Nurse Quacky questions

[bal704]

I'm relatively new to effects building, and I'm working on my second project, the nurse quacky. 

http://www.home-wrecker.com/nurse-quacky.html

Originally, I misoriented all the polarized parts (3 caps, 2 LED's).  This led to a great clean sound when the 3PDT switch was in one position, and no sound at all when in the other.  I fixed the polarized part problem (with all new parts), and now I get clean sound out of both 3PDT positions (i.e. no auto-wah).  In my first project, the fuzz face, if I had no power applied, I still got the clean sound in one switch position, but no sound in the other.  This is what I would expect out of a dead battery situation, and I expected that on this project as well.  I have the 3 pots in middle positions, and rotating the pots doesn't seem to have an effect.

When I do insert the battery, the LED attached to the 51ohm resistor flashes and goes off.  The LED attached to the 10k resistor comes on and stays on. I'm using 2 green LED's out of the same box.  When I switch the 3pdt, the LED attached to it comes on and stays on in one position, stays off in the other.

now the questions:

1) Should the 2 LED's on the PCB come on and stay on?  If not, what's the fix?
2) Should I expect no sound at all in one switch position if no power is applied? (i.e. a dead battery situation)
3) Any obvious problems I should look for?

Thanks in advance for any help

[Mark Hammer]

I'm relatively new to effects building, and I'm working on my second project, the nurse quacky. 
http://www.home-wrecker.com/nurse-quacky.html
Decent choice of project for #2.

Originally, I misoriented all the polarized parts (3 caps, 2 LED's).  This led to a great clean sound when the 3PDT switch was in one position, and no sound at all when in the other.  I fixed the polarized part problem (with all new parts), and now I get clean sound out of both 3PDT positions (i.e. no auto-wah).  In my first project, the fuzz face, if I had no power applied, I still got the clean sound in one switch position, but no sound in the other.  This is what I would expect out of a dead battery situation, and I expected that on this project as well.  I have the 3 pots in middle positions, and rotating the pots doesn't seem to have an effect.

When I do insert the battery, the LED attached to the 51ohm resistor flashes and goes off.  Normal  The LED attached to the 10k resistor comes on and stays on. Also normal. I'm using 2 green LED's out of the same box.  These should really be red.  Red and green LEDs have different voltage drops so the circuit will behave differently.  When I switch the 3pdt, the LED attached to it comes on and stays on in one position, stays off in the other. Wired correctly.

now the questions:

1) Should the 2 LED's on the PCB come on and stay on?  If not, what's the fix?  One should stay on and the other should light up when you strum.
2) Should I expect no sound at all in one switch position if no power is applied? (i.e. a dead battery situation)  Yes
3) Any obvious problems I should look for?

Thanks in advance for any help

A perennial problem is what I affectionately call "undersignalitis": the insufficient signal level to drive the envelope follower into providing an audible sweep.  That can be remedied by either providing a bit of boost ahead of the pedal, or by increasing the gain in the envelope follower.  In the schematic shown, that is already taken care of via the replacement of the usual 2.2M resistor with a 3.3M resistor.

[bal704]

If the change in resistor value from 2.2M to 3.3M was to fix the 'undersignalitis' problem, I apparently still have it.  Is there anything I can check on the board to confirm this is the issue, and if it is, how else can I fix it?

thanks

[Mark Hammer]

Plug in, strum, and measure the AC voltage at the wiper of the sensitivity pot, after the 47k resistor, the .01uf cap, the output of the op-amp (i.e., the junction of the 3M3, LED and output pin).  Then measure after the second LED (you should have something about 1.5V lower than what was coming from the op-amp output), and again at the base of the transistor.  If you can get at least 50mv at all of those points (and several times more at some of them), then you know that your envelope follower is doing its job.  If the envelope follower is working fine, then the culprit is clearly the filter section or in/out switching. 

It is also possible to vary the filter frequency with the trimpot even when the sensitivity is set to minimum.  If varying the trimpot under those circumstances produces a wah-like sweep, then you know the filter section is working fine.  If you get signal coming through but no change in frequency with trimpot tweaking, then it is probably time to verify the transistor pinouts.

[bal704]

I get no voltage drop (AC or DC) across the 3M3, 0.01uf  or 47k.  On the 3M3, I get a voltage of about 1.7VDC with respect to ground on both legs (i.e. no drop).  I see no voltage drop across the sensitivy pot either.  On the 10k LED, I get about 1.8VDC drop across the legs (and the green light is on), on the 51ohm LED, I get 1.6VDC drop across the legs (and the green light isn't on).  I'll try changing the LED's to RED to see if that changes anything.

What do you make of the fact I get sound out of either 3PDT switch position, with or without power?  Does that indicate that I'm bypassing the effect altogether? 

