YANDP (yet another Neovibe Debug Post)

[dxm1]

Howdy,

I'm having trouble solving a problem in the preamp stage of the Neovibe.

I'm using the board from General Guitar Gadgets. The schematic is here:

http://www.generalguitargadgets.com/diagrams/neovibe.pdf

I built the board as per the BOM, using 2N5210 transistors for everything
but the bulb driver, for which I used a 3904. The oscillator seems to be
working properly.

The problem is with the output - it is about half of the bypassed volume,
and heavily distorted. Voltages on Q2 and Q3 are screwy...

Q1
C=5.3
B=2.5
E=2.0

Q2
C=8.3
B=5.3
E=8.2   !!!

Q3
C=7.7  !!!
B=8.3
E=7.6

This is with a 15.1 volt V+ rail, and a 1 volt drop across R5.

I've verified all component values in this circuit, and double checked
solder connections (reflowing any that looked questionable). I replaced
both Q2 and Q3 to no effect.

I also replaced C1, C2, and C18, substituting non polarized 1uF units. None
of these changes have had any effect.

On the bench, I'm feeding a 1V sine wave from a signal generator, but the
sound is the same with a guitar plugged in.

Any ideas? Could C4 be causing this kind of problem (I didn't want to start
shotgunning components...)

Thanks!

[boogietube]

Not sure if GGG has the updated schem and layout, but www.geofex.com does. Don't know if that has anything to do with your problem though.Hope this helps.

[R.G.]

It's clear that Q2 and Q3 are not amplifying in any normal way, so it's not surprising that it would be low output and very distorted.

I think that there is something wrong with Q3. It's possible that either the emitter of Q3 is not being pulled down by its emitter resistor, or its collector is not being pulled up.

Just to clarify things, can you read and report the voltages on each side of resistors R4 through R11 and both sides of C6 and C18? I think that will clear it up.

[dxm1]

Thanks RG,

Just to clarify things, can you read and report the voltages on each side of resistors R4 through R11 and both sides of C6 and C18? I think that will clear it up.

Sager words have not been spoken...

While probing the ground side of R6, I noticed some movement of the lead. Looking
at the solder side, there was a nice MIL-2000 solder peak floating about 1/16 inch
away from the ground bus   

Nice pad, nice bus, but no trace between the two. In the past GGG boards have
been OK. However, on this one, I've had to drill two missing holes - and now this.

I don't know why this did not turn up when I was inspecting solder connections, during
the orange stick "poke and listen" test, or when I measured the values of all components
in this section. Missing it once I can see - but three times   

So, now I have full volume Vibe action. I just need to figure out why Chorus mode
attenuates all of the high frequencies. Back to the schematic...

Thanks for the help!

[R.G.]

My pleasure. You're well on your way.