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

[jsteiger]

The switch and t-pad issues possibly sound like bad solder joints to me. I never recommend a new preamp build with a new opamp build. Too many things can be wrong. You need to first make 100% sure that opamp is good. Do you have another module you can test it in?

Might be a silly question but have you dbl checked the lead colors on the output transformer? That would explain both issues you are having.

There should be no level change at all when engaging the polarity flip switch.

Some pics may help.

 

[olafrec]

Thanks for the fast reply Jeff!

I reflowed the t pad and three switches, no change.

This is, unfortunately, my only opamp currently (hope to have more soon  )

Interesting, but my only other high-end pre (vintech 573) exhibits the same level bump (and high end shift) if I invert its phase. Inverting the phase on my rme interface exhibits the normal minor change you'd expect. There's perhaps other issues with my set up so I'll just concentrate on the output attenuation issue for now as that's obviously not working.

I can buy another 2520 to test with that as I'll certainly use it in a future build. Do you have any other suggestions? Are there tests for the output trans?

Below is a pic of the transformer leads (I guess I can only post one pic at a time?)

Thanks,

Jason

 

[jsteiger]

There should be no measurable level change when flipping a polarity switch. It may be a perceived change with the ears but not by measurement. I would look at other things in your setup especially if it is happening with another preamp.

If you scroll backwards thru this thread, you will find how to measure the DCR of the output transformer and check for shorts. I would do that and if there is no change, I will most likely send you another t-pad. If memory serves, I have never had a bad t-pad from Bourns but sh!t does happen. The t-pad should be checked with a DMM as well.

 

[olafrec]

Thanks Jeff,

I'll check the transformer and t pad and report back. I assume I need to desolder the t pad and monitor resistance while actuating the knob, correct?

Thanks!

Jason

 

[jsteiger]

Yes on the t-pad.

I would check the 2623-1 first.

 

[olafrec]

So, both the 2623-1 and t pad seem to be fine.

For the 2623-1 I get the following resistance values:

red to orange = 14.6
green to blue = 22.6
brown to black = 22.4

I did not get any reading when checking each lead against all other colors (except for the above, of course).

Unfortunately, I butchered the t pad trying to desolder it but I don't think it was defective: I get the stated resistance values or near zero resistance depending on knob position and terminal combination.

Since these components are currently decoupled are there any tests I can perform on the pcb that would help narrow anything down?  Should I start to suspect the opamp at this point?

Thanks,

Jason

 

[jsteiger]

It could possibly be the opamp I suppose. The t-pad is just across the 2623-1's secondaries. Do you have access to a known good working 2520 style opamp?

This is definitely why I don't like testing a new preamp build and a new opamp build at the same time. Can be a lot of tail chasing.

 

[olafrec]

Problem solved 

So, I've learned that logic and careful thought are your friend. Long story short: my vp26 build never had a problem at all. I soldered in the new t pad and the output attenuate still did not work, which I mostly expected as my tests suggested that the original one was fine. But, there I was, staring at the schematic wondering what could possibly be wrong. It just didn't make sense. There had to be a bridged connection somewhere. The fact that I decided to look past the issue of the polarity switch causing an extreme level bump started to creep into my conscious. I grabbed an xlr and ran the preamp output directly into my interface, thus bypassing my patch bay which I normally (pun) employ in my set up. (Yes, I should have done this test much earlier in my troubleshooting). Low and behold, the polarity switch now works as expected: no more level bump. And.... the output attenuation works too! So, there seems to be a bridge or short somewhere downstream in my set up. This probably also explains the horrible hum I get when using the DI of one of my other pre amps.

While it's frustrating that I've been troubleshooting a perfectly good pre amp, this process has exposed a rather gnarly but veiled issue and my rig will be better as a result. This is why DIY rocks!

Thanks so much for your help Jeff. I didn't expect the already great value of this product to come with tech support  You rock!

Cheers,

Jason

 

[jsteiger]

No problem Jason. At least you know what is what. Everyday is a learning experience! 

 

[bigevil]

You can bypass the t-pad to rule it out. Just remove the t-pad by desoldering the 3 leads at the main PCB. Then solder a small jumper wire from pad A to pad C while not touching B.

