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

[jsteiger]

Just finished my first build of (2) VP28s. Not sure why, but on one of the units the polarity switch must be engaged to get any decent amount of gain. When disengaged the signal is very weak and lacks any low end. No idea how to track down what component is to blame without a schematic. Second unit, which I built at the exact same time, works just fine.

Check the DCR of the leads of the first output T2. Check for open windings and/or shorts. This has been posted about above, but not sure how far back.

 

[thisisntjohnv]

In need of some help.

I loaded my newly built VP28 preamp into my 500 series rack today. It's passing signal but very low and only when the mic switch is engaged. The channel fader gain is working but the preamp gain is not affecting the signal.

I took the casing off and desoldered the pre gain resistors and check the opamp connections but the pre amp gain is still not working and the signal is low.

Can anyone point me in the right direction or help with this?

 

[jsteiger]

In need of some help.

I loaded my newly built VP28 preamp into my 500 series rack today. It's passing signal but very low and only when the mic switch is engaged. The channel fader gain is working but the preamp gain is not affecting the signal.

I took the casing off and desoldered the pre gain resistors and check the opamp connections but the pre amp gain is still not working and the signal is low.

Can anyone point me in the right direction or help with this?

You never told us what opamps you are using?

It could be many things. With DIY the biggest trouble is typically bad solder joints or misplaced resistors.

 

[jbmacdo3]

I took a look through this thread and could not find any guidance on this.  I very well could have missed it, so if I did I apologize in advance.

I tried to salvage a build for a buddy of mine and am having some issues getting the unit to output volume.  The unit has a tested GAR2520 in the preamp slot with a GAR1731 in the Fader slot, Litz transformers.

I hooked it up to my signal generator per the AC spec in the test point guide and am getting proper readings except for: 0.003VAC at TP6 and TP7 with 0.05VAC across the output.    Any help would be greatly appreciated to try to narrow down where I should be looking for the issue(s)

Thanks,

Jeff Macdonald
Is there some component or at least area of the PCB I should be focused on to try to hunt down the problem? 

 

[thisisntjohnv]

You never told us what opamps you are using?

It could be many things. With DIY the biggest trouble is typically bad solder joints or misplaced resistors.

I’m using two  pre built red dot opamps and Litz transformers.

 

[jsteiger]

I took a look through this thread and could not find any guidance on this.  I very well could have missed it, so if I did I apologize in advance.

I tried to salvage a build for a buddy of mine and am having some issues getting the unit to output volume.  The unit has a tested GAR2520 in the preamp slot with a GAR1731 in the Fader slot, Litz transformers.

I hooked it up to my signal generator per the AC spec in the test point guide and am getting proper readings except for: 0.003VAC at TP6 and TP7 with 0.05VAC across the output.    Any help would be greatly appreciated to try to narrow down where I should be looking for the issue(s)

Thanks,

Jeff Macdonald
Is there some component or at least area of the PCB I should be focused on to try to hunt down the problem?

Dbl check the wiring for the T3 output transformer. If that is correct, desolder the T3 leads completely and see if TP6 comes up to normal.

 

[jsteiger]

I’m using two  pre built red dot opamps and Litz transformers.

You need to follow the steps on the Rev B Test Points guide and post the results here so we can narrow down your problem. Same as the dude just before you did.

 

[thisisntjohnv]

You need to follow the steps on the Rev B Test Points guide and post the results here so we can narrow down your problem. Same as the dude just before you did.

If I can reach the test points when the unit is in my 500 series rack is it safe to carry out without the jig? Or should I order this for testing: http://capi-gear.com/catalog/product_info.php?products_id=406

JV

 

[jsteiger]

If I can reach the test points when the unit is in my 500 series rack is it safe to carry out without the jig? Or should I order this for testing: http://capi-gear.com/catalog/product_info.php?products_id=406

JV

No problem without a test jig if you can safely and accurately get to them.

 

[thisisntjohnv]

No problem without a test jig if you can safely and accurately get to them.

I went to test my unit. I was only able to get a reading of 1.2V AC (connecting the red probe to pin 2 and black probe to pin 3) when the mic button was engaged. Even with the sine wave signal generator cranked in my DAW there was not enough signal when the mic button wasn't pressed in.

Should I still carry out the test on all the test points? Also, how do I get the reading for TP1, TP2 etc. I'm just a little confused of where to put the common and red when testing for each point. I'm using my 6 space 500 series chassis while testing with no other units so I'm able to reach the test points.

