I didn't have the mic button pressed in.  : Completely forgot about that. 

So I now get clean, beautiful signal, but I have to crank the gain on input and output both to get the levels anywhere decent (which may explain why I didn't get ANYthing in line mode).

I'm a bit relieved I've not screwed things up as badly as I thought I had.  Still have something screwy with the output and the LEDs being really dim.  I'll hammer on those a bit more after I can check voltages.

Brandon

 

[S-O]

Hey there! Thanks for putting out a rad kit that is super easy to follow! I had very few troubles assembling! The only problems I had were my own fault and were pretty easy to fix!

I have made 2/4 VP28's I bought. Halfway there!

I have one odd issue that may never really be much of a problem in the long run but I would like to see if there is an easy solution. I searched a bit and didn't find anything that matched my problem exactly.

So When I have the VP28 in the CAPI 511-VPR Filtered rack with DB25 cables plugged into it and into my patch bay the green signal LED is on. As soon as patch something in the input or output all is well and it turns off! If I plug headphones in I can hear that something seems to be oscillating.

Maybe something is grounding out or solder bridged but I can't seem to find out where or how! Any input would be rad and appreciated!

 

[jsteiger]

Hey there! Thanks for putting out a rad kit that is super easy to follow! I had very few troubles assembling! The only problems I had were my own fault and were pretty easy to fix!

I have made 2/4 VP28's I bought. Halfway there!

I have one odd issue that may never really be much of a problem in the long run but I would like to see if there is an easy solution. I searched a bit and didn't find anything that matched my problem exactly.

So When I have the VP28 in the CAPI 511-VPR Filtered rack with DB25 cables plugged into it and into my patch bay the green signal LED is on. As soon as patch something in the input or output all is well and it turns off! If I plug headphones in I can hear that something seems to be oscillating.

Maybe something is grounding out or solder bridged but I can't seem to find out where or how! Any input would be rad and appreciated!

What opamps are you using and who built them?

 

[S-O]

What opamps are you using and who built them?

Thanks for the speedy reply!

Lil ol' me built them! Not my first DIY attempt, but one of the more complex projects I have done!

I am using your GAR2520 v4.1 opamps!

 

[jsteiger]

Thanks for the speedy reply!

Lil ol' me built them! Not my first DIY attempt, but one of the more complex projects I have done!

I am using your GAR2520 v4.1 opamps!

That said, its quite possible that there is an error on one or more of the opamps so the green signal LED is doing its job due to maybe some noise or oscillations. I would test them with prebuilt or solid known to be working opamps.

 

[S-O]

That said, its quite possible that there is an error on one or more of the opamps so the green signal LED is doing its job due to maybe some noise or oscillations. I would test them with prebuilt or solid known to be working opamps.

Will do and will report back when I do! Thanks!

 

[resilient]

Ok, finally got everything put together and took some test measurements.

Here is what I have:

TP1:  110mv
TP2:  109mv
TP3:  192mv
TP4:  53mv
TP5:  53mv
TP6:  53mv
TP7:  54mv

Any thoughts on what I may have screwed up?  This is with Lieber red dot op amps, too, FWIW.

Brandon

 

[jsteiger]

Ok, finally got everything put together and took some test measurements.

Here is what I have:

TP1:  110mv
TP2:  109mv
TP3:  192mv
TP4:  53mv
TP5:  53mv
TP6:  53mv
TP7:  54mv

Any thoughts on what I may have screwed up?  This is with Lieber red dot op amps, too, FWIW.

Brandon

Try disconnecting the primary leads (PCB labels red and orange) from T2 and see if TP2 comes up to normal.

FWIW, to anyone who is troubleshooting the signal path...anything AFTER the first TP that does not measure right is not important until we get the FIRST place the signal is incorrect sorted.

 

[resilient]

Thanks for the reply, Jeff.

This made TP1 379mv and TP2 374mv.  All others were 0.

Brandon

 

[jsteiger]

Thanks for the reply, Jeff.

This made TP1 379mv and TP2 374mv.  All others were 0.

Brandon

This should not have changed TP1 at all. Something is strange possibly with your input level?

 

[resilient]

Well I tried again tonight.  Verified input was 1.22v and this time I got 111mv at TP1 and 109mv at TP2. 

Brandon

 

[jsteiger]

Well I tried again tonight.  Verified input was 1.22v and this time I got 111mv at TP1 and 109mv at TP2. 

Brandon

OK so the issue is between TP1 and TP2.

This means you have a misplaced resistor from R5-R8 OR something is wrong with your gain switch resistor string. That is R1, C1 and then RG1-RG11.

Are you sure the preamp gain knob is orientated the proper way?