[BUILD] CAPI VP2X~500 Series~Preamp Kit~Official Support Thread

I am out of town for a few days so won't be able to post any full details like that until next week.

Hey there,
So I built a few of these a couple weeks ago and all of them were working perfectly, however today I went recall a session from yesterday and ended up just switching the order of two of the pres and noticed the ones I used today on a particular instrument was about 4db quieter than the other I had used. I double checked the pres against each other and it was in fact quieter. I checked all cables, level settings, mics, knobs were aligned correct, etc etc so I know it is the preamp thats causing the signal different.

So I was wondering what I should check? I thought it was the opamp because I had one start making some noise before but that was just from a bad solder joint that was fixed. Thanks for the help

Is it variable or stepped gain?

If stepped, I would first compare all the R's behind the GH switches. And if stepped, I would check the 2 pre's against each other at all preamp gain settings.

Next check all other R's. Maybe something got switched around accidentally.

Jeff, I'm not getting adequate signal from my VP26. There are three issues here. Sending a mic level signal through my lunchbox only comes through when my gain pot is set to full, and does not result in enough gain, the phase switch kills all signal, and the attenuate pot works in reverse where the pot set to 0 on the face plate results in a fully attenuated signal, and 0 passes signal. I have passed all tests listed at the end of the install guide, and am passing signal with both dynamic and condenser microphones, but the signal is distorted and requires the stepped gain as well as the input attenuate pot, (in reverse,) to be set to full. The output transformer is wired to the PCB properly, and I have also swapped out the pre built GAR op amp with a JH990C that I know is in good working order. I also know my lunchbox and the connections from it to my UA Apollo to be tested and in good working order.

Can you send me the exact link for the posting or page number on this thread that involves signal injection for testing the VP26? What other suggestions do you have for getting this unit to go?

  Picture two.

Some of these symptoms combined makes me wonder about the output transformer. Desolder all of the leads and measure DCR between them. Set your DMM to a range of less than 100 ohms. Start at one end of the colored leads and measure each lead to all of the others. You should have DCR between only 3 sets of windings.

Jeff, measuring the output transformer leads has me slightly concerned. For this measurement I have to say that my DMM never quite settled on any of the measurements, and every time I measured I noticed significant changes compared to the last test. I also noticed that the orange and red was the most significant/solid reading by my DMM, but moving the red lead, (without touching or allowing any of the other connections to touch one anther), caused my resistance level reading to change significantly beyond the less than 30 ohm level.  If I had to average my findings I would say the following:

BLACK BROWN 36 Ohms
GREEN BLUE 26.6 Ohms
ORANGE RED         21.7 Ohms

After your last suggestion to test the output transformer I realized that I had done this at your request months ago, these are the numbers that I came up with then. I didn't document it, but I suspect that my measurement numbers were inconclusive then too, and that these numbers were taken as an average.

Black & Brown: 22.7 Ohms
Red & Orange: 14.4 Ohms
Green & Blue: 23.4 Ohms

-P

Was your DMM set to a low range like less than 50 ohms? Depending on the DMM, the readings should not change when moving them around. Make sure you are not touching the leads with your fingers.

Otherwise these results are correct. That tells me the polarity switch is most likely a bad solder joint. All that switch does is flip the red and orange leads to the 2623-1. My guess tells me that it could also be a bad solder joint on the t-pad.

Jeff, My DMM is a full auto sensing meter. I'm using an ideal 61-312. If I need something that tracks in a lower range to perform this test with better accuracy I might be able to borrow one.

I checked and reheated all t-pad and switch terminals with the same results. I see no bridged connections, and have ruled out cold joints.

I'm running out of ideas here for how this unit continues to not function. Please advise on how I can get it to go.

-P

You can scroll back and find the injecting signal pointers. It is pretty easy to do. You need to send a sine wave tone into the pre (-40dBu @ 400Hz) and use the schemo as a road map to "follow" the signal and see where it stops.

While doing some testing posted by Haima on this forum with my signal probe I was able to determine some questionable connections at the output section. See below:


F) now test the output all the way to the end - with your signal generator still connected as before, connect your probe "0v/sheild/ground" to the "black" lead on the output transformer or card edge pin 4.

G) probe the "blue" lead of the output transformer - got signal? if so,

I get no signal at this point.

H) probe the card edge pin 2 - turn the output attenuator - got signal? if not look at the attenuator wiring, everything on the output transformer secondary.

The turning of the output attenuator in any direction makes no difference.

Suggestions?

Do you have good signal at the primary of the 2623-1? That would be the red lead of the 2623-1 with the polarity switch out or disengaged.

Jeff, With my signal entering at the O socket of the op amp and my probe set at the red lead of the output transformer I get good signal with the phase switch disengaged and no signal with the phase switch engaged. This is done without an opamp in circuit and the pre disconnected from the power supply.

OK this is good. With the phase switch engaged, the signal should be flipped over the orange lead.

Yes, that's correct. Flipping the phase flips the output to the orange lead. Ideas?

Pardon me but it's hard to remember, have you fully desoldered the 2623-1 to test for internal shorts and open windings?

Yes, I have done that. Our back and forth about this is actually on the last page of this support forum.

OK I just found that. Did you measure each lead to all of the other leads or only verify the winding pairs?

Right now we are at the t-pad and output transformer. There is nothing left it can be and I highly doubt its the t-pad.

  Jeff, I checked all leads to one another and posted only the resistances that were noticed. Here's what I found:

BLACK  BROWN  36 Ohms
GREEN  BLUE  26.6 Ohms
ORANGE  RED          21.7 Ohms

OK, well next remove the t-pad. Move the blue wire from the 2623-1 over to the hole near the gold fingers that says opt no t-pad. You can just splice/solder a short lead on if its too short.