Neovibe Problem

[elpucho]

Hi,

I just finished building a Neovibe using the pdf from Geofex.com.  I didn't make any modifications, and used 2n3904's for all the transistors, along with a 12v 0.08A 0.96W bulb.

There is no sound at all coming through, yet all the controls seem to work as expected (judging by the bulb lighting up faster and brighter when the speed and depth pots are turned), I also checked the suggested voltages from the pdf and they all seem to check out OK.  I tried working through the signal path with an audio probe and the last time I get a sound is going into the base of Q1, I get no sound from the Collector or Emitter.

Would this mean that Q1 has a problem?  I've never really worked my through a circuit before and I'm not that clued up.

I checked the continuity of the circuit before I hooked up the pots, so I think that should be OK.

Any help, or suggestions would be appeciated.

[R.G.]

The most likely problems are the transistor pinout, electrolytic capactor direction and wiring.

Beyond that, read "Debugging Thread: What to do when it doesn't work" and follow.

[elpucho]

Thanks RG,  I spent a bit of time today and I have managed to get it working now.

One thing I did notice was that Q13 (I used a 2n3904) gets pretty hot and I wondered if that could be a problem.  I used a 12v 0.08A 0.96W bulb, is that pushng the 2N3904 a bit too much?  As far as I have read the 2N3904 can handle 100mA and my bulb is drawing 80mA,  I also have a 12v 60mA, or a 5v 45mA bulb I could use, if they would be better suited.

Cheers.

[elpucho]

I changed over the bulb to a 12v 60mA to see if that stopped Q13 from getting as hot, but it still seems to be heating up (if I place my fingertip on the top of it I can definately feel heat, not enough to burn me but enough to make me thinks it's pretty hot).  Does anyone know if this is likely to cause problems?  I hunted around and found a post where it said a 2N3904 in Q13 should not be getting hot as it can handle up to 100mA.

I'm almost done now and it's sounding great, just need to figure out how to balance the volumes when I use the vibrato/chorus switch, set them both to unity gain and box it!

I also meant to say:  I tried replacing Q11 and Q12 with 2 MPSA13's but when I tried that the bulb would not flash at the highest speed setting (it just stayed lit), I'm not sure if that's normal or not.  I ended up using a MPSA13 in Q11 and a 2N3904 in Q12

[Thomeeque]

I changed over the bulb to a 12v 60mA to see if that stopped Q13 from getting as hot, but it still seems to be heating up (if I place my fingertip on the top of it I can definately feel heat, not enough to burn me but enough to make me thinks it's pretty hot).  Does anyone know if this is likely to cause problems?  I hunted around and found a post where it said a 2N3904 in Q13 should not be getting hot as it can handle up to 100mA.

Hi!

I'd say it's normal, bulb-driving transistor at my Forum Vibe gets hot a lot as well - until it's "not enough to burn" I would not worry.. Anyway, smaller current bulb (and/or some heatsink for this transistor) could help here (I have 12V/40mA there now and I'm planning to switch it by 12V/20mA bulb, it just arrived).

"Q13 should not be getting hot as it can handle up to 100mA" - how much Q gets hot is influenced by effective voltage drop between it's Emitter and Collector and current coming thru (P=U*I ~ power, turned to heat) (Base currents are small enough to be ignored here) and by surrounding condtions, influencing it's cooling (package, surrounding temperature, airflow etc.), not by it's max. current limit. So, it would get hot same even if it was able to handle 10 ampers if nothing else has changed.. Maximal Power dissipation (and conditions for this value) is important here.

I hope I'm not bringing too much confusion here, somebody please correct me, if I do

T.

[R.G.]

What you're noticing with Q13 getting hot is going to be expected if not actually what we'd call normal if you go to the higher current bulbs.

A 2N3904 can conduct 100ma, and it can withstand 40V, but it will overheat if you try to do both at the same time. The maximum amount of heat that can get out of the package is 625mW, and this is a limitation caused by the fact that the transistor chip inside is of a fixed size, and the epoxy covering it can only get a certain amount of heat out into the air. With a 20V power supply (roughly) in the Neovibe, the average current it can sustain without burning up is 0.625/20 = 3.125ma. ACK! How does it ever NOT burn up?

That's because the voltage across it drops as the current causes the bulb to heat up. You have to get off into time average voltage/current and some minor calculus to get to the real answer, but you can see how thin the margin can be.

The to-92 package is just barely capable of doing the job, heating up as it goes. As long as you can touch the transistor and keep your index finger on it without involuntarily pulling it back or raising a blister, it's marginally OK. For long term life, it's not OK. That transistor, Q13, is the single most common failure in real, manufactured Univibes. I've replaced many of them in Univibes I've repaired over the years. I would have expected the bulb to burn out more often than the transistor, but it doesn't seem to work that way.

So what you're seeing is correct - 2N3904 works, but it gets hot, especially with the bigger-current bulbs. If you can get a 12V/45ma or 12V/25ma bulb, it would be better. I also put in pads on the Neovibe board for a bigger transistor package. You can put in a TO-126 mini-flatplack transistor like the 512-KSC1173YTU (Fairchild's "2SC1173" Mouser part number. $0.31) which has almost as much gain as the 2N3904, but can conduct 1A and dissipate several watts, or a TO220 package, even a darlington like the TIP127 (Mouser 512-TIP127-TU, $0.47); the extra holes in the PCB are there for doing exactly this. But the 2N3904 will last a while, maybe even long enough.

[elpucho]

Thanks for the advice Thomeeque and RG.  I'll use what i have at the moment and look around for a either a replacement bulb and/or a replacement transistor as RG specified.

I'm currently messing around with R37 and R38 to see if I can balance the Chorus/Vibrato volumes.  I managed to get them both around the same volume, but I think it has changed the sound somewhat, which I don't like.  I may just put R37 & R38 back to the stock values and use the volume control to balance them out.

I appreciate the help RG, you must have answered these type of dumb questions a thousand times.  You must have the patience of a Saint!

[R.G.]

I'm currently messing around with R37 and R38 to see if I can balance the Chorus/Vibrato volumes.  I managed to get them both around the same volume, but I think it has changed the sound somewhat, which I don't like.  I may just put R37 & R38 back to the stock values and use the volume control to balance them out.

It may be that by changing R37 and 38 you're changing the loading on the R35/36 pair that adds the dry and wet signals to make the phase sound. If that happens, the cancellation of dry and wet on the phase sound is affected and the phasing is not as deep. Just guessing. But the R37/38 pair is 47K and 220K, so if you lower R38 much, it will cause more loading on the phaser resistors all the time. Try only going higher on R37/R38 for volume adjustment, (that is, bigger R38 for more volume, bigger R47 for less volume) and then once you get the volume right, readjust R35/36 for best phase sound. They're slightly interactive.

[head_spaz]

R.G.
Which is it?
Mouser part number 512-KSC1173YTU -- (2SC1173) is NPN
Mouser part number 512-TIP127-TU -- TIP127 is PNP

[R.G.]

R.G.
Which is it?
Mouser part number 512-KSC1173YTU -- (2SC1173) is NPN
Mouser part number 512-TIP127-TU -- TIP127 is PNP

Sorry. I was looking here: http://www.mouser.com/catalog/catalogUSD/640/501.pdf
Which shows the 512-TIP127-TU under the NPN darlington column (upper right corner). Given that they made the mistake twice (KSH127 is shown in NPNs as well) I don't feel quite as bad, but I admonish myself like I do other people: go look at the datasheet, R.G. !

The TIP122 is a much better choice, since it has some hope of working! 
Good catch. Thanks for correcting my lookup error.