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

[dubbySwords]

Howdy!

I’m new to building boards. I new most of the theory a long time ago and thought I’d try brushing up my theory and put it into practice. I started with the VP28. It is now assembled and with the 2 op-amps. When I plug it in I get the +/- 16V power to the op-amps but nothing on the signal inputs. I tested plugging in a mic but nothing out.

What should I expect on the inputs (with or without the mic)?

/D

 

[jsteiger]

Have you scrolled thru this thread? Your questions have been answered before earlier in this thread.

 

[dubbySwords]

Hey All,

Really enjoyed building this unit!

Quick question regarding DC voltages on the op amp connections. I'm getting the +/-16 without issue, but I have some very small DC voltage readings at all of the other points as well (this is with everything built, but op amps and green LED driver IC not yet installed). I'll list them below. Do I need to go hunting for a trouble spot?

Preamp
C = -7.3 mV
+ = -7.3 mV
0 = 251 mV
- = 251 mV

Fader Booster
C = -7.3 mV
+ = -7.2 mV
0 = 251 mV
- = 251 mV

As you can see, there are obvious relationships between [ C & + ] and [ 0 & - ]. I'm not sure if these are things that will resolve themselves once I install the op amps, but I don't want to do that before I know more.

EDIT: I guess this is unnecessary information since they're connected, but the 0 and - voltages match what I'm reading on the positive side of C18.
EDIT AGAIN: So, I should say, these voltages are all in reference to chassis ground. I'm not sure what I should be looking for, or if I should be measuring between different points instead. Since I can't find any documentation of this part, and I've not built one of these before (nor in fact anything with a discrete op amp), I'm just feeling uncertain.

Thanks for any help you can offer!

Have you gotten an answer to this i.e what signal to expect in the in terminals?

 

[JMan]

Have you gotten an answer to this i.e what signal to expect in the in terminals?

No one ever answered me as far as I can recall, so I went ahead and installed the DOAs and fired it up and everything worked fine.

 

[dubbySwords]

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

Did you manage to get a schematic. I have a problem with my build and could really use it.
/D

 

[LoreM]

Howdy guys,
greetings from Italy.

I'm building my frist VP28 but I encountered a tremendous issue... I really don't know how it happened but the polarity toneluck switch got split in two and then started the nightmare...
I tried to remove the switch but it wasn't a quite journey, at all.
At the moment I was able to clean only 9 pads of 12, two are still stuck with a little solder and what remains of the pin, one seems to be only solder but doesn't want to clean up. Desoldering braid didn't help, just scratched the surface (for sure my fault).
I'm waiting for a hotair gun and I hope with it to complete the cleaning but I really fear to have screwed up the main pcb.
A little pad remained stuck to the pin of the switch on the upper side of the pcb, I was able to recover it but I don't know if it's still usefull (it's the third from right in the upper row in the pic).
I was wondering, do all the pins work or some of them are just there for "stability"?
May this pcb be still of use? After clean up I'll test the single connections but I fear for internal layers and so.

@jsteiger would it be possibile to order some spare parts (main pcb and the various elements I wouldn't be able to recover from the older one)?

I've attached some pics, there's a little dirt caused by the solder pump , all the joints were solid and shiny.

Thank you very much guys!

I was proceding quite smoothly and this problem really stuck me, I hate desoldering things! eheh

P.s. How much is important to completely flow with solder on the upper side of the pcb? I mean, except of the pads that shows clear path of connections. I had to rework some of the joints from above but I was wondering if it's worth it.

 

[LoreM]

Hi guys, I don't know if anyone is reading at the moment and I know Jeff is quite busy too with the building of the new house.

As an update, it seemed to me that the pcb was completely ruined, so I tried to recover as much components as I could, I'll need to reorder from Jeff a few caps and resistors, as the main pcb and the switches.

There's one thing in particular I'm worry about, the input transformer. While desoldering it one pin got free from the chassis, maybe because of the high temp. I could apply it back to the transformer just heating it up but how can I check if it's alright? I have a second one at home, I'm starting the new build just this evening. I hope the 2622 it's alright as it's a quite expensive piece in the entire build.

Thank you for your help!

UPDATE: I tried mesauring the transformer with the multimeter and got the same results as the new one (and something was telling me I was measuring them the wrong way)...
By the way I decided to open it up and that was a good idea because the wire from pin 1 (that was the one that got stuck to the old pcb) was disconnected... Resoldered and now I should have a working CA2622... well I hope so, I'll figure it out only in future when I'll get the spares I need...
Finger crossed for the second unit, it's growing smoothly!

 

[paulnajar]

Hi Everyone. Well after building 4 VP28’s Litz and 10 CA-0252’s over about 6 months I finally have my first build errors it would seem.

