Stompbox build failure

[voskarp]

I've built a ZenDrive clone from a kit and there is something wrong with it (it's the 9:th build, and first one to malfunction ). The Volume goes from nothing to about unity and the Gain does nothing, so I guess there's something wrong with the gain part of the circuit.



I've tested the diodes and transistors with my multimeter (diode test setting), not removed from the circuit. Diodes shows 419-422 (ohms?) and transistors S+ to D- 687/686, S- to D+ 1291/1140 (S to G the same), G to D 0 (beep).

Are both transistors blown (static shock?), or have I toasted them (in previous pedals I have managed to solder 7 germanium transistors without destroying them, using same method with heatsink) or is it something else going on?

[GGBB]

the Gain does nothing

Have you checked the gain pot itself? That's the first place I'd look.

[voskarp]

Yes, measured from 1 to 2 & 3 0-0.45 M ohm, about as it should.

[ElectricDruid]

Bad soldering on R3 or the Voice pot, perhaps? If there's a broken connection in that string of components to "Va", then that op-amp is basically an unity-gain buffer built with far too many parts

That was my first thought. There are other possibilities, I'm sure.

Tom

[voskarp]

OK, I'll look for that.

The Voice pot changes the sound though, but I don't know if it is like it's supposed to be.

[voskarp]

Here are some voltage measurements.

[voskarp]

I get 0v at 1, 2 & 3 on the Voice pot.

At R3 it's 4.78v.

[ElectricDruid]

Deeply suspicious!

Sounds like you're getting warm to me.

T.

[voskarp]

I can't see any bad solder joints and the Voice pot measures (in circuit):

1 to 2 0 Ohm
1 & 2 to 3 0-9.7 kOhm

The strange thing is that on the VA side of C3 I have 4.88 v but on the Voice pot side (and on all pins of the pot) there's 0 v, so 0 v on Voice pot pin 3 BUT 4.77 v on both sides of R3!

Broken strands and short circuit to GND at the Voice pot?

[voskarp]

[Kipper4]

OK, I'll look for that.

The Voice pot changes the sound though, but I don't know if it is like it's supposed to be.

Yep that's the idea. It's an RC filter to tune the distorted signal.

It's a wierd one.
Did you swap the tone and gain pots by mistake?
Are the IC1B pins all around 4v5?

[voskarp]

Gain 500k and Voice 10k.

IC:
1. 4.8 v
2. 4.78 v
3. 4.58 v
4. 0 v
5. 4.76 v
6. 4.78 v
7. 4.79 v
8. 9.8 v

[ElectricDruid]

There's no way you've got 4.77V at one end of R3 and 0V at the same end of the Voice pot! They're connected together!

Unless they're not? Is that a clue?

Tom

[voskarp]

 Yes!

But I really can’t see that the solder joints are bad.

Could just resolder anyway, no big risk of toasting resistors I guess...

Otherwise it must be the PCB track that is broken, or..?

[antonis]

<The Volume goes from nothing to about unity and the Gain does nothing>
That shows a unity gain non-inverting buffer on IC1A - something in the R3-Voice-C3-VA order is "open"..

Try to disconnect C3 upper leg from VA and connect it to any convenient GND..
(although I agree with you and Tom for a broken connection between R3 & VOICE pot so just place a wire jumper between them.. )

[voskarp]

Thanks for the tips, I'll try that.

[voskarp]

The jumper wire between R3 and Voice pot worked, now I'm off to play some guitar... 

Thanks everybody!

[ElectricDruid]

Excellent! Glad to hear you got it sorted!

T.