[Mark Hammer]

You're getting (and getting me) confused.  The diodes provide a voltage drop, but what we are looking for is evidence of an AC voltage showing up in the places where we expect to see it.  The sensitivity pot varies the amplitude of the audio input signal going to the envelope follower, so you should see an AC voltage at the wiper of something in the 30-100mv range with the pot turned up full, depending on your pickups and how hard you strum.

The LED on the input of the chip provides for a fixed DC offset aded to that audio input, which is then subtracted by the 2nd LED.  What gets subtracted by the 2nd LED is not AC or DC specific.  It's just something like 1.5v less than what you observed at the output of the op-amp whether you were feeding an audio input of not.  Think of it like chopping a foot and a half of the stilts someone is walking on.  You may not know how high up their head is (human height plus stilt height), but whatever they were, it's now 1.5ft shorter.  It should be possible to detect AC output from the op-amp and at each of several points after it, with the AC voltage corresponding to expected levels.  We "expect" that the 2nd LED will chop off half the waveform, plus an additional 1.5V.

[oldrocker]

This only being your second build I would bet that there is a mis-wired circuit here.  I just built this last week.  It isn't too difficult to build but I would still consider this an intermediate project.  Did you perf or vero board this?  I would assume you did and in that case without many builds under your belt it's easy to make a mistake.  It's good to check voltages but even if you find a voltage problem you'll still be trying to figure out how to fix it.  I know it's a pain but you need to go over your build many times and study this circuit and schem until something clicks or jumps out at you.  Many times I thought I had all the components connected in a circuit and after closer examination I'll see where I missed connecting something like grounding a pin or forgetting to bring plus voltage to an IC.  In my Green Ringer I had my trannys in backwards for weeks before I realized it.  So it's so easy to forget or miss something simple.  And remember cold solder joints can be your worst enemy.

[bal704]

Good news, sort of.  I changed the LED's on the board to RED.  Now the 10k LED stays on, and the 51ohm LED flickers when I strum, the intensity varying with how hard I strum.  Also, the 3PDT now has no output when I switch the effect into the circuit (yes, this is an improvment).  I measured the 100k sensitivity pot, with one probe on the middle  post, and the other on the right post and I get about 30mv AC when I strum it.  The LED that stays on measures about 1.7V across it, and the one that flickers shows about 1.1 V when I strum it.

Also, I measured the transistor output.  From B-C and B-E I see about 5mv.  Is there another way to verify the transistor is working, and if it's hosed, could it be causing a no output problem?

Oldrocker
I'm using a PCB purchased from JD Sleep.  I've quadruple checked this board, but obviously, that doesn't mean I haven't missed something 4 times.  Check my original post with the caps.

[Mark Hammer]

OK, I think we're getting somewhere.  All voltages reported prior to the transistor are well within expected values.  I realize the change of colour may not seem relevant but in this case it is, since different colours depend on different internal electro-chemistry and result in diferent voltage drops.

As for the transistor, you're not actually using it in the normal fashion.  Instead you are using it as if it were a voltage-controlled resistor.  Indeed, if you completely eliminated everything on the bottom half of the schematic and replaced the transistor with a pot to ground, you could control the filter like a wah.  All of which begs the question: Are we certain the transistor is oriented appropriately and of a suitable type?

[bal704]

I'm using a 2N3904 transistor (called for on the parts list).  The diagram JD provided had the curved edge of the transistor toward the lower edge of the pcb.  What I found online shows the 3904 should be CBE (left to right), with the E going to the 220ohm resistor.  So I think it's correct. 

Also, I'm going to double check my connections.  Now that it's behaving somewhat normally, I'll make sure the pots etc have good connections.

[Mark Hammer]

The transistor is correct.  If you are looking at the "front" of the transistor where the printing is, flat side facing you, pins pointing down, the pins are (from left to right) E - B - C.

[bal704]

Great news.  I am now getting wah-wah like sound out of my speakers.  I've wired everything with alligator clips so far, and squeezing them into the 3x3 grid of the 3pdt is tough. I think my problems now lie with the kluged up temporary wiring I'm using.  I'll fix that and go from there.

Thanks for the help!

[oldrocker]

Great now that you have that working you should think about building a Dr. Quack down the road.  I've built both of them and I like Nurse Q's simplicity.  The Dr. Quack goes further with a bass switch and more sensitativity with some extreme attack settings.  Give it a try after you run out of things to build.

[bal704]

Does the Dr. Quack bass switch allow it to work with bass guitar, or just accentuate the bass on a regular 6 string?

[Mark Hammer]

It is neither.  It reconfigures the filter from one topology to another.  The result is that it sweeps in a different way.  The "normal" sweep doesn't sweep quite so high, nor linger up high quite so long.  The Bass mode has a more "normal" sweep; it goes up and comes down.  There is no change in the actual filter range, only in how it moves through the filter range.  The Nurse only uses the Bass mode for a more reasonable sweep pattern.