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

I would certainly believe that my DMM isn't giving accurate AC readings.  However, the readings are very consistent, so while accuracy is in question the precision not so much.  I wouldn't trust the exact voltage levels, but I do trust the discrepancies between good/bad modules that I measured.  I went over those test points several times and got the same exact values on each board each time. 

I don't really have a way to rig an audio probe unfortunately.  I wish I had the budget for a Fluke right now, but I don't just at the moment.

It is strange, and maybe telling, that the polarity switch cuts out the signal completely. 

I am convinced that this is not an issue with the output transformer.  I tried swapping it out with another and there was no change in the bad module's behavior whatsoever.

I checked the solder joints on the polarity switch and they looked fine.  I re-heated them again anyway but got no change.

I'll try measuring the signal at the secondary of the transformer, I haven't done that yet.

I did probe the pins on the polarity switch and they seem fine. 

From the schematic and what you've told me thus far I'm starting to think it might be the T-pad.  Besides lifting those leads from the output transformer (which I will try) is there anything else I can do to check the T-pad?

mxz174 said:

.....From the schematic and what you've told me thus far I'm starting to think it might be the T-pad.  Besides lifting those leads from the output transformer (which I will try) is there anything else I can do to check the T-pad?

Click to expand...

The best and most accurate way is to lift the blue/black output transformer leads from the PCB and take your balanced output signal from those leads.

This is a Litz transformer so I take it that you mean the gray and orange leads?

I will try that out as soon I get a chance, thanks!

Either way, the pad labels on the PCB will be Blue and Black.

Sorry it's taken me so long to get back to this!

Ok I checked out the VAC between the black and blue leads and here's what I've found:

Good unit:  12.9mV

Bad unit:  4.4mV

Quite a difference!  Does this point to the T-pad as the problem component?

jsteiger said:

The best and most accurate way is to lift the blue/black output transformer leads from the PCB and take your balanced output signal from those leads.

Click to expand...

To quote myself, desolder the 2 leads and use those for your balanced output. This will eliminate the t-pad from the circuit.

Oh I see, so then since I am seeing the level difference at that point that tells me it is NOT the T pad.

I've already verified that it isn't the output transform either by swapping.  What else is there?  Could it be th input transformer?

Well, did you physically desolder both of those leads from the board? If so, the next previous spot to measure for a difference is just before the opamp which is at the 1k R.

I can't recall if you found a dif just after the opamp or not?

Sorry if that wasn't clear.  Yes, I de-soldered those leads and took the measurements from the leads.

I don't believe I checked the 1k resistor you are referring to (is that R2?).

I did make a handful of measurements comparing good and bad modules.  Here are those measurements again:

In High (J1-10)  11.7mV  11.7mV
In Lo (J1-8)        11.6mV            11.7mV
R11 (top)                  11.4mV    11.6mV
C10 (pos-bottom) 4.2mV      4.0mV
C10 (neg-top)            4.2mV              4.0mV
Out Hi (J1-2)            5.1mV      3.0mV
Out Lo (J1-4)              1.8mV      3.0mV

I'll do a similar comparison for R2 later tonight and post the results.

Thanks for all your help Jeff!

OK, so the voltage across R2:

Good unit: 3.2 mV
Bad unit:  3.2 mV

No difference.  You said that point is before the op amp?  So then that maybe points to the op amp.  But that doesn't make sense to me.  These are SL2520 red dots, and I did already try swapping the op amp with another one and it made no difference.

Now I'm really stumped!

I spent some more time with this last night, going through the schematic and probing a good unit and the bad unit back and forth.

The two units have identical measurements through the entire circuit until you get up to the output transformer.  If I compare measurements taken at any point before that transformer I find no difference at all.  However, after the output transformer, right from the transformer leads and in the final output on the card's edge, there is significantly less voltage on the bad unit.

What has me stumped is that I have already pulled the output transformer off of the bad board and replaced it with another, with no change whatsoever.  I used the output transformer that I pulled on another module and it works fine.  So it would appear that the issue isn't in the transformer itself. 

I touched up the solder points from the transformer leads to the board but got no change.  However, I'm starting to think that for some reason the connections into the output transformer from the board might be messed up.  I have had many PCBs of all shapes and sizes made myself and have never had a single issue, but do you think there could possibly be a defect in the board itself?  Somehow I think some trace is mistakenly connected to ground, or traces/leads shorted together where they shouldn't or open where they should be shorted.  But I can't find any spot on the board where that is happening due to soldering.  The VP26 is not that complicated of a circuit, there aren't very many solder junctions or components so it's easy to check each one, still I can't find a mistake anywhere.

I'm still pretty stumped at this point

Just to confirm, when you run signal to both units, the AC measurement for each connection point for the output transformer primaries (Red and Orange solder pads) are the same, correct?  If this is true it can only be a handful of issues:

Bad solder joint(s) on pads to transformer leads

Broken transformer lead

Incorrectly wired transformer leads

Bad solder joints on output attenuator

Bad Output attenuator

Like you said, it's a pretty easy circuit. So unless something else is causing a short on the transformer secondaries between themselves or ground or a copper trace has been broken, I don't see how it can be anything else.


Thanks!

Paul

I have read through this entire thread and have not found a solution to my particular issues. Before I bother anybody for advice on how to track down my problem, firstly I want to try one thing.

What is the method for bypassing the T-Pad on the Rev C VP26?

Z-Chord Studios said:

What is the method for bypassing the T-Pad on the Rev C VP26?

Click to expand...

It is just above here post #845 https://groupdiy.com/index.php?topic=33501.msg821820#msg821820

jsteiger said:

It is just above here post #845 https://groupdiy.com/index.php?topic=33501.msg821820#msg821820

Click to expand...

I guess what confused me there is that you said to desolder the blue and black and use them as the output.

I assumed it was pertaining to something else. As I said I've read this entire thing, and I think I may have been a bit hazy towards the end.

After I take them off of the PCB, what do I do with them at that point? Leave them hanging? I guess I don't know what you mean by using them as my output.

Leave them hang temporarily and use blue for + output and black for - output. You will need to use alligator clips or solder an XLR directly to them etc.

jsteiger said:

Leave them hang temporarily and use blue for + output and black for - output. You will need to use alligator clips or solder an XLR directly to them etc.

Click to expand...

Oh I see. Yes yes.

I meant permanently though. It sounds like you were talking about just to run the tests?

I don't have the cash to afford to put together a test rig. So for now I just want to do a hard bypass on the attenuator so that I can put it back into my rack and see if it works.

I do get what you were saying, and I suppose I could sacrifice an XLR cable and just plug it straight in to my converter, see what happens.

I have a VP26 with a phantom power LED out, the preamp seems to be working fine including phantom power.

Is there something I should test or would just replacing the LED be a reasonable route?