Dr. Quack problem

[warmachine]

Hi!
The schematic and layout are from GGG (R.G. Keen). I get normal signal, the bass switch works, and both LEDs light up: one is on all the time and the other lights up faintly when I strum hard.
The problem is that I get nasty distortion instead of quacking.. it's not even distortion it's just noise like when you're scratching the cable against the guitar jack.
The parts are all in the correct spot and I've thoroughly checked that everything is connected and there are no cold solders.

The different parts I've used are:
- .047 instead of .05 caps
- .0047 instead of .005 caps
- f245b JFET
- 1N4148 instead of 1N914

pin voltages on the op-amp:
1 - 1.43
2 - 1.29
3 - 1.6
4 - 0
5 - 2.77
6 - 2.75
7 - 2.75
8 - 8.87

My bet is that one of the semi conductors got burned out when soldering. Any suggestions before I order new parts?

[peterg]

The readings on mine are:
1 1.60
2 1.60
3 1.81
4 0
5 4.62
6 4.73
7 4.73
8 9.27

Your numbers aren't far off but 5,6 and 7 seem low.

I used an NTE 451 JFET and a TL072CP op amp.

Not sure if that helps but given my lack of circuit understanding its all I cam do!

[mistahead]

So you are JUST getting noise (as described) or instead of proper quack you are getting that sort of noise OVER/WITH signal?

[Mark Hammer]

jack Orman's JFET input buffer certainly helps to achieve a better tone, but the original Dr. Q "didn't need no steenking JFET" to actuallt work.  Try feeding your input signal to the circuit after the 10uf cap following the JFET.  If you get sweep, then you know the problem lies with the JFET input stage.

Incidentally, you're sure you didn't bend one of the op-amp pins underneath when pushing it into a socket, right?

[warmachine]

I'm terribly sorry I didn't have access to the internet for a couple of days so I didn't see the replies.

That does look off, thanks!

So you are JUST getting noise (as described) or instead of proper quack you are getting that sort of noise OVER/WITH signal?

No, the signal is clean when I strum lightly and I get this noise whithout signal when I strum harder - when it should quack.

jack Orman's JFET input buffer certainly helps to achieve a better tone, but the original Dr. Q "didn't need no steenking JFET" to actuallt work.  Try feeding your input signal to the circuit after the 10uf cap following the JFET.  If you get sweep, then you know the problem lies with the JFET input stage.

Incidentally, you're sure you didn't bend one of the op-amp pins underneath when pushing it into a socket, right?

I'll try it , thanks.

And I didn't bend any pins I checked a million times.

[WannabeGeek]

My Dr. Quack is still on the breadboard, but I'm having almost the identical problem.
I'm using a 4558 op amp.  And I have tried a few of them just to see if that might be the problem.  My voltage readings are:
1.  1.69v
2.  1.75v
3.  1.75v
4.  0
5.  4.45v
6.  4.46v
7.  4.45v
8.  8.73v

I also had to substitute all the .05 and .005 caps with .047 and .0047uf.  I'm using a 2n5457 for Q1, a 2n3904 for Q2 and a 1n4148 instead of a 1n904.
I've tried inputting after the 10uf cap behind Q1 but there's no change.  No matter where I turn the pots, I'm getting a DJ scratching sound where the quack should be.  There's also almost no attack.  If I strum or pick real hard, I get a scratch, but if I play quietly, no signal makes it to the amp.  The LED that connects directly to ground is always lit.  The other one flickers when I strum.  I'm thinking about bypassing the Bass/treble switch, but I'm not sure what the best connection would be for a test.  Any suggestions?

[Transmogrifox]

pin voltages on the op-amp:
...
5 - 2.77
6 - 2.75
7 - 2.75
...

Something is definitely wrong here.  Make sure your 47k resistors are actually 47k.  Maybe for troubleshooting take off the 10 uF capacitor to ensure it doesn't have an excessive leakage current.

Another thing you can do is remove the op amp and make sure the 47k divider measures 4 to 4.5 volts.  If it measures correctly when you take out the op amp, then the op amp is bad.

I don't foresee much trouble around the JFET unless the pinout was incorrectly applied (check the datasheet to make sure the same pinout).  As long as the JFET source voltage is around 4 to 7 volts and the gate is ~1 to 2 Volts lower than the source, you should be ok there.

Another resistor to verify is the series 47 ohm to op amp pin 8.  This could really ruin your day if it's several hundred ohms or more (for example mistaken 470 ohm)

[duck_arse]

photos! you get to show us photos!

check yr transistor orientation. check the range pot connections, resistance, setting. can you measure the V at the junction of the diode and range pot, no signal and then when you give a thrash? and the voltages on the transistor, no sig and thrash, please.