Since I hit a wall with testing I went back and cleaned the board, removed excess flux and reflowed parts of the board that looked like they could be causing the problem. Still no luck getting the Preamp gain switch to affect the signal. When I engage the mic button I can get about 24db of signal max. When I disengage the signal drops to where it barely hits my meters on my symphony mk2.


JV

 

[jbmacdo3]

Dbl check the wiring for the T3 output transformer. If that is correct, desolder the T3 leads completely and see if TP6 comes up to normal.

Hey Jeff,

Thanks for the quick reply!  I took a look at the wiring for T3 and it was correct so I desoldered the leads and TP6 is still not reading correctly, it is still showing 0.005VAC.

Thanks,

Jeff

 

[jsteiger]

Hey Jeff,

Thanks for the quick reply!  I took a look at the wiring for T3 and it was correct so I desoldered the leads and TP6 is still not reading correctly, it is still showing 0.005VAC.

Thanks,

Jeff

TP6 is post the fader booster amp so I would look at that.

 

[thisisntjohnv]

Can anyone direct me or guide me through testing my VP28 test points? I set up my DAW to send a 400hz sine wave at +4db and I got my multimeter to read 1.2V AC when  connecting DMM red probe to pin 2 and black probe to pin 3, but Im not sure how to carry out the rest of the test. I can only get enough signal to  coming through to set up and carry out the test when the mic button is engaged.

I'm lost here. Could use some help on the proper way to test. I'm referring to the CAPI VP28 Rev B Test Points PDF.

JV

 

[jsteiger]

I can only get enough signal to  coming through to set up and carry out the test when the mic button is engaged.

If you follow the steps in the Test Point guide, all measurements shown are done in Line mode, not Mic mode. What is your reading at TP1 when following the proper procedure?

 

[frenkonio]

Hi,
built several VP28 but it's the first time I am encountering this issues. On two VP28 the volume is higher than regular ones.
These are the measurements I have done with two Red Dots as opamps.

TP1: 112.5mV {1} 112.3mV {2}
TP2: 1.42V {1} 1.42V {2}
TP3: 2.60V {1} 2.50V {2}
TP4: 1.25V {1} 620mV {2}
TP5: 1.25V {1} 620mV {2}
TP6: 1.25V {1} 620mV {2}
TP7: 2.60V {1} 1.24V {2}
OUT:  2.60V {1} 1.28V {2}

Out has been measured with line in at unity gain and fader at 0.
Where could I begin troubleshooting?

 

[jsteiger]

Hi,
built several VP28 but it's the first time I am encountering this issues. On two VP28 the volume is higher than regular ones.
These are the measurements I have done with two Red Dots as opamps.

TP1: 112.5mV {1} 112.3mV {2}
TP2: 1.42V {1} 1.42V {2}
TP3: 2.60V {1} 2.50V {2}
TP4: 1.25V {1} 620mV {2}
TP5: 1.25V {1} 620mV {2}
TP6: 1.25V {1} 620mV {2}
TP7: 2.60V {1} 1.24V {2}
OUT:  2.60V {1} 1.28V {2}

Out has been measured with line in at unity gain and fader at 0.
Where could I begin troubleshooting?

My guess would be you have misplaced resistors around the A1 opamp. Specifically R5-R8.

 

[frenkonio]

My guess would be you have misplaced resistors around the A1 opamp. Specifically R5-R8.

Nope. R5 to R8 were good. I misplaced wrongly RF12 (it was an error of the small plastic bags specific of 24.9R values, probably one wrong resistor went inside the  bag and I didn't checked it).

 

[jsteiger]

Nope. R5 to R8 were good. I misplaced wrongly RF12 (it was an error of the small plastic bags specific of 24.9R values, probably one wrong resistor went inside the  bag and I didn't checked it).

We have a very specific way of sorting the 11 and 12 pc resistor bags so not sure how we could screw that up...especially the way your bulk orders were packed...but sh!t happens. 

 

[frenkonio]

We have a very specific way of sorting the 11 and 12 pc resistor bags so not sure how we could screw that up...especially the way your bulk orders were packed...but sh!t happens. 

Anything that comes to a good end... is good don't worry about that!

 

[radtunez]

Hey,
I've got a problem with one of my Gar2520s in a VP28. The thing started giving off a wicked smell the first time I put it in my lunchbox, and it seemed to be coming from the op-amp. I tested that same op amp in my other VP28 that has been working perfectly, and same thing happened. This time around, however, I noticed the R9 and R13  56R resistors had been totally fried black :/ And may have singed the neighboring caps.  I double checked all of my resistor placement, transistor placement, solder bridges, and I'm not coming up with any reason.

Any ideas? Thanks in advance!