Preamp Studio Projects VTB-1 - No signal, only hum.

[stonerbox]

Hi guys! Back with another none stompbox related issue.  

My VTB1 broke a while ago and I am trying to repair it (since it is my only preamp). I opened it up a long time ago and a metal surface touched the board, connecting something and sparks where jumping.

I need help in how to locate the broken parts, I am a newbie but have built several guitar pedals. I sent a email to Studio Projects asking for the schematic and are waiting for their response.

What can / should I do?


Crappy images of the circuit:
http://bounav.free.fr/wp/wp-content/uploads/2009/04/vtb1_inside_1.jpg

http://cdn.head-fi.org/8/86/861c4f1d_img001.jpg




L7812CV (positive voltage regualtor) datasheet http://www.datasheetcatalog.com/datasheets_pdf/L/7/8/1/L7812CV.shtml

Pins:
1. 12+dcv
2. 0dcv
3.9+dcv


I marked up all the op amp positions here:
All op amps:

"U1" 4580D JRC Datasheet http://pdf1.alldatasheet.net/datasheet-pdf/view/204246/KODENSHI/KK4580D.html
1. 0dcv [OUTPUT 1] (get signal from mic here, fairly strong with hum)
2. 0dcv [1IN-]
3. 0dcv [1IN+]
4. 12-dcv [GROUND]
5. 0dcv [2IN+]
6. 0dcv [2IN-]
7. 0dcv [OUTPUT 2] (get signal from mic here, fairly strong with hum)
8. 12+dcv (VCC+)

"U2" 2082D JRC datasheet pdf http://pdf.datasheet.su/njr/njm2082d.pdf

1. 0dcv [OUTPUT 1] (get no signal from mic at all)
2. 0dcv [1IN-]
3. 0dcv [1IN+]
4. 12-dcv [GROUND]
5. 0dcv [2IN+]
6. 0dcv [2IN-]
7. 0dcv [OUTPUT 2] (get signal from mic here, strong with hum)
8. 12+ Dcv (VCC+)

"U3" LM339N Datasheet http://www.alldatasheet.com/datasheet-pdf/pdf/3071/MOTOROLA/LM339N.html

1. 0dcv [OUTPUT 1] (get no signal from mic at all)
2. 0dcv [OUTPUT 2] (get no signal from mic at all)
3. 9+dcv [VCC]
4. 6.1+dcv [INPUT1-]
5. 0dcv [INPUT1+]
6. 2.9+dcv [INPUT2-]
7. 0dcv [INPUT2+]
8. 0dcv [INPUT3-]
9. 0.1+dcv [INPUT3+]
10. 0dcv [INPUT4-]
11. 0dcv [INPUT4+]
12. 0.2dcv [GROUND]
13. 0dcv [OUTPUT 4] (get no signal from mic at all)
14 0.9+dcv [OUTPUT 3] (get no signal from mic at all)

"U4" LM358 Datasheet http://www.alldatasheet.com/datasheet-pdf/pdf/194750/STMICROELECTRONICS/LM358N.html

1. 0dcv - Output 1 (get no signal from mic at all, only loud hum)
2. 0dcv - Inverting input
3. 0dcv - Non-inverting input
4. 0dcv - VCC-
5. 0dcv- Non-inverting input 2
6. 0dcv - Inverting input 2
7. 0.2+dcv - Output 2 (get no signal from mic at all)
8. 9+dcv - VCC+

"U5" 2114D JRC Datasheet http://www.alldatasheet.com/datasheet-pdf/pdf/7259/NJRC/NJM2114D.html

1. 0dcv [OUTPUT 1] (get no signal from mic at all)
2. 0dcv [1IN-]
3. 0dcv [1IN+]
4. 12-dcv [GROUND]
5. 0dcv [2IN+]
6. 0dcv [2IN-]
7. 0dcv [OUTPUT 2] (get no signal from mic at all)
8. 12+dcv (VCC+)

"U6" The 4508D was broken so I swapped it for a TL082CP Datasheet http://www.alldatasheet.com/datasheet-pdf/pdf/25407/STMICROELECTRONICS/TL082CP.html

1. 9+Acv [OUTPUT 1] (the loudest microphone signal from this one with lot of hum)
2. 0dcv [1IN-]
3. 0dcv [1IN+]
4. 12-dcv [GROUND]
5. 0dcv [2IN+]
6. 0dcv [2IN-]
7. 0dcv [OUTPUT 2] (get no signal from mic at all)
8. 12+dcv (VCC+)

[stonerbox]

-

[tempus]

Do you have/know how to use a signal tracer? That's where I'd start. Essentially all you're doing is applying a signal to the input of the mic pre, and touching the signal tracer probe to various points in the circuit:

Audio signal (could be a mic placed in front of a speaker in the other room, or your mp3 player output turned down VERY low) => mic input. Now touch the probe of your signal tracer to the input jack. You should hear the signal coming through the tracer. Now move it to the next point in the circuit, and keep moving away from the input jack until you have no signal. That's the first place to start looking for damaged components. There's a pretty simple way to adapt your amp to a signal tracer here: http://www.musicfromouterspace.com/analogsynth_new/HOT_TIPS/ampsignaltracer.html

[stonerbox]

Tempus, thanks for sharing some helpful information!

I guess that's kind of what I have been doing, I connected a wire to the output xlr and tapped in on different op amps, transistors and resistors, with various results, some times with electrical sparks and pops.    

I'll keep on trying to figure out where the faulty component/s are but it's really hard without a proper schematic (Studio Projects seem to refuse on sending me one).

