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

[New Soul Rebel]

Well, when you have phantom on, there is usually a pop or thump when engaging the standard pad. After all, when switching DC there will be some popping. It has always been a standard practice to engage phantom and the like with a pre's output muted by some means or not connected, It really can also be said for the mic/line switch.

Except that there must be something wrong... as I don't get this thump at all on my other VP28's.

Its an almighty thump Jeff... I know what you mean about a pop or thump when engaging mic lines or pads, but this is an unusually big thump! Any other ideas what might be causing this. Like I say, the pre works perfectly in operation. But I want to resolve this.

 

[kante1603]

2 different problems:

1) The preamp with the LED that stayed on the whole time:  I switched out the 4558 and that didn't do anything.  Other than that, works perfectly fine.

Hopefully the chip was inserted the right way this time.I don't have schematics for the preamp,I can only guess that surrounding parts might have been damaged too.Maybe Jeff can clear this up?

2) preamp 2: green led works fine on this one and overall seems to work fine, but by Shure KSM 32's will not pass signal.  Any other mic I have including other phantom powered mics work fine, but these mics will not work on this particular preamp.  The Shure's both work fine on my OTHER VP28, but not this one.  Do you think it's a phantom power issue on this one?  Maybe it's not quite enough voltage on it?

It shouldn't be a too lowish voltage,the Shure KMS32 starts working at 11vdc,positive voltage on pins 2 and 3.Read this from their manual.At 48vdc it draws a current of 4,68mAs,so totaly standard stuff.
So yes,it seems there's something wrong.Check at least every part,maybe you have swapped resistors or so.Since you have a working identical mic preamp I would lay them side by side and use a magnifying glass and good light.Compare the resistor colours then.

But what confuses me most is that other phantom powered microphones do work on this preamp.....strange......


Udo.

 

[Terryb]

I lost one of the grayhill stop-pins.  Not sure how, but I did.
Can i just use a trimmed lead to replace it?

 

[jsteiger]

Yes, exactly.

 

[George Adamski]

Hey all,

I have two VP28s that I built at the same time, using GAR2520 opamps. They both work and sound great, but with one constantly being roughly 10dB quieter than the other.

here's what I've tried so far :
- swapping opamps between both pres (all four of them)
- checking boards on both sides for build error, they are fine.
- checking transformer leads.
- trying the modules in different 500 series racks with different input signals from different converters and mics
- checking that the gain goes up and down the amount its supposed to for every step of the gain knob and output fader.

no luck with any of that. If any of you guys has any idea of what I could try next, it would be truly appreciated.

thanks!

 

[dgbt]

I didn't see anything about this in the thread, but are the VP28 resistances supposed to read as mentioned in the VP2X assembly guide between the op-amp pins (V+,V-, C), over 200k?  I'm getting around 106k, and its rising to this measurement...SLOOWLY.  I just wanted to make sure everything is fine before I plug in the opamps.

I should note the card is not connected to a power supply, nor are the opamps installed.

Hopefully someone has an answer to this.

Thanks

 

[kante1603]

I didn't see anything about this in the thread, but are the VP28 resistances supposed to read as mentioned in the VP2X assembly guide between the op-amp pins (V+,V-, C), over 200k?  I'm getting around 106k, and its rising to this measurement...SLOOWLY.  I just wanted to make sure everything is fine before I plug in the opamps.

I should note the card is not connected to a power supply, nor are the opamps installed.

Hopefully someone has an answer to this.

Thanks

Hello,welcome to the forum,

first thing is that you measure in circuit.Can't compare to mine because I'm on holidays but it doesn't look bad so far.
Rising resistance is normal in circuit because caps are charging.Remember that your dmm supplies a voltage when doing measurements!
More important is to look at the voltages at the milmax sockets for the DOAs.If you get +/-16vdc (a bit less it will be) then you're good to go,plug those little beasts in.

Tell us what you get and have fun,

Udo.

 

[Craig32768]

My first VP28 build was going great... until I made a silly mistake. I soldered the 6db/12db filter switch on the wrong side of the board... In the process if trying to get the sucker of, I broke the switch and stripped a couple of PCB contacts / track. I'll attach the embarrassing photo.

So.... Firstly.  Can anyone tell where I need to connect the two pins shown in the photo - to which component, bypassing the PCB tracks?

And then.. I guess I'll need to get another switch off Jeff so I can complete the build on the 2nd VP28 as I've grabbed the switch from that kit to replace this one.

I'm kicking myself... But I'm sure I can recover from this hopefully without ordering a new PCB etc...

Thanks for your help in advance.


