Neovibe debug: "Pop"

[frokost]

Hello, good people! I have several issues with my Neovibe, I think. Any help or hints would be greatly appreciated. My head is heavy after hours of soldering and debugging. Following the debug thread:

Here is the checklist to fill out:
1.What does it do, not do, and sound like?

Plugged in power for the first time, the bulb shone really bright. Then I realised I that the board was lying on a metal surface, and I sort of panicked. Didn't think of disconnecting power, but pulled the board away instead. I heard a small "pop", and then the bulb didn't light up anymore. That's how far I've come. I haven't tested for sound yet.

I took the bulb out and tried it on a 9V battery, it lit up, although very dimly. I'm using this lamp: http://www.smallbearelec.com/Detail.bok?no=716

2.Name of the circuit =

Neovibe.

3.Source of the circuit (URL of schematic or project) =

Board bought from GGG.

4.Any modifications to the circuit? Y or N

N

5.Any parts substitutions? If yes, list them.

No. All transistors are BC548, except Q13, which is a 2N3904.

6.Positive ground to negative ground conversion? Y or N

N

7. Now, using the original schematic as a reference for which part is which (that is, which transistor is Q1, Q2, etc. and which IC is IC1, IC2, C1, and so on) measure and list the voltage on each pin of every transistor and IC. Just keep the black lead on ground, and touch the pointed end of the red probe to each one in turn. Report the voltages as follows:

I'm using an adaptor from elextro-harmonix, it's supposed to output 18V/500mA, but this is what I measure at the board input: 25.5V before the diode bridge.

Ok, here we go:

Q1
C = 3.6
B = 1.3
E = 0.8

Q2
C = 4.6
B = 2.0
E = 1.4

Q3
C = 10.7
B = 4.6
E = 4.0

Q4
C = 14.9
B = 4.7
E = 4.3

Q5
C = 11.2
B = 4.3
E = 3.7

Q6
C = 14.9
B = 4.7
E = 4.3

Q7
C = 11.2
B = 4.7
E = 3.7

Q8
C = 14.9
B = 4.7
E = 4.3

Q9
C = 11.2
B = 4.3
E = 3.7

Q10
C = 14.9
B = 5.9
E = 5.3

Q11
C = 24.1
B = 11.7
E = 13.6

Q12
C = 24.0
B = 13.6
E = 13.0

Q13
C = 0
B = 0
E = 0

Q14 (78L05)
I = 24
G = 0
D = 14.9

Q11 seems to be the problem, but I'm not sure how to proceed from here. I've searched and found that this causes the LFO to never start. EDIT: I did reflow the solder in the LFO section. Still whacky voltages on Q11.

Q13 is apparently dead. It wasn't when I measured the voltage on it before I decided to post. Weird. It did get a little bit hot, though, but not so hot that I couldn't touch it. I wish I remember the voltages from before it died. But isn't it a bit strange that it reads 0 volts on all pins?

Anyway, the pop I heard worries me. Something was shorted when I dragged the board along the metal, and I don't know what. Could it be the bulb? As I said, it worked when I connected a 9V battery to it, so I put it back in, and after that Q13 doesn't seem to work anymore. Should I just replace the bulb?

[fhsueh]

Hi, I can't help you too much since I'm pretty newbish myself, but I did successfully build the Neovibe recently, after following R.G.'s instructions very carefully.  I had a similar experience on my build, and realized that R.G. even states the lamp driver transistor (Q13) is one of the most commonly killed components and the one under the most power stress.  He even suggested having some spare 2N3904's, as this transistor is easily killed even just by probing (see p. 5 of his build instructions).  And I think I actually killed Q12 somehow.  (I did hear two pops during my debugging and there was a funny smell). After replacing both, my lamp driver and LFO worked again. 

[frokost]

Yeah, I know about the issue of Q13, which is why I put a 2N3904 there. It didn't get THAT hot, so I assume that wasn't the problem.

So far, this is a very fun build, because of it's size and complexity. And since it's such a classic I want to do everything right. Just need to get the thing to work  . I think the bulb is dead, and the 3904 too, so I took them out. It sure is hard replacing components with the GGG board. Are there any good tricks? I'm having problems clearing the holes for solder.

[trixdropd]

Yeah, I know about the issue of Q13, which is why I put a 2N3904 there. It didn't get THAT hot, so I assume that wasn't the problem.

So far, this is a very fun build, because of it's size and complexity. And since it's such a classic I want to do everything right. Just need to get the thing to work  . I think the bulb is dead, and the 3904 too, so I took them out. It sure is hard replacing components with the GGG board. Are there any good tricks? I'm having problems clearing the holes for solder.

Flow the solder then hit the side of the board against the table, the solder will fall to the table. R.G. posted this a bit ago and it's an awesome solution.

[frokost]

And I've run out of 2N3904's... I do have some 2SC1815's though. They should be okay for Q13, right? The datasheet states that I_C is 150mA (max), V_CEO is 50V (min). They are listed with a hFE of 200~400.

trixdropd: Ah.. I've seen that advice before, but somehow I missed the "hit the side of the board" part of it. Thanks  .

[R.G.]

For single sided boards, get a wooden toothpick, then heat the solder on the pad and poke in the sharpened end of the toothpick. Leaves a nice open hole. Obviously, don't use the toothpick for a toothpick after this - Doh! 

The 2SC1815 will work fine, probably better than the 3904, but it definitely has a different pinout than the 2N3904. The 3904 is EBC, the 1815 is ECB.

Bipolar transistors fail in microseconds when asked to discharge large capacitors - which is probably the "pop" you heard.

[frokost]

Ok, thanks for getting me on track again. I replaced the 2N3904 with a 2SC1815. These are the voltages I get on Q13 now: B: 6.0 C: 22.9 E: 5.4. It gets a little hot, but it's touchable.

Every store is closed here during easter, so I bought a 12V/1,2W bulb from a gas station. When I rotate the trimmer all the way clockwise, it glows just a little bit.

Back to the oscillator, Q11 and Q12 readings are the same. Any hints? The oscillator obviously isn't running.  Could that cause the dim bulb as well?

[frokost]

Hah! I went over the wiring to the pots again and found a loose wire on the speed pot. I have oscillation! 

Now for the biasing. I can get the bulb to glow dimly when I dime or almost dime the bias pot. I'm unsure of how much the bulb is supposed to light up when the depth knob is cranked, because it really doesn't light up that much. Is it supposed to be status LED bright or oh-my-god-take-the-sun-away-bright?

[frokost]

You know, don't worry. I think I'll get it from here. Search is my friend. Thanks to everybody for the conversation. If I run into trouble again I'll be back.

[Jargo]

I remember being taught to flow the solder and then hold the board with your hand and tap your hand against your bench...virtually no stress on the PCB itself...plus it gets the holes cleared pretty much every time!!  Also, to clear the holes, flow the solder and introduce a toothpick - that's in a post here somewhere. I use this Dental Probe that I bought at CVS pharmacy (like $4) - it's dual-pointed and both tips are lighted - also comes with a dental-style mirror.

[frokost]

Yep. Good techniques. Finished it today. Sounds just great!