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

[drumski]

kante1603 and weiss, thank you for the kind assistance. Nice to know this gar2520-thread 

This is first time when I built doas. Yes, I can test with other gar2520 doas. They are in the one VP28 in our small studio of our church. So I can use these, if necessary.  I need now to wait over weekend when I get my new and better multimeter in HFE-feature. So my doas are now in a half way.

My feeling has been very positive to build these babies and my opinion is that these VP28 and gar2520 doas are not difficult to build. Jeff and Gary has really done a great job in creating these products. Of course, the gar2520 is very dense and it need very stable hand, good eyes and good soldering iron… I have these all  .  My faithful servant from late 80s, Weller WECP-20 with tiny soldering tip  8).

 

[kante1603]

Great to hear you can use working doas!This is an immense help to sort things out.
And you have the Weller....Mine at home is even from the early 80s.....and still working like a horse!
So you are well prepared!


Cheers and have fun,


Udo.

 

[drumski]

Set your dmm to dc voltage reading.
Black probe to ground ("C"or "common").
Red probe on V+ should give around +16vdc,probe on -V should be -16vdc.
You might have a little drop on both rails due to the protection diodes (roughly -0,7v),this is o.k. then.
Test both doa locations.

Before I connect 16V DC to terminals 12 and 14 I need to make sure the location of the poles & order. See the attached picture. Have I understood correctly this order and reading direction? P.S. This is first time for me so I don't want to burn anything… 

 

[weiss]

just insert it and make sure the upper connectors go right in the upper socket. so if you don't use the additional voltages the lower two sockets are left empty!

 

[drumski]

just insert it and make sure the upper connectors go right in the upper socket. so if you don't use the additional voltages the lower two sockets are left empty!

I do not have in my workshop  the system 500 rack. I would just like to make this above-mentioned test with laboratory power source and I want to be sure that I put DC 16V to correct terminals.

 

[kante1603]

Looks o k. to me!


Best,


Udo.

 

[drumski]

Set your dmm to dc voltage reading.
Black probe to ground ("C"or "common").
Red probe on V+ should give around +16vdc,probe on -V should be -16vdc.
You might have a little drop on both rails due to the protection diodes (roughly -0,7v),this is o.k. then.
Test both doa locations.

Tomorrow I will go to the studio and to test these my first two VP28 preamp. But first, before I will put DOAs in place, I tried to do this test in my workshop but I get the multimeter readings as follows:
Black probe: C
Red probe: +V  (I got +0.59 VDC)
Red probe: -V  (I got -14.43 VDC)

What goes wrong? I put +16 VDC to pin 12 and -16 VDC to pin 14.

 

[kante1603]

Set your dmm to dc voltage reading.
Black probe to ground ("C"or "common").
Red probe on V+ should give around +16vdc,probe on -V should be -16vdc.
You might have a little drop on both rails due to the protection diodes (roughly -0,7v),this is o.k. then.
Test both doa locations.

Tomorrow I will go to the studio and to test these my first two VP28 preamp. But first, before I will put DOAs in place, I tried to do this test in my workshop but I get the multimeter readings as follows:
Black probe: C
Red probe: +V  (I got +0.59 VDC)
Red probe: -V  (I got -14.43 VDC)

What goes wrong? I put +16 VDC to pin 12 and -16 VDC to pin 14.

The supply setup is wrong.You need a dual rail supply with ground (0v).It will not work like this.


Udo.

 

[drumski]

OK. I will made tomorrow this voltage testing (without all DOAs)  with our studio's 500 rack. If I get correct voltages from pins then I will put DOAs and IC to PCB. Thank you for your fast help!

 

[Commander Fluffypants]

Woohoo. I just built four more VP28's. I had built two a while back. Those two came out perfectly. These four... not so much... Two seem to be fine. The other two have problems.

When I plugged one of the VP28's in, there was a real cool glow and a wonderful smell... Turns out I fried a few resistors in the Opamp in A3, a GAR 1731. Specifically, I can see that R15 is fried and possibly R8 next to it. (R8 might just have soot on it from being so close to R15.) All the way on the other side of the board R14 also fried. I can't see if anything else fried. So... how do I know why they fried? I don't want to stick another 1731 in there and have it fry if it's something on the VP28 that's causing it. I hope it's just the Opamp itself and I can just build another or even fix the one I have.

One of my other VP28's seems to be working well except that there's a low level distortion, like a kazoo, happening all the time. It's present when the knobs are all the way down and does not get louder as I turn the knobs up. It's subtle, but it's there. I've eliminated the microphone and cabling, etc., as the source of the problem. I will now attempt to post a link to a sound file:

https://dl.dropboxusercontent.com/u/1643668/Kazoo.mp3

I started with both knobs all the way down and then turned up just the top knob and then turned it all the way down again before doing the same with the bottom knob.

As always, I appreciate all the help from you guys!

 

[Commander Fluffypants]

Found the answer to my 1731. I managed to reverse Q7 and Q8. I shall do the necessary repairs...