//////// EDIT //////////

OK - I seem to have traced where the poles go from the switch. So I'll mod this one a little(!) and ask Jeff for another switch for the other VP28.... Hopefully I've not buggered up any of the tracks around the switch and I'll post a lovely pic of the successful build in a day or so

///////

 

[Craig32768]

OK completed my first VP28 but I cannot get the knobs to go onto the greyhill switches.  I've unscrewed both the set screws so that the knob goes over the shaft. But when I tighten them, it does not completely tighten up, there's a lot of play in the switch and you can just pull the switch off. I saw in Chunger's post that there was a collar adaptor and there doesn't look like there's room to put one on any way. Anyone else had the same issue / could offer some advice please?

 

[jsteiger]

Craig, the shaft adapter is no longer needed since the knob holes are 1/8" and so are the Grayhill shafts. They must be aligned properly but they will tighten correctly.

 

[jakmusic]

Hey Guys,

Newbster McGee here, just finished testing the sockets on my VP28, the tests resulted as follows

Seated in my 500 rack, Powered.  The voltages read:
A1
O/V-  15.5V
O/V+ 15.6V
C/V-  15.4
C/V+ 15.7

A3
O/V-  15.9V
O/V+ 15.2V
C/V-  15.4V
C/V+ 15.7V

How close to 16V are the sockets supposed to read when powered?

thanks

 

[jsteiger]

This is pretty normal since there is a .7V drop over the cards protection diodes.

 

[George Adamski]

Hey all,

I have two VP28s that I built at the same time, using GAR2520 opamps. They both work and sound great, but with one constantly being roughly 10dB quieter than the other.

here's what I've tried so far :
- swapping opamps between both pres (all four of them)
- checking boards on both sides for build error, they are fine.
- checking transformer leads.
- trying the modules in different 500 series racks with different input signals from different converters and mics
- checking that the gain goes up and down the amount its supposed to for every step of the gain knob and output fader.

no luck with any of that. If any of you guys has any idea of what I could try next, it would be truly appreciated.

thanks!

anyone?

 

[jsteiger]

You are gonna have to inject a test signal and measure the level at a few different test points thru the circuit. This will narrow down where the issue lies. You will need to be able to power the module flat on a bench or table in front of you. Make sure all controls are set the same for each module and compare your results. If you use mic mode, inject a -43dBu signal. If you use line mode 0dBu or +4 is fine. You can measure at the test points with a DMM set to ACV if you don't have a scope. I would use 400Hz as the test signal if using a DMM otherwise 1kHz should be good.

Attached is a pdf with specific points to measure at. Unfortunately, I do not have a built VP28 here to tell you what a properly built unit will measure at these points. But in your case, comparing should should get us in the right area to look at.

 

[Craig32768]

Hi Jeff - thanks.

I found there was a bur inside one of the holes that wasn't helping. Cleared that out and all is good in the world again! Onto the op amps now.. wish me a steady hand and en eagle eye..

Craig, the shaft adapter is no longer needed since the knob holes are 1/8" and so are the Grayhill shafts. They must be aligned properly but they will tighten correctly.

 

[jakmusic]

Hey Jeff,

I just tested my API 500VPR rack and found out that the +16V Rail is delivering +16.4V and the -16V Rail delivers -16.1V.  When I questioned one of the service techs at API about it he said don't worry about it.

The only thing is that on one of my preamps I'm having trouble getting the DOA sockets to read the correct voltages.  They are:

A3
O/V+ 14.8V
O/V- 16.5V

A1
O/V+ 15V
O/V- 16.25

The C/V+ C/V- voltages all seem fine, within 15.5-15.8V 

I've gone over the main PCB and HPF PCB numerous times, trying to make sure there are no solder blobs, and checking it over for cold joints.  I've double checked all my resistors to make sure the right values are in the correct places. 

Any ideas or tips of where to start?

thanks much,

James

 

[jsteiger]

I would probe at the 1N4004 diodes and see what you have. On the rack side of them, you should have readings that match measuring inside the rack's card edge connectors. On the module side of the diodes, you should have appx .7V less (closer to 0V) than on the rack side. You can check around Q1, C2 and R1-R3. Also make sure A2 is in properly. You can compare readings between the 2 preamps.

 

[jakmusic]

I measured and compared between one of my preamps that has the correct voltage reading on the sockets and the one in question. 

I put one probe on the hole that connects to the ground pin and the other on each connection for the following components
CR1
CR2
R1-3
Q1
C2
Everything matched within between the two preamps to .02V 
A2 was a little loose so I pushed it further in, but that didn't change anything.

Anything else I should do or check?

Thanks Jeff

 

[jsteiger]

So the 2 pre's match now? I thought they did not match. If they are the same I would say to proceed and install the opamps.

 

[jakmusic]

I must be breathing too much solder.  I forgot to say they match except the one preamp in question still has incorrect voltages at the A1 A3 sockets.  The voltages at those sockets hasn't changed since my first post.

A3
O/V+ 14.8V
O/V- 16.5V

A1
O/V+ 15V
O/V- 16.25