Neovibe Debug

[Craig V]

Hi,

I have an old Neovibe project from a few years back that needs to be fixed up.  It probably only needs a few tweaks to get it up and running, but it was never up and running in the first place...  There are 2 problems:  one, the chorus/vibrato switch gives the same sound regardless of where it is set - the vibrato setting.  Secondly, the LFO and bulb do not flash.

Here is the info in the format Aron and RG recommend:

1-  Neovibe from GGG
2 - http://www.generalguitargadgets.com/diagrams/neovibe.pdf
3 - I followed it as closely as possible, on a homemade PCB, and followed the layout exactly.
4 - The only part switched is R38, a 100k was used because I'm out of 220k.  Also, no voltage regulator was used.
5 - I believe this project is negatively grounded.
6 - The chorus/vibrato switch give the vibrato sound, regardless of the position.  The Bulb does not flash - some sort of LFO problem.

Here are the voltage measurements:

Q1
C = 2.34
B = .99
E = 1.08


Q2
C = 2.40
B = 2.37
E = 1.74

Q3
C = 12.65
B = 5.90
E = 5.33


Q4
C = 18.21
B = 5.16
E = 5.25

Q5
C = 13.6
B = 5.25
E = 4.61

Q6
C = 18.19
B = 13.9
E = 15.04


Q7
C = 14.35
B = 15.08
E = 14.34


Q8
C = 18.21
B = 5.21
E = 5.31


Q9
C = 13.52
B = 5.31
E = 4.66

Q10
C = 18.20
B = 6.36
E = 6.52


Q11
C = 18.16
B = 3.35
E = 9.95

Q12
C = 18.20
B = 9.95
E = just drops when I place probe on it

Q13 (the unnamed one)
C = 16.92
B = 4.12
E = 3.52

Both D1 and D2 have voltage falling, much like the emitter of Q12.

This project has been idle for a few years, and I'd really like to get it working.  The Neovibe is one of the best sounding pedals ever made, IMHO. 

Are there any clues as to whats wrong?  I noticed a couple of transistors have the emitter higher than the base, "doh."  I'm really lost as to what is causing all of that to happen.  The lamp doesn't flash either, which worries me.  I really don't know what to look at first.

Help would be much appreciated.  Thank you!

-Craig

[Craig V]

I just want to add that the lamp lights up, albeit very dimly, and adjusting the trimmer doesn't really do anything.

-Craig

[R.G.]

OK. A couple of questions first.

Are the transistors exactly the part numbers in the list, not substitutions? (That's my sneaky way of saying are the pinouts correct...)
Is the sound of a reasonable volume in the vibrato settings? As loud as or nearly as loud as the bypassed volume?
Are your LDRs inside a dark enclosure?

Can you supply the voltages at
- junction of R39/R40/R41
- junction of R5 and R7?

[Craig V]

Hi R.G.,

Thanks for replying.  The transistors used are indeed 2n3904.  These particular ones came from Small Bear, and the flat side has grey paint on them.  So pinouts should be correct, but maybe these particular transistors have different pinouts?  I used your diagram to double check, and they're in there correctly.  Maybe different pinouts?


The volume is pretty close to bypassed volume, a bit quieter though.  That is expected with a Neovibe, since the original Univibe wasn't true bypass.

The LDRs have a film canister enclosure, but when I talk about the sound of vibrato, I take the canister off and point a light and use my hand to vary the amount of light.  So the bulb isn't providing any modulation, only my hand.

The voltages:

R39/R40/R41 = 10.59
R5 and R7 = 15.50

I do not have the voltage limiter in, just a jumper.  Do you think I should install it?

Thank you very much!

-Craig

[R.G.]

The transistors used are indeed 2n3904.  These particular ones came from Small Bear, and the flat side has grey paint on them.  So pinouts should be correct, but maybe these particular transistors have different pinouts?  I used your diagram to double check, and they're in there correctly.  Maybe different pinouts?

It's possible.You could test that by using your ohmmeter. Test your ohmmeter on a known diode with the band to help you. The red lead on the un-banded/anode end and the black lead on the banded/cathode end should show conduction on the lowest resistance range. If that's true, then clip the red lead on the center pin and test for diode-style conduction on both the two outer pins. Then put the black lead on the center pin and verify no conduction to the two outer pins. If both those are true, the pinout's reasonably verified. Are Q11-13 the only 2N3904s?

The volume is pretty close to bypassed volume, a bit quieter though.  That is expected with a Neovibe, since the original Univibe wasn't true bypass.

Yeah, that's what I expected. I think your audio path is fine, maybe a bit of problem with the mixing at the end.

The LDRs have a film canister enclosure, but when I talk about the sound of vibrato, I take the canister off and point a light and use my hand to vary the amount of light.  So the bulb isn't providing any modulation, only my hand.

And this is yet more info suggesting that the problem is just the LFO.

I do not have the voltage limiter in, just a jumper.  Do you think I should install it?

Not now. You're running at an OK voltage for the audio path. It will run OK without the regulator if the DC level is under about 20V.

The voltages:
R39/R40/R41 = 10.59
R5 and R7 = 15.50

Not too bad.

Q12
C = 18.20
B = 9.95
E = just drops when I place probe on it

Both D1 and D2 have voltage falling, much like the emitter of Q12.

I think that this is a symptom of whatever is wrong. That voltage at the emitter of Q12 should be quite firm, not dropping when the probe of the voltmeter touches it. I believe there's a connection, pinout, or soldering problem with Q11/Q12 or the diode/capacitor setup. I can't call it any closer from the info here, It's a small enough section of circuit that you could take an ohmmeter and verify the whole mess in a few minutes, though.