Mad Professor Snow White Fail

[nickbungus]

Hi Guys

Although I visit every day, its been a long time since I posted anything.  I suppose that's because all my builds have been working or I have debugged them myself.

Anyway, I've just built the Mad Professor Snow White Autowah and its not working.  I'm not at the posting voltages stage at the moment as I have a question about a substitution I made.  C11 and C9 are both 1uf polarised caps, I only had non-polarised ones.  Would this make any difference in this circuit?

Here's the  Schematic and PCB layout I used.

[Mark Hammer]

Near as I can tell, in those particular locations in the circuit, no.  Non-polarized should work fine.

[M.A.P]

Hi Nick

I agree with Mark. The two caps are used to block DC voltage and let audio signals pass. Is the whole circuit not working or doesn't it sound like it's supposed to?

Greetings from Germany
Marcus

[nickbungus]

I'm getting a bassy hum which I can alter the pitch of with the pots.  Sounds like a Rage Against the a Machines solo.  This is without me playing anything on the guitar. 

Sounds like dc in the signal to me, hence why I was suspecting the caps, although my knowledge is poor.

[Kipper4]

Have you tried probing it? Except for A1c A1b Through Q1 Q2 upto pin 1 and 16 which will be DC (rectifier)
A2a and A2b form the filter. The bit that goes wah.
Maybe post some voltages too.
You know the drill. What to do when it doesn't work. Debugging.

[M.A.P]

You cannot hear DC voltage/current because the human ear can only hear changing signals with a frequency   But enough of being a smartass.

If you have some other polarised caps you could solder them in instead the caps you are using atm. Is the FET soldered in correctly? The diodes and transistors, too? Are the ICs noses facing in the right direction? Do you power it with battery or wall wart? I once had a Fuzz that only amplified the deep strings because my battery was low
Is there an electrical device that might throw that hum in your circuit like a transfromer for example?

[nickbungus]

Thanks Rich and Marcus.  Yes Rich, I know the drill, I'm going to try to figure it out myself first,  I was just curious about the caps and polarity.

I've got my probing kit out.

[Kipper4]

No worries.
As Mark and MAP said those non polarised caps should be ok.
Good luck.
Sometimes it's just good to write it in a thread.

[nickbungus]

I've had a good probe about and expect voltages!!  I'm shattered now so going to beddiebyes so I'll get on it again tomorrow night!

Just one thing to add if it gives anyone any clues, following the probe around, I get and very good signal at pin 8 on the LM324, a good signal at pin 7, but its really faint at the base of Q2 and then nothing really audible anywhere else at Q1 and Q2.

[M.A.P]

Maybe you picked a wrong value for R13 accidently? Have you tested D6? Is it built in right and not broken? Q1 and Q2 form a Darlington circuit so there should be something to measure except the signal doesn't get to the base of Q2. It's connected to ground over R27+P3 and C8 so it's not amplifying but maybe has a low leakage which is amplified by Q1 and that's causing the hum you're hearing. Some kind of white noise.

I would bet any money that D6 is causing the malfunction

[anotherjim]

By Pin 7 of the 324, it should be losing audible signal. Amps C & B form a full wave rectifier. Q2 base will have the DC value of your inputs envelope on it. 0v when silent and rising when you strike a chord - I guess it should just about manage about 4v with strong playing & Decay pot maximum resistance.
I suppose it's what it is, but I don't like the way the Decay works. With Decay pot minimum resistance, for sure it will decay fast, but it will also limit how high the envelope sweep gets from the initial note attack too.

The rectifier is basically this old databook scheme...

Explained here...
http://www.freecircuits.net/circuit-61.html

[nickbungus]

Thanks as always Jim.  I'll have a read.

Here are my voltages with no input signal.
Dc from Battery: 9.26v

LM13700
----------
1   1.2
2   0
3   4.1
4   4.1
5   4.5
6   0
7   4.5
8   3.8
9   4.4
10  5.6
11  8.1
12  5.6
13  4.1
14  4.1
15  0
16  1.2

LM324N
---------
1   4.1
2   4.1
3   4.1
4   8.1
5   4.1
6   4.1
7   0.166
8   4.1
9   4.1
10  4.1
11  0
12  4.1
13  4.1
14  4.1

7805
------
In  8.1
Gnd 0
Out 5.1

Q1 - 2n5457
--------------
D   3.6*
S   7.2
G   8.1*
*  Modified (I had the D and G the wrong way round)

Q2 - BC550C
--------------
E   .001
B   .001
C   5

Q3 - BC550C
--------------
E   1.2
B   0
C   5

[PRR]

Your transistor numbers don't seem to match the schematic in your first post.