I thing I noticed are that the line in/D.I works perfectly when tapping into an op amp, no hum no noise, but the mic channel (when the phantom power is engaged) cause lots of hum and noise, but mostly hum.

[tempus]

So you've narrowed the possibilities considerably - it's likely something in the mic section. Have you been starting from the input or the output of the micpre? You want to apply a signal to the input, then start tracing from the closest point to the input moving to the output.

Are you getting 48v at the input with phantom applied?

[stonerbox]

Yup, I read 48 volt off the female XLR!

I haven't been able to go from point a to b, at all, mostly just checked for faulty op amps and transistor. I will try to track the signal through the circuit. Will not be a easy task, but I'll try my best

[stonerbox]

Wow, wait! I get 36dcv to the mic all of a sudden. The drop from 48 to 36 is after R18 and R17 (down to the left in picture). Is that normal?

When I feed the mic with full 48 volts it hums a lot!

[tempus]

The standard value for phantom power resistors is 6.81K, and I would think unloaded you should measure 48v, although many phantom mic circuits on the con- prosumer market don't actually have 48v there. The quickest way to rule out the phantom power as being an issue would be to try a dynamic mic and see if that works.

[stonerbox]

I don't know why I didn't post this earlier, sorry! I hooked up a dynamic mic yesterday and the signal was clean. No hum, no noise.

Now if only SP could be kind enough to send me that damn schematic, guessing not though.

[tempus]

Dude - that's huge! So you're saying that the pre works perfectly with a dynamic mic, but not with a condenser? This means one of two things: either the condenser mic you're using is not working properly, or there's something wrong with the phantom power circuitry. Even if it's the latter, it should be a fairly easy fix from here on in. Do you know what the phantom circuitry should look like?

[stonerbox]

Dude - that's huge! So you're saying that the pre works perfectly with a dynamic mic, but not with a condenser?

I'm pretty sure it's not the mic. I have tested both my condenser and ribbon microphone and it's the same thing on both of them.

Do you know what the phantom circuitry should look like?

No comprendo, what it should look like?

[tempus]

Ribbon mics don't need phantom power, and in fact most will be damaged by it. What mic is it? There's a schematic about halfway down the page here: http://www.audiomasterclass.com/giant-killing-5-mic-preamp-its-secrets-revealed. Your micpre front end probably looks similar - from the XLR up to the diodes, possibly only to the caps is likely where your problem is.

[stonerbox]

Killer man! I'll look into this right away.

The ribbon I use is specifically built for phantom power, a must when on a low budget preamp like this one.

[stonerbox]

Ok, replaced the 47uf caps and it didn't stop the sh*tsstorm of hum... BUT one of the 2N3904 (Q8) reads 48v and the electrolytic caps around there is marked 35v, probably a HINT.

[tempus]

Before you replace anything else, are you actually tracing the components to see that they're connected to what you think they are? I'm guessing that the 2 47 uF caps are there to keep the DC from the phantom out of the preamp circuit, but you should check to see that they are actually connected to the XLR on 1 leg and the preamp in on the other. Those 35V caps may be fine if they're not part of the 48V circuit. Are the resistors in spec? Is there actually 48v sitting on both legs of the resistors? If so, trace back to the source of the 48v supply and check that area out.

[stonerbox]

Yeah, I traced them before changing them.

The strange thing about the two 35v 47uf caps is that they are connected to the phantom section but is supplied with 48v even with the phantom power is switched off. One of them are connected to the emitter leg of a 2n3904 which also reads 48+dcv (you'll find them in the middle/top of the circuit picture). They are connected all the way over to top/left, to the R66 where the phantom power part is located.

Update: The R53 is damn hot, just below the Q8 (top/middle in pic), reads also 48dcv.

[tempus]

OK, well, if they've got 48V on them and they're only rated for 35, there's a problem; either they're underspecc'd or the 48V shouldn't be there in the first place. Have you been able to trace back to find where the 48V circuit is actually located on the board? Also, I just caught something in your readings - that 7812 should have more than 12v on the input if it's going to put 12v on the output, so something's amiss there too. What is the pre powered by?

[stonerbox]

Yeah, it's definitely weird.. I don't think they would underspec the caps if they where intended to be feed 48v.

Got a answer from SP and they said they won't send me the schematic and that it would be too pricey to send it in for repair (CA, USA) and literally ending the mail telling me to go out and buy a brand new one instead. Wow, thanks SP Im so grateful for your awesome support! Cheap bastards..  

So.. right now I'm working on a schematic, part of it (the 48volt affected parts) will be up later today. Wish me luck!  

Also, I just caught something in your readings - that 7812 should have more than 12v on the input if it's going to put 12v on the output, so something's amiss there too. What is the pre powered by?

7812? Don't have any IC with that number, perhaps you meant the LM339N?

[tempus]

That customer service is just insulting. If you end up having to get a new one, I'd let them know that you're buying something else.

Hopefully you don't have to though.
This is the voltage regulator I was talking about:

L7812CV (positive voltage regualtor) datasheet http://www.datasheetcatalog.com/datasheets_pdf/L/7/8/1/L7812CV.shtml

Pins:
1. 12+dcv
2. 0dcv
3.9+dcv

There should be at least 14 or 15v at the input for it to pump out a regulated 12v at the output. If there isnt`12v there, it`s possible that the low voltage is affecting other areas of the pre. Is it powered with a wallwart, or how is it powered?

[stonerbox]

ah, the L7812CV! Yeah, should be at least 15v. I bet that when I get the circuit down on "paper" it will be easier to spot what could be the failing component/s.

This is what I got so far, not that great, but I'm working on it.

Making this one is part revenge because of the sh*tty attitude from SP, they should have given me that schematic from the start, now, they lose either way.