[Build] CAPI VP312~500/51x Series~Preamp Kit~Official Support Thread

[jsteiger]

Hello,

I recently built a couple of these however one of them doesn't have any output signal. For reference I've tried swapping op amps ( currently using one of Capi's prebuilt 2520 clones) According to the guide c7 should be 120uf 50v nichicon however I received a 100uf 63v kingtronics gf2323 PET capacitor instead in the one that isn't working- besides that I've triple checked that all the resistor and capacitor orientations are the same/ correct.

I de-soldered the output transformer leads of the non working preamp and compared it to the others I have uninstalled and the non working card's output was in the expected ranges. For reference:

Orange & Yellow - 11.4 ohms
Green & Blue- 11.6 ohms
Violet & Gray- 11.2 ohms
Brown & red- 11.7 ohms


After ensuring both have the same gain settings (stepped input middle , max output attenuator) and the phantom is off-
without uninstalling the input transformers and comparing eachother unpowered, I did notice a discrepancy however

Good Installed input:
Pin 1 & 3 -12 ohms
Pin 2 & 4- 11.5 ohms
Pin 5 & 8- 1.89k

Non Working 312 installed input transformer:
Pin 1 & 3- 11.5 ohms
Pin 2 & 4- 11.5 ohms
Pin 5 & 8- 9.8 Ohms

Any further troubleshooting I can try here? Pretty stumped

I would remove the input and measure DCR while its not installed.

 

[geardude]

I would remove the input and measure DCR while its not installed.

Removed & here are the measurements:

Pin 1 &3 - 23.3 Ohms
Pin 2 & 4 - 24 Ohms
Pin 5 &8. 1.948k

The extra uninstalled input I have measures really close to these figures as well

 

[jsteiger]

Removed & here are the measurements:

Pin 1 &3 - 23.3 Ohms
Pin 2 & 4 - 24 Ohms
Pin 5 &8. 1.948k

The extra uninstalled input I have measures really close to these figures as well

This looks correct.

 

[geardude]

This looks correct.

What should my next step of testing be?Could it be a pad or mute switch or the grayhill,maybe the op amps aren't connecting in the plugs properly? I have the schematic and couldtry taking measurements following the diagrams between both the working and non working modules to see where there's a difference. I notice both of the following capacitors are different than the others I have built but I'm not sure if that would cause radical reading differences.

c7 should be 120uf 50v nichicon however I received a 100uf 63v kingtronics gf2323 PET capacitor instead
c2 is an older wafer style cap that looks similar to those found in the old school op amp kits

Not sure if it impacts anything really but I’m not schooled in these matters and have no clue but I did Omit C5 & C6, C1 & R2
and Put the 100k resistor in R3 just like the other I have built.... in either case not sure how to proceed next and any insight would be appreciated!

 

[justinmantell]

Matt, no gain tells me that there is a cold solder joint in the inverting opamp to ground string. I suggest following the schematic and looking at the 200Ω R, the 330µF cap, the string of Grayhill resistors and the switch. If something in there is floating, the gain switch will not function.

Is there a possibility that the floating item could be the GH switch itself? This is the second Stepped (Litz) VP25 I've built, and I am experiencing the same issues of everything working on both except for the GH switch on the fresh build. Tonally & with input/output level measured against a 1kHz sine wave I can confirm that both units function exactly the same in respect to mute, phase, pad, phantom, and they both react as expected and equally smoothly when using the attenuation knob.

All pre-flight testing of the new unit checked out, and both opAmps for both units work equally well. I also achieved a 25K overall resistance when checking the outer most pins of the GH switch and visually all my solders are shiny and appear to have decent to good flow in the work itself. I experience this no gain issue no matter in which slot of my 6 working lunchbox slots this faulty VP25 sits. I also checked the GH switch in the circuit by placing an alligator clip on GH pin 1 and then moving my DMM probe up the resistor ladder while switching to a higher position on the GH and the values read out to the DMM to match the BOM re: resistor values. Not sure if this is a real test or if it means anything about the switch, but I wanted to share.

I spent the weekend tracing back all items in the 330uF cap and 200 R string, tripled checked and confirmed all components are in their correct place and of correct value and orientation (including all leads from the Litz transformer), desoldered and re-soldered the board end to end several times but I can't seem to solve this issue of GH switch and no gain. Both VP25's sound gorgeous in stereo with GH in position 1 (off?) -- the first step on the GH switch -- and both sound insanely gnarly on position 12 (full bore) on the GH switch, however my newly built unit lacks the ability to switch to any of the gain stages between positions 1 & 12 -- in other words, the new unit has two modes: 1. No gain, and 2. Full gain.

I am by no means without a pre, as I wouldn't use the gain section for reamping sources from interface out to pre and back in, and I wouldn't use gain for stereo DI sources, such as synth, etc., and I can probably get by just fine sticking this VP25 in front of a loud guitar or bass amp cranking into an SM57, but I would love to be able to solve the issue of the gain stage mystery and have two fully working units in my rack.

Thank you in advance.

 

[jsteiger]

You can easily test if the Grayhill switch wiper is making continuity with each of the respective 12 pin positions as you turn it. Just check with a DMM.