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

[Seditionary]

Cool - will grab my camera in a second. Looks like a tiny piece is still there. I remember losing the whole bottom ring on another part of the board though, which flaked off easily.

 

[Seditionary]

attached is the broken gold ring.  you can see there is a tiny bit still there on the right… i guess as long as i don't lose that i'm cool… but i remember losing a whole ring on some other connection.  soldering from the top wouldn't help me if i lost the bottom, huh?  and please explain more about how to check those windings for a direct short.

thanks so much for your immediate help!  i just posted and i already have a response   even though this has been a long process, your quick responses are amazing and will leave me recommending your kits to friends (as soon as i can get the first one working)

 

[Seditionary]

On testing for a direct short on the output transformer... Do you mean test each color against each other coming out from the bobbin? For example... I read 16 ohms between red and orange, but read zero ohms between red and blue or red and black etc. is this what you mean?  Is my output transformer bad (and maybe I've lost some of my traces on the board)?

Thanks!

 

[jsteiger]

On testing for a direct short on the output transformer... Do you mean test each color against each other coming out from the bobbin? For example... I read 16 ohms between red and orange, but read zero ohms between red and blue or red and black etc. is this what you mean?  Is my output transformer bad (and maybe I've lost some of my traces on the board)?

Thanks!

Yes, this is what I mean. If anything is less than 1 ohm, there is a problem with the transformer.

 

[Seditionary]

ah man -- yeah, it reads zero. 

 

[Seditionary]

Man -- I just tried the other output transformer... Same thing -- it is shorted out.  Feel better that it might just be bad parts... Unless I did lose all of that one gold pad.  Should it be the same test with the 312 transformers?

How do we move from here?  Should I send you in the bad transformers?  Would you send me some working guys back along with the screws that were missin in the input knobs?  Thanks Jeff! 

 

[jsteiger]

Hmm, the odds of 2 bad transformer together are huge, especially with the exact same symptoms. It's hard to imagine but not impossible I suppose. I know Ed will want those back to investigate. We have had very few bad out of the many that we have sold. Either way, I am not happy about it and I know Ed won't be either.

Send me an email and I will get you replacements out pronto. Also remind me of how many set-screws and for which knob. I have them all here so no problem.

The 2503's are similar to test. All 4 windings will be around 8.5 ohms. Measure red to brown, blue to green, orange to yellow and violet to gray.

Also, be sure of your DMM auto ranging settings. I have had many guys have many problems over the past all related to cheap DMM's that are set the the wrong range and so on. This could surely give a false reading here if not careful.

 

[Seditionary]

on transformer readings -- i made a stupid mistake.  i read about 15.8 between red/orange and 32 resistance on all other pairs... between the other ones, i wasn't reading 0.0... i was reading 0.L, which i interpreted as zero... when really this means that there is no resistance!  oops  :-[

anyways -- i re-soldered every single connection on the bottom… i have some sound but it is hiss / white noise.  as i increase the stepped gain, the hiss gets worse, so the stepped gain seems to be working.  the output attenuator knob brings the hiss down when backed off.  the mute button kills the hiss completely… so that all seems to be working.

i tried different opamps -- tried two of scott's.  tried taking it apart and re-soldering for a 3rd time... still with the same results.  already tried the voltage test with the opamp removed to test the rack...

thanks!!!

 

[jsteiger]

The hiss you hear is most likely from the opamp so the input pad will not change it. That is not surprising but normal.

At the point you are at, I would recommend injecting signal and tracing thru the circuit to see where it stops. There are pointers for doing this near the beginning of this thread.

Are you 100% sure this has nothing to do with the rack cabling? I have heard of this happening a lot.

 

[Seditionary]

cool -- so with the hiss, you wouldn't still recommend swapping the transformer? 

 

[jmpizzoferrato]

I'm having a strange problem with my vp26. Using an API 2520 as a test opamp, my unit causes the other units in my lunchbox NOT to work, yet my vp26 seems to be working.

Ive tried it with two different lunchboxes. Where should I begin testing?

Thank you.

 

[Seditionary]

i just tried michael, mjenki14's test... same thing... when i go out of the vp26 and into another preamp and put the gain up, i can hear signal!

 

[jmpizzoferrato]

Scratch mine. I traced the problem - seems that it was a cold solder joint to a capacitor. I was able to trace it and fix it right up. Sorry for the false alarm. Excited for the next pair! Hoping those input transformers get back in stock soon! Thanks.

 

[Seditionary]

dang, scratch that it was the output transformer (unless i have 2 bad ones, which i probably don't). 

 

[Seditionary]

so i should try and build a listening signal probe? 

 

[jsteiger]

so i should try and build a listening signal probe?  is that the best way to move forward?  ???

Yes or something similar. A scope or DMM set to ACV can also be used. Think of the circuit as a highway. Using the schematic, inject signal and probe along the "highway" to see where your injected signal stops.

You can use two 4.99k R's and a 160R as a U-pad to inject line level signal into the mic input.

 

[Seditionary]

i see that r5 connects to r12 on the schematic and can follow the traces on the back of the board.  when i set my dmm to ac volts, it reads 0.000 V between then.  is that what you mean by acv?

thanks.

 

[jsteiger]

The unit must be under power and you need to be sending a signal to the input that can be measured.

 

[Seditionary]

okay -- but how can i get to the back of the board when the unit is powered on?

also, when you say sending it signal... you mean In Hi on the schematic?  J1-10, which if i'm reading the board right is the 6th gold marking from the top?  so i'd solder those resistors and a cap together and just place them on the 6th gold finger?

also, if it is the rack... would it make sense i'm still getting signal through after going in and out of the rack and then getting gain from another preamp?  signal is passing through the unit then i'm guess... it just isn't getting amplified properly.

thanks!

 

[jsteiger]

When trouble shooting like this, the module needs to be flat on a bench or table, laying flat in front of you. It cannot safely, effectively or scientifically be carried out while installed in a rack.

Not sure what "cap" you are referring to.

An audio mic level signal needs to be applied to the typical input for the module. This needs to be something like a 1k sine wave from a signal generator of some sort.