HELP with CAPI VP28 and VP312 builds (including gar2520 opamp and 1731 opamp)


I recently (this past weekend) put together 2 CAPI VP28s and 2 CAPI VP312s. The VP28 and VP312 can be found here:

and here:

I have tested one of each and neither work. It appears the issue is one or both of the opamps (which I soldered together myself), according to information on the support thread here:

The documents for the vp28,and vp312 preamps, as well as the documents for the 2520 and 1731 opamps are found here:

Specifically, the vp28 overlay can be found here:

And the VP312 overlay can be found here:

The schematic for the VP312 is here:

The assembly manual for the gar1731 opamp is here:

The assembly manual for the gar2520 is here:

I will attempt to post pictures of the builds. I will also be attending the open house this evening. I am hoping someone has experience with these and can point out the problems. Thank you in advance for your time!


Not sure how I can add any images. Obviously those would be necessary in order to receive helpful feedback.


Hi Mario,

I bumped the ‘trust level’ on your forum account. I assume there’s some image uploading limit on new accounts to discourage spammers. Can you try again and see if you have a file upload option in the edit toolbar?



Those resistors look pretty toasty!

Without seeing it in person, I can’t spot any obvious errors. Better
pictures of the solder joints might help. Make sure there are no short
circuits between those joints. Double check that all of your components are
the right polarity - Transistors, some capacitors, and diodes are all
susceptible to this.

And we definitely have spares of those resistors, that’s not a big deal.


Hey Mario - wow, that is an impressive project and a ton of soldering. If you haven’t already done this it might be worth coming in tonight and taking a look at the opamps under our magnifying scope (it’s across from the #3D printer area). It’s great for clearly seeing the joints. Maybe you have a cold joint/bridge or something?

We have a mixed group of members in on Tuesdays but someone may be around who can help you troubleshoot further. So come in and show off this project and use our gear!



Hi Janet - Thank you! I will be coming by this evening. Hopefully with some guidance I can figure this out. Maybe enlisting superior soldering skills is what is necessary for the opamps.


Thank you, Jarrett. Based on a naked eye inspection there does not appear to be any solder joint bridges but a closer look may prove otherwise. I have also double checked that all of the components are positioned properly re: polarity. Based on others having a similar problem it would appear that the opamps are the source of my misery. Hopefully someone much wiser than myself can take a look tonight and help me solve this. I really want to continue these sorts of projects and learn more about the electronics behind much of my audio gear, as well as the products I use day to day. Unfortunately this project failure has deflated my proverbial tire.


Don’t feel deflated Mario! Even seasoned electronics folks run into troubleshooting issues. It’s good you’re sticking with it!


Apologies if you’ve already checked this but there was this warning at the head of both of your linked docs.

“To avoid massive build errors, make sure the BOM Revision you follow matches the Revision of the PCB you are building!!”


Thanks for looking into it! Yes, I made sure I had the correct BOM. I also confirmed with the company prior to building. I believe the issue is solder bridges or components touching one another, maybe both.


It was a pleasure meeting everyone tonight. You are all quite friendly and hospitable; I felt right at home.

Special thanks to Andrew for the introduction and to Bob for the guidance. Thank you Jackson and Mark for the company and assistance finding material. Here’s to hoping it works!


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