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

[GregNey]

Hello, I recently purchased a VP28 locally that someone tried to build but couldn't (I've built 3 successfully). Long story short, it didn't pass signal at all when I got it. I took it apart, cleaned it, re-heated some pads and now it's passing a very small amount of signal. I ran a 1k test tone through it and I'm getting good signal at points 1, 2, 3, but I have a pretty substantial level drop at 4. My natural instinct is to replace the components on the sub-board and Q1 (due to it's proximity) but just wanted to run it by the board before I did anything.

Any help is greatly appreciated.

Thank you, Greg

 

[jsteiger]

Greg, if things are good at #3 but low at #4, that indicates it's something with the fader switch and related R's.

If it was good at #4 and low at #5, I would say to look at the HPF board.

Q1 is part of the constant current source for the LED's so don't do anything with that.

 

[GregNey]

Weirdly, the fader switch is the best looking part of this build... Not exactly sure what I'm looking for here...

 

[jsteiger]

Dbl check the values for all of the fader R's. Maybe some got switched around.

Next, turn the fader all the way up, fully CW. Probe the right end of the row of fader R's just below the #3. The level should decrease as you go down the row.

These points will also match what is happening at test point #4 if you turn the switch with each point down the row. Test point #4 is equal to the switch wiper.

So,
+12dB switch setting = TP#3 = wiper = TP#4
+10dB switch setting = right end of RF11 = wiper = TP#4
+8dB switch setting = right end of RF10 = wiper = TP#4
and so on....

 

[GregNey]

Weird stuff happening.... My setup:
-EL500 rack with 2 VP28's in it. I built one and it is working perfectly, has been for over a year. The other I bought locally as non-working.
-Red Dots in both positions for both preamps.
-When I first bought the preamp it did not pass signal. I've cleaned alot up (it was not built very well) and I just plugged it back in to test. Things looked good at first.
-At line unity level and 0db on the fader, the 2 units track within .1db of each other.
-At line unity level and -20db on the fader, the 2 units track within .2db of each other (great so far.... or so I thought)
-At line unity level and +2db on the fader, I switched my build and I saw an appropriate level increase. When I switched the new build to +2db on the fader I got an incredibly HOT signal in BOTH channels... ??? I just turned it off as I didnt want to subject anything to what looks like a dangerous signal. Here I am now.

Thank you for your help! There is definitely something wrong at the fader switch.

 

[kante1603]

Hello,

I didn't get what the "level increase on both channels" means,are they linked somehow?
Anyway,an immediate increase leads to something is going wrong on the resistance curve/array on the switch.
I'd look at three points:
1.:Are all resistors in the correct place?
2.:Any shorts at the grayhill pins?
3.:Are the stop pins set right?

Start with nothing else attached to the rack,apply a signal,say 0dB at 1Khz,set to line mode and unity gain.What do you get on the output,is there any offset between in and outputlevel?

Let us know,good luck,

Udo.

 

[technoteo]

Hi everybody
I need some help beacuse I'm stuck!! 

I just finihed my first VP28 kit and everything was smooth and easy.  (BTW Thanks Jeff Steiger, the kit is awesome!!)

I'm assembling the 2 GAR2520 but realized that I used one of the matched BC550 for Q5 ....    .....

https://www.dropbox.com/s/6w3vl2xoic2hrh4/bc550-2.jpg

Now I'm trying to desolder the Transistor to put it in the right position, I try not to heat the transistor too much, used a pump and also some copper solder remover, but I still have a lot of tin on the silk screen side and there's no space to work on....

https://www.dropbox.com/s/5p832j4j8r1kx3b/bc550-1.jpg

Is the matched pair vital for the 2520 to function properly?

Any suggestion how to take away the remaining tin on the transistor? SOLVED!  ;D

I spent almost 2 hours to desolder the transistor trying to warm it as little as possible, and making it slowly not to damage the PCB

now my question is : do you know if the BC550 is sensitive to temperature? I mean, may I have damaged heating it too much?
Do I have to buy another matched pair of BC550 ?

thanks everybody

 

[jsteiger]

They are sensitive to too much heat. I would just try it and see if it works.

 

[technoteo]

They are sensitive to too much heat. I would just try it and see if it works.

OK, I don't want to get crazy again in the case they're gone and since I have another pair of gar2520 I will change the transistors and order a new matched pair from you.    I will wait for one of the kit to be completed.....

 

[mbjr]

Hi everybody,
first time builder from Denmark. Built 2 VP28's fairly successfully, they pass audio and all buttons work incl. the HPF.
I made a mistake with the DOA opamps: 4 gar2520. I switched the direction of Q7 & Q8 so the text is facing the wrong way.
They did pass audio, all though very distorted so now I'm going to desolder the Q7 & Q8 and turn them the right way but I'm not quite sure whether they have damaged them or any of the other parts of the opamps?
Jeff mentioned in a earlier post that they are quite heat sensitive and I'm having a hard time desoldering and removing them so I'm going slowly and try to apply as little heat as possible and still desolder them.
The question is whether the opamps can be salvaged if I manage to desolder the Q7 & Q8 or if rest of the components have been damaged so I just should build some new ones without making mistakes?
How do I test whether the Q7 & Q8 or any of the other components have survived my desoldering?

