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

[Potato Cakes]

I have been combing through the thread to see if anyone had similar issues but I can't seem to find anything.  My VP28 build does not seem to be passing any signal.

I'm pretty sure that on this thread and most all the other build threads here on the GDIY forum there are several posts about builds not passing signal. Also, just saying it is not passing signal is not going to be useful in diagnosing the problem. Unfortunately you will have to go back through this thread a little more closely and read all the posts on this thread regarding op amps, solder joints, transformers, etc. There are also voltages posted for the test points on VP28 Support page on the CAPI website to aid in troubleshooting. I know it doesn't sound like fun, but trying to find errors in one's build rarely is.

Thanks!

Paul

 

[jsteiger]

Hello All,

I have been combing through the thread to see if anyone had similar issues but I can't seem to find anything.  My VP28 build does not seem to be passing any signal. The green LED does not light up and there seems to be nothing passing through. The other LEDs are working and everything seems to look fine but not sure where to start. Does anyone have any suggestions on where to look first? Thanks!

As always, please tell us first about the opamps. What are they and who built them?

Also, have you tried the Test Points doc? Where does your module stop showing the same results as what is posted in the TP doc?

As Paul mentioned, read back thru this thread and you will find a plethora of excellent info for you to at least start with.

 

[resilient]

Did you also build your opamps?

You will want to get known working on amps to start with.

Brandon

 

[noiro]

Did I said that already CAPI freakin' ROCK 8) 8) 8) 8) 8) 8) 8) . Building new one today and yes having working op-amp to test with is a must, but also if you start building lots, it may be worthed to have the FiveFish test JIG or even better check this thread
https://groupdiy.com/index.php?topic=60558.0

 

[Leon]

My VP28 has been working great. Today I used it to test a newly built GAR2520. It did not passing signals, and I smelled something. Well looks like PR1 and PR3 got burnt (see pictures). Any idea what  could be wrong with the GAR2520? I did check the critical points mentioned in the guide botg virtually and using multi meter. There is no short.  Any help will be greatly appreciated!

 

[Leon]

Here is a photo of the DOA

 

[Leon]

bottom side of the DOA:

 

[Leon]

Just saw it in the GAR2520 thread, I switched BD138, BD140, thinking they are the same. Making mistakes at the very end has become a habit to me . Too bad   

 

[TillM]

No worries,
This can happen.
Just change the burned resistors and swap the BD139 with the BD140 and everything will work.

 

[Leon]

No worries,
This can happen.
Just change the burned resistors and swap the BD139 with the BD140 and everything will work.

Yeah, just did that and now it works again. Thanks!

 

[lalalaetc]

Hello,
I've had a few VP28's that have been working excellently for a couple of years now. I recently swapped some modules around in my rack, and now one receives, but won't pass signal.

Here are my TP measurements:

Tp1- 105mV
Tp2- 668mV
Tp3- 20mV (that's right, .020V)
Tp4- 0V
Tp5- 0V
Tp6- 0V
Tp7- 0V

These measurements seem to clearly demonstrate the area of the problem, but I have been unable to find a published schematic to effectively trace the circuit. Any thoughts?

Thank you in advance!
Wyatt

 

[jsteiger]

Wyatt

If things were working fine and then they stopped...this is likely a cold solder joint. I have heard of this exact situation more times than I can count.

 

[lalalaetc]

Thanks for getting back so quickly, Jeff.
I'll definitely have a close inspection on the pcb, and re-flow any questionable spots.
From the values reflected in my reading of the test points, does it seem logical that I could isolate this to a particular area of the circuit?

Thank you again for your support

Best,
Wyatt

 

[lalalaetc]

Welp... Traced between tp2 and tp3, touched up a few questionable joints, and I'm back up and running. Thanks again, and sorry to bog down the thread with such simplistic problem solving measures.

Best,
Wyatt

 

[Spiritworks]

What's the purpose of the small washer placed between the transformers and the PCB? I forgot to install it on two builds. Do I need to take them apart to place the washer?

 

[jsteiger]

What's the purpose of the small washer placed between the transformers and the PCB? I forgot to install it on two builds. Do I need to take them apart to place the washer?

The transformer lams are connected to the chassis ground. The top copper layer is 0V audio common. The washers just help to make sure chassis ground is not directly shorted to 0V.

 

[Spiritworks]

The transformer lams are connected to the chassis ground. The top copper layer is 0V audio common. The washers just help to make sure chassis ground is not directly shorted to 0V.

Thanks Jeff.  So I better install them.

 

[jsteiger]

Thanks Jeff.  So I better install them.

I would, just to be sure.

 

[ramtazz]

Hello Everyone,

While I am not a noob to DIY I am to building these CAPI V28's.  I purchased 2 and put one together with 2 2520 op amps (kits) and then the fun began.... I have signal going in through the mic (both reg and condenser w/ 47v), signal Led lights up... and upon listening to the output... its a constant 60 cycle hum ( the knobs do not reduce or make it louder)....  (although if I crank the gains you can slightly hear a distorted signal way in the background).  I've tried swapping the 2520's, I've tried it without them... and swapped cards in different ports and working cards in those ports so I know the ports are good. I get the same hum.

I am awaiting more banana plugs to do further testing (published test points), but this is what I've tried so far....
Resoldered both amps and main PCB board
Traced back the output to the last of the transformers.

Without a schematic I cannot do anything more. I'm assuming since I have the hum with and without the op amps, that they are not an issue.  I don't know if its a bad trace, bad part or what.

Any guidance would be very much appreciated. 

Thank you in advance!!

Bill

 

[jsteiger]

Bill, I think we emailed about this. Unless you have 2 solid, known to be working opamps, trying to troubleshoot a new preamp build with unknown opamps is almost pointless. You will have no way of knowing what if anything is working or not working. Could be a problem with the preamp, one or both opamps or all of the above.