It concerns the filter sub board on one of the VP28’s. 6 and 12dB slope works, 40 and 80Hz works but 160Hz (both buttons in) all sound stops. I’ve had a very good look at the solder points and it all looks clean and right. I've noticed that if I lightly tap the 80Hz button with both filter buttons in I can get sound to come through in 160Hz mode so does that mean there's an issue with the switch contact? In 80Hz mode tapping the same button while in there is no such intermittency.

I have also discovered another led issue on the same channel. If mic, phase or pad buttons are in (LEDs on) and I press +48 button all other lit buttons LED's turn off. It's just the LED's that have the issue. The unit is sonically perfect and the buttons do what they are meant to.

Any hints where the problem of these two issues might be?

 

[mike_richter]

Hello everyone!

Sorry guys... newbie "testpoints" question here...

Trying to follow the CAPI VP28 Rev B Test Points guide.
I am using DAW and a DMM for the test. Setting 400Hz sinewave no problem.
Nothing is connected to the output of the generator and DMM is set to ACV red probe goes to pin 2 and the black probe goes to pin 3 (see picture). Setting the generator level so it reads 1.228 V AC. So far so good.

Now to the test points. Setting my DMM to mVAC. Putting my black probe on the "GND" holes on the PCB and red probe to the TP1, TP2 and etc...

Results are:
TP1: 109,8 mVAC
TP2: 193,3 mVAC
TP3: 343,2 mVAC
TP4: 8,53 mVAC
TP5: 8,53 mVAC
TP6: 8,53 mVAC
TP7: 17,95 mVAC
Not sure how to measure the module's output.

My results are not even close to what the "Proper results" should be....
Guys, please tell me what am I doing wrong?

I've built 3 VP28-s now and they all work perfectly fine, no complaints... (In fact they sound VERY nice) But the test points! I am clearly missing something and it drives me crazy...

It may be something very simple, I get it but please point me in that direction even if it's super-obvious. I really want to learn.

Thanks
Mike

 

[Clbraddock]

How important is this washer? (See pic) I've built 4 VP28s and I dont think I have been adding this washer. Is it worth taking these out of the 500 rack and taking them apart to add the washer or can I just leave it be?

 

[jsteiger]

How important is this washer? (See pic) I've built 4 VP28s and I dont think I have been adding this washer. Is it worth taking these out of the 500 rack and taking them apart to add the washer or can I just leave it be?

Its there to help keep the laminations of the transformers from making contact with the top "audio" ground plane. The transformer lam's are grounded to the bottom copper "chassis" ground plane thru one of the mounting screws.

 

[oldskool1]

I just finished 7 of 8 VP28 kits. one of the Litz output transformers has a factory defect and can't be mounted due to the mounting hole being obscured by a few displaced laminations, which are glued into place wrong. Should I a.). drill out the mounting holes, b.) take a hammer to the lamination in an attempt to break the glue and knock it into place, c.) return the transformer to CAPI for replacement?

 

[jsteiger]

I just finished 7 of 8 VP28 kits. one of the Litz output transformers has a factory defect and can't be mounted due to the mounting hole being obscured by a few displaced laminations, which are glued into place wrong. Should I a.). drill out the mounting holes, b.) take a hammer to the lamination in an attempt to break the glue and knock it into place, c.) return the transformer to CAPI for replacement?

Just drill it with an 1/8" bit. That is rare but does happen sometimes.

 

[Forcemusicgroup]

I have a VP28 that partially works. if i activate the 48v or the mic button. I get no signal coming in. any clue where to look?

 

[jsteiger]

I have a VP28 that partially works. if i activate the 48v or the mic button. I get no signal coming in. any clue where to look?

The 48V switch has nothing to do with signal path. It applies 48V to the pair of 6k8s thru a 200R ONLY if the Mic/Line switch is in the Mic (out) position. If in Line mode then 48V cannot be applied. Maybe you have some bad solder joints on the switches.

 

[Forcemusicgroup]

Definitely checked.. same issue...

 

[jsteiger]

So pushing the 48V switch cuts off any audio? As I mentioned the 48V switch is not on the audio path. Maybe its a cold solder joint somewhere else on the board?

You can also beep out the switch pins for continuity to make sure they are working as expected.

 

[Forcemusicgroup]

Resoldered the entire board. Just to make sure. Same issue. Any place to start looking

 

[jsteiger]

You can also beep out the switch pins for continuity to make sure they are working as expected.

 

[dubbySwords]

Error in BOM or KIT capacitors

I'm building CAPI VP28 Rev. B. A problem has arisen with capacitors C14 and C5 which are given in the BOM as 68pF and 82pF.
The only 2 capacitors I have left which have not been placed are an NC 441J and an NC112J.
Can someone help here?
/D