If the symptoms are the same as before, I would skim backwards thru this thread. I had the last guy inject a sine wave signal into the pre and trace along the audio path to see where it stops.

You can also measure the DCR of the output transformer and test to make sure nothing is internally shorted. That is all explained and very recent in this thread as well.

Good luck and keep us posted.

Thanks again for the continued help!!

Removed tpad and soldered jumper from A to C (no b!) but no improvement unfortunately....
Removed  output transformer and measured resistance on the 2503 and all were 8.6 Ohms...

next step seems to be the signal tracing with an audio probe/signal generator along the schematic path and seeing where it stops....

I will keep you posted.
Thanks

Well.... after many hours of signal tracing and other misc troubleshooting I've decided my best bet is to grab another PCB and components. I can make use of all of the rest of my stuff so the cost is minimal and I'll be able to start fresh and see what happens. That being said, do you think this is a problem (see pics)



while desoldering/removing my parts the EA2622 seems to have lost a leg. Its still there, just really short, I think it will actually go into the PCB and be fine, just a harder solder job. My concern however is that the short lead almost came out, i pushed it back in with needlenose pliers, is there any way to test that it still works before soldering it?




I beat up the old pcb nicely (for giving me such trouble!)

but the new one went together very well and will test this evening

Thanks much!

 

[Pliplo]

hey guys, is it common for the VP26 to make some noise when you engage phantom switch? im pretty sure my vp312di doesnt make any noise when i turn phantom on, even at high gain... could that be a mistake i made?

 

[jsteiger]

You will usually here a pop when engaging the switch but no noise. That part of the circuit is identical to the VP312DI so they will work exactly the same. I would re-flow the solder on the 200R, 6k8's and the 120uF cap. My guess is you may have a shady solder joint on the cap.

I always use the mute switch before flipping the phantom toggle on or off.

 

[Pliplo]

You will usually here a pop when engaging the switch but no noise. That part of the circuit is identical to the VP312DI so they will work exactly the same. I would re-flow the solder on the 200R, 6k8's and the 120uF cap. My guess is you may have a shady solder joint on the cap.

I always use the mute switch before flipping the phantom toggle on or off.

yes yes a pop! well, more like 3, a big one and 2 small ones heheh... ill roll a joint and check my joints... (im sure no one made that one EVER BEFORE)

 

[andre tchmil]

I build two REV2 boards, both with stepped gain.
On one of them , the output attenuator doesn't work, CCW or CW , output is at full.
I measured both boards and att. pot side by side. All values are pretty much the same.
I'm stuck.

 

[jsteiger]

I would look at reflowing the solder to the 9 inner pins of the t-pad. I would also check the lead colors to solder pads for the output transformer.

 

[andre tchmil]

Aaaaaaaarhhhgg ! Solved.

one of the legs on the T pad wasn't properly inserted.

thanks for your time.

 

[OOF!]

Hi Jeff,
I bought a pair of VP26 a while ago.  one has worked perfectly from the start and the other never worked right- always a faint output.  i have both on the bench so i can A/B.  While following the tracing path outlined by HAIMA on thread #113, I have found that:
I get signal fine to the input of the opamp.
I get signal from the output pin of the opamp on the board to the RED transformer wire, but then when checking at the BLUE wire i get the faint signal.  i have once again resoldered the tranny wires and they are indeed correct (!).  The attenuator does work, but it's only getting the faint signal to work with.  Does this mean the problem is inside the transformer? 
Funny, it only took a couple of hours to build these, but troubleshooting this module has added another 8 hours.  Frustrating to say the least.
thanks very much for any help,
David

 

[OOF!]

just to update testing my output tranny.  all leads desoldered from PCB.
red to orange= 33 ohms
brown to black= 17 ohms
green to blue= 1k ohms

this green to blue reading is way off, correct?

 

[jsteiger]

just to update testing my output tranny.  all leads desoldered from PCB.
red to orange= 33 ohms
brown to black= 17 ohms
green to blue= 1k ohms

this green to blue reading is way off, correct?

Indeed, this should also be around 17 ohms. Does your 2623-1 have a white sticker on the side or no markings at all? Send me an email thru the store and I will get you squared away.

 

[OOF!]

will do.  thanks Jeff.