Thanks!

 

[qmp audio]

For all you folks de-soldering... i can recommend the hakko 808.  With proper use, it makes de-soldering a quick and easy task.  I got one a while ago and i can say it was worth the investment.... it has paid for itself with time saved.

 

[GregNey]

I didn't get what the "level increase on both channels" means,are they linked somehow?

There was a crosstalk issue that has been remedied and ALL IS WELL!!! Thank you all for your help!!

 

[kante1603]

There was a crosstalk issue that has been remedied and ALL IS WELL!!! Thank you all for your help!!

Cool,congrats and have fun

Best,

Udo.

 

[erol]

Just finished my first DIY VP28 and everything works!!
BUT...

I've got some problems installing the knobs on the grayhill switches.
First of all, what's the mm Allan key I need for both knobs?

just did a test, when I put the channel fader on +4, I can not go further. When I go back in value, the volume is getting softer so it looks like +4 is actually +12.
But this is the only way the knob fits on the greyhill switch?

Maybe someone here could explain to me in which position the grayhill switches has to be before installing the knobs. (flat side down and knob up??)
Thanks again

 

[kante1603]

Just finished my first DIY VP28 and everything works!!
BUT...

I've got some problems installing the knobs on the grayhill switches.
First of all, what's the mm Allan key I need for both knobs?

just did a test, when I put the channel fader on +4, I can not go further. When I go back in value, the volume is getting softer so it looks like +4 is actually +12.
But this is the only way the knob fits on the greyhill switch?

Maybe someone here could explain to me in which position the grayhill switches has to be before installing the knobs. (flat side down and knob up??)
Thanks again

Hello.

Yes,looks like the pot caps are mounted wrong.
The answers are all on the first page,please read through the thread,it's not too long.
Page 4 points 15 & 16 of the assembly guide describe the how-to,and our friend chunger even made fantastic pics in his documentary (to be found on the first page in this thread too).
And about the allen wrench keys-they're not metric but imperial,again to be found on the first page,reply #16:
http://groupdiy.com/index.php?topic=48610.msg614212#msg614212

Udo.

 

[erol]

Just finished my first DIY VP28 and everything works!!
BUT...

I've got some problems installing the knobs on the grayhill switches.
First of all, what's the mm Allan key I need for both knobs?

just did a test, when I put the channel fader on +4, I can not go further. When I go back in value, the volume is getting softer so it looks like +4 is actually +12.
But this is the only way the knob fits on the greyhill switch?

Maybe someone here could explain to me in which position the grayhill switches has to be before installing the knobs. (flat side down and knob up??)
Thanks again

Hello.

Yes,looks like the pot caps are mounted wrong.
The answers are all on the first page,please read through the thread,it's not too long.
Page 4 points 15 & 16 of the assembly guide describe the how-to,and our friend chunger even made fantastic pics in his documentary (to be found on the first page in this thread too).
And about the allen wrench keys-they're not metric but imperial,again to be found on the first page,reply #16:
http://groupdiy.com/index.php?topic=48610.msg614212#msg614212

Udo.

Thanks.
Any idea were I can find both Allen keys? (In Europe)

 

[erol]

1,3 mm (0,05")
1,6 mm (1/16")
http://www.conrad.nl/ce/nl/product/824038/Wiha----263-13005-X-40-PicoFinish-Griff----00521----Inbus-schroevendraaier----1?

http://www.conrad.nl/ce/nl/product/407101/BernsteinInbussleutel-116-16-mm-6-852?

right?

 

[Patrick from Davis]

I tend to lose things like allen keys very fast.  I have an assortment of rarely used and odd sized ones that come with guitar bridges and cheap furniture, but those don't seem to be useful for anything other than their originally intended purpose.  So I got a couple of these.  This one linked is for the small sized imperial hex keys, there is one for the larger ones as well.  They color code the handle yellow for imperial, and red for metric, so it is easy to tell from a distance.  I am unsure if you have somewhere (Hardware store) that would carry something like this in stock, but I have saved quite a bit more time (and frustration) than the 5-10 dollars it cost me is worth.  Another one of those tools that doesn't get much use, but man o man does it save time when I need it.
Patrick

 

[kante1603]

1,3 mm (0,05")
1,6 mm (1/16")
http://www.conrad.nl/ce/nl/product/824038/Wiha----263-13005-X-40-PicoFinish-Griff----00521----Inbus-schroevendraaier----1?

http://www.conrad.nl/ce/nl/product/407101/BernsteinInbussleutel-116-16-mm-6-852?

right?

Looks about right.
I know the WIHA stuff,pretty good for the price.
Once I bought a complete set in imperial size from the "magic ring"-series,very good but a bit expensive.But since I wanted to build a lot of modules I went for them.You can't beat good tools.
Just don't use too cheap tools,most of them are weak and might break.Be sensitive when tightening.
Hopefully you haven't damaged the screws using the wrong keys.

Best regards and good luck,

Udo.

 

[antoniowas]

Hello folks,

I've been getting into DIY audio gear and I found this thread. I went to the api website but I wasn't sure on how to buy the kit. I think it might be sold out, is that true?

Cheers,

Antonio