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

Hi guys.

Just built my second VP-28. The first one works perfectly, but the second has an issue.

All lights light up when hitting the knobs, but there is no sound or no green signal light lightning up when feeding sound to it.

To be honest I have no clue what so ever how and where to start the troubleshooting so any help is highly appreciated!

Hello everybody.

This is my first post and i'm glad to entered the diy group with those very nice preamp.
I recently build 8 VP28 and finally had 2 defective units.
For the first one, the green led was on all the time. I found i had a bad strip between C13 and R14. This problem is now resolved.
For the second one, the sound goes in and out of the preamp but my gain knob have any impact on the preamplification. Only the fader stage seemed to work properly. I tried with other "good" opamp (2520 and 1731) but the result is the same. Any ideas?
Cheers

Arno

Arno

When you say "bad strip between C13 and R14" what do you mean exactly? PCB trace or a short? C13 and R14 should not directly connect to each other. I'm interested to see what you did here as a few other folks have had this same issue. I have never had the problem or been able to reproduce it.

For the other one, it sounds like you have an intermittent solder joint somewhere on the PCB. Sometimes the physical impact of the switching action can cause the bad solder joint/audio to go in and out.

Hi Jeff,
My "bad strip" was a short due to a bad solder from one point of R14. I also forgot to precise that i found another bad solder on the EA2622.
I will check all my solder joint and i will let you know.
Thanks for your help.

Arno

Ok, so i checked my solder joint. I plugged my mic and...no sound...until i play with the grayhill knob (gain knob). I tried to "move" the knob and sometimes i have sound , sometimes not. I verified the solder and it's ok( it seemed ok). Now i have serious doubt on the knob. Do you think it's possible Jeff?

This still sounds like a bad solder joint. I had the exact same thing on a proto build. I naturally thought it was the Grayhill but turned out to be a iffy solder joint on a coupling cap clear across the PCB. You could run signal thru and start touching components around the board with you finger and see if you can find the intermittent joint.

So no reply for me  :-\

SgtAlexx said:

So no reply for me  :-\

Click to expand...

I think Jeff once posted a guide how to trace the signal through specific points.
Must be somewhere in this thread.
When you've found it look where the signal stops.
Also you can do the basic measuring like if the suplly voltages are correct on the doa sockets etc.
Maybe you can trace it with the good module side by side?
Then report back.


Best,


Udo.

Thanks.

@Jeff: Could you point me to the guide?

SgtAlexx, many people post with the same problems. If you go back thru this thread, the advice on where to start never changes.

First, rule out any outside possibilities like rack slot, associated cabling, converter settings and so on.

Next rule out opamps. Swap with other known good working opamps and see if the problem follows the opamp(s) or stays with the pre. Are the opamps from kits? It is very hard to test a newly built pre with newly built opamps. That's too many variables. Also, are they seated properly? http://classicapi.com/catalog/DOA_Install.php

The next step is as Udo says. Find the test points PDF and follow your signal thru the circuit.

There is not always a magic bullet or free lunch when a kit doesn't work. Most every time it will require some systematic diagnostics by the builder.

Wasn't looking for a free lunch, just a point in the right direction. These two beauties are the first ones I ever made and apart from checking rack slot, DOAs and so on I'm stuck on troubleshoot.

The test points PDF was what I was looking for. Thanks.

Hi everyone.

Just to let you know that i found the problem. The grayhill gain knob was the problem. I  changed it with one from another module and everything is going well now.
Bye

Arno

I've had my pair of vp28s for a few months and really love them. These are a must have!
Now I just need more.

I'm getting closer to finding the level discrepancy between my pair - I just noticed it only happens in Mic mode, not line. So I just wondered which components are engaged when Mic button is pressed since I guess they're the culprits (or joints thereof)?

Hallo i have a problem:
I finished my VP28 and :
the HP does not work properly. :-[
when 80HZ is engaged the signal is muted.
40HZ and 6 + 12  works great
if 40 HZ is engaged and i add 80HZ signal is getting a bit lower, but not much.

The rest works great and i like the sound very very much
Also gain and impedance fit´s perfect to my Royer Ribon Mic

Sounds like a bad solder joint on that pushbutton switch or one of the caps on the filter board. I would try reflowing those.

Thank you Jeff!

Did it!

Works great!

Andreas

Can't wait for my two vp26 to be delivered

Hi. I’m building my 6th vp28! I’m getting no output. Green light shows signal, reflowed solder and I’ve switched opamps. Any advice on where I should be placing my anxieties?

Thanks.

David J.

Update: Checked all polarity and resistor values and nothing. I notice if I push my board I can hear signal that is faint. Preamp gain changes volume but fader knob has no effect. Also, if I pull opamp 2 out it has no effect. Could it be a bunk transformer?

Jeff,

On one of my VP28 builds, with the HPF disengaged, the -6/-12 switch cuts the output when in the -12 position but not for -6. When the HPF is turned on, everything works fine. I didn't see a schematic for the preamp to aid in chasing down the culprit. I resoldered any suspect connections, but to no avail. Any thoughts?

Thanks!