Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
« Reply #1000 on: March 05, 2019, 12:20:19 AM »
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

Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
« Reply #1001 on: March 05, 2019, 01:42:43 AM »
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

Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
« Reply #1002 on: March 05, 2019, 02:11:12 AM »
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


 

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