The JFET with S up against D does not make sense. Suspect missing connection, short, mis-identified FET legs.

Overall, while this "should" work with 8.1V, I would like to see numbers for 9.0-9.4V supply. Basically all the "4.1" should change to 4.5V, but it would be interesting if any voltage changed a LOT.

[nickbungus]

Hi Paul

Your transistor numbers don't seem to match the schematic in your first post.

Yeah, they differ from the schematic to the PCB layout.  Sorry for the confusion. 

The JFET with S up against D does not make sense. Suspect missing connection, short, mis-identified FET legs.

Whoops, wrong pinout
D  3.6
S  7.2
G  8.1

Overall, while this "should" work with 8.1V, I would like to see numbers for 9.0-9.4V supply. Basically all the "4.1" should change to 4.5V, but it would be interesting if any voltage changed a LOT.

I've got 9.1v from the battery, after the 4007 Diode(D4) its down to 8.4 and then after the 47ohm resistor (R20) I've got just over 8v

[Cozybuilder]

Nick- Use a 1N5817 diode instead of the 1N4007 for D4 and you'll get about ½ volt back.

[nickbungus]

Ok, but I'll have to order some in,  I've got some 4004s too

[PRR]

> Whoops, wrong pinout
> D  3.6
> S  7.2
> G  8.1

Still don't make sense.

Drain goes to +9V (whatever that is).

Gate goes to V/2 through a resistor. It should read about 10% down (meter loading) from V/2 voltage.

Source should be roughly a Volt higher than Gate.

FWIW: you can actually build it with D and S reversed. JFET is basically symmetrical. But the Gate has to be the Gate.

If you have a Diode Checker, poke the JFET 3 legs all 6 ways. The lead which reads "good diode" to both other legs is the Gate.

[nickbungus]

I'm going to make this again!  I've tested all my connections, looked for bridges etc.  I know I've messed up somewhere but the time I've taken to debug this is less than the build time, so its the obvious conclusion.

Also, if I get the 2nd build working, then I could probably use it to debug V1.

I'll report back. 

[Elijah-Baley]

Sorry to hear that, but good luck for your new building.
My Rat on veroboard just built didn't work. Then I seen the output cap in a wrong position.

I have to say my Snow White seems to have the decay pot completley useless, with no effect. Weird... same thing with a DOD FX25 clone I was building early.

[nickbungus]

So the failure continues....

I decided to rebuild and decided to use a different layout.  I'm not a fan of the flood fill on pcbs so I tried this one:



I have audio output but its just high-pitched awful noise.  This is what it sounds like (sorry there is a lot of bleed from my actual guitar and I only had a short opportunity to record the vid).

Here are the voltages  (I've put them on the layout too if you have keen eyesight):
DC=9.04v

LM324
1      4.07
2      4.08
3      4.05
4      8.12
5      4.06
6      4.06
7      0.02
8      3.99
9      4.07
10    4.06
11    0
12    4.07
13    4.07
14    4.07

LM13700
1       1.2
2       0.01
3       4.04
4       4.05
5       5.2
6       0
7       5.2
8       4.04
9       4.04
10     5.2
11     8.07
12     5.23
13     4.04
14     4.03
15     0.1
16     1.2

Q1 - BC550C
C      5.02
B      0
E      50mv

Q2 - BC550C
C      5.02
B      1.3
E      1.2

Q3 - 2n5457
D     8.1 
S     7.2
G     3.6

5v Voltage Regulator
I      8.1
G     0
I      5.02

How annoying, this one seems to be my nemesis.  I've re-checked all the components (checking the cap and resistor codes), everything is correct.  Where possible I have checked that all components are connected correctly using the continuity feature of my multi-meter (some of the electrocaps dont have their legs exposed so I couldn't double check these).

I'm all out of ideas, please help?