I'd still appreciate help with the kazoo sound, though.

 

[drumski]

OK. I will made tomorrow this voltage testing (without all DOAs)  with our studio's 500 rack. If I get correct voltages from pins then I will put DOAs and IC to PCB. Thank you for your fast help!

Everything works perfectly!  ;D

 

[Commander Fluffypants]

One of my other VP28's seems to be working well except that there's a low level distortion, like a kazoo, happening all the time. It's present when the knobs are all the way down and does not get louder as I turn the knobs up. It's subtle, but it's there. I've eliminated the microphone and cabling, etc., as the source of the problem. I will now attempt to post a link to a sound file:

https://dl.dropboxusercontent.com/u/1643668/Kazoo.mp3

I started with both knobs all the way down and then turned up just the top knob and then turned it all the way down again before doing the same with the bottom knob.

As always, I appreciate all the help from you guys!

Trying to bump this to get a response... I also figure I should clarify that that is my awful voice in the link. If you listen carefully, however, you'll hear, especially when the volume is down, a bit of distortion. There is no distortion if there is no signal, hence the need for the silly vocalization.

Thanks again.

 

[Chuckles]

With no load it is common for 48V supplies to rise above their rated voltage by 10 to 15%. 55V is nothing unusual. When a multiple output regulated power supply is used like in the Workhorse, the regulation is usually monitoring the + side of the lowest voltage output. In this case the +16V line.

This means that with no load or a light load on some lines, the other lines will be high or sometimes a little low.  Seeing 55V on a phantom output is OK as when the phantom powered loads are turned on, the voltage will fall to the correct range.

 

[Unit7]

Jeff,

On one of my VP28 builds, with the HPF disengaged, the -6/-12 switch cuts the output when in the -12 position but not for -6. When the HPF is turned on, everything works fine. I didn't see a schematic for the preamp to aid in chasing down the culprit. I resoldered any suspect connections, but to no avail. Any thoughts?

Thanks!

I have this problem too on one of my recent builds (that I emailed about, but decided to bring it here instead). But only occasionally and the -6/-12 switch is 'scratchy'. I've reflowed the switch, all pads on the HPF and the 5 pin header and replaced op amps (just in case..) but still no change. HPF works fine btw.

Just a brief follow up that after two failed attempts reflowing all pads around the HPF I've now replaced the -6dB/-12dB switch, and problem seems to be solved!

 

[Mbradford6288]

Just built my first VP28.  Can't get any signal indication from the green led. 

I reflowed all my solder joints.  I have proper voltage in the opamp pins.  I double checked my resistors.  Everything seems fine.  So, I figure the problem is the DOAs.

I pulled the 990s out of my LOLA, and when those go in, none of the LEDs will light up.  I put the Gars(two 2520) in the LOLA, and it's LEDs illuminate very dimly.  Is there a difference between the two that I can't use the 990s to test the VP28?


Thanks!

Matt

 

[Potato Cakes]

Just built my first VP28.  Can't get any signal indication from the green led. 

I reflowed all my solder joints.  I have proper voltage in the opamp pins.  I double checked my resistors.  Everything seems fine.  So, I figure the problem is the DOAs.

I pulled the 990s out of my LOLA, and when those go in, none of the LEDs will light up.  I put the Gars(two 2520) in the LOLA, and it's LEDs illuminate very dimly.  Is there a difference between the two that I can't use the 990s to test the VP28?


Thanks!

Matt

The 990's that I have from Hairball work in my VP28s.

 

[dmp]

Are your 990s for 24v or 16v?

 

[Mbradford6288]

I used the resistors supplied with the kit for the 990s.  It says that makes 'em 16v.  I'm reading 15.8 across the common and v- v+ terminals on both opamp slots on the 28. 

All that is to say, I have no idea, but I think 16v.  Which I thought would be compatible with the vp 28.  But, the Gars at least let the vp 28 turn on.  No sound, or signal indication, but my two working 990s won't even give me lights on the other leds.  The gars do. 

Hmmmm.  Any thoughts?  I greatly appreciate the help.

 

[benediwa]

Hi!

I have just built two CAPI VP28 kits but Im having trouble with one of the units.
It does not register the input signal though there is clearly power. I've tried changing the OP-amps and using the ones from the build that works, but its still not registering input.

Using the test point image from an earlier post I have measured the volts at the test points with the following results:

Test of CAPI VP28 does not register input signal:
(+/- 0.5 mV)

Point 1 : - 1,5 mV
Point 2 : - 14, 35 V
Point 3 : - 1,5 mV
Point 4 :  - 1,5 mV
Point 5 : - 1 mV
Point 6 : - 8,8 mV

Test of CAPI VP28 in working condition:

Point 1 : - 0.6 mV
Point 2 : -  9, 2 mV
Point 3 : - 0,5 mV
Point 4 :  - 14,5 mV
Point 5 : - 11 mV
Point 6 : - 8 mV

Im not sure where to go from here though.

Help would be greatly appreciated!

-B