Mad Professor Snow White Fail

[anotherjim]

Stick at it lad!

Q3 still isn't happy. Source voltage should be closer to the gate voltage. Check resistance from source to power ground wire - should be 10k (with power off!). +Try other N-channel j-fets, J201 etc. Since it's not an amplifying stage but just a buffer and normally only passing low level guitar signal, it isn't tragic if the source isn't at exactly Vcc/2, but I'd like to see it somewhere between 3 & 6v. I'd get that going so your have clean signal leaving Q3 source.

After that, isolate the envelope control by lifting R31 (6k8). The bias pot alone should work as a manual filter sweep, so you can concentrate on getting that part right before worrying about the envelope control.

[nickbungus]

Thanks Jim as always.

I've still got hundreds of n channel jfets from the fOXX phaser!

Will try it all and report back

[nickbungus]

Ok, I didn't get the opportunity to do a sound test but here's what I've found so far.

Resistance from source to ground was a tad over 9k.

I tried some JFETs and heres the results

                    2n5485             J201         J112           J113            2N4302        2n5457
D                        8                 7.95         7.93           8.02               7.96              8.1
S                       5.8                4.8           7.6             5.4                4.72              7.2
G                       3.6                3.6           3.5             3.6                3.58              3.6

Which one do you suggest would be the best to try?

[anotherjim]

J201

[Cozybuilder]

With the source voltage lower than the drain in each case, I wonder how any of these would work?

[Cozybuilder]

With the gate voltage lower than the source in each case, I wonder how any of these would work?

ERRR, ummmmm-     thats gate lower than source- need some coffee!!!

[nickbungus]

I haven't sourced a set of working voltages but there's quite a few voltages posted here of the non-working efforts.  Looks like I'm not the only one to have issues with this. 

[duck_arse]

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:


.....
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

pin 7 shows 0V02. how come? there is a point under pin 7 connecting to pin 4 [which is at 8V12] and R20. does the under-point really read 0V?

[Cozybuilder]

I just spent awhile verifying the traces on the PCB to the schematic, and it looks like the PCB follows the schematic except for the following labelling differences:
1) All 4 pots are labeled backwards according to the convention we use
2) Transistors Q1 and Q2 are labeled Q2 and Q1

Assuming the schematic is accurate:
The voltages on the base and emitter of Q1 and Q2 are just weird, and the Q3 likewise is way off on the gate and source, and the chip voltages are very odd: First thing I'd check are correct values of components. (e.g.: R5 should be 10K, if it is a much higher value, or open, that could be a source of the observed values of Q3).

Can you post clear photos front and back of your build? There may be something one of us can spot and hopefully help you resolve this.

[Ice-9]

The PCB two posts up drawn by Dimebag is verified and works just fine.

Why do you have 8.1 volts for the power rail when it should be closer to 9v with a psu or fresh battery ?

[anotherjim]

With the gate voltage lower than the source in each case, I wonder how any of these would work?

ERRR, ummmmm-     thats gate lower than source- need some coffee!!!

Vgs is negative isn't it - so higher source is right. After all, these can work with a single ground bias gate resistor.

Yes, the FET is wrongly drawn as a P-channel - it really is an N as shown in this alternative version...


Power comes via a Si series protection diode, it's about right as it is.

[anotherjim]

Addressing LM324 pin7 &  Q1&Q2 voltages.

It looks like there is a track break between Q2 emitter and Q1 base - they should have the same voltage.

My take on the operation of the envelope part...

Assuming no signal, the 324 has a pull-up to +V via R19 on its pin6 -ve input, which drives the output pin7 to ground unless there is signal which will produce a lower voltage from the rectifier to over-ride the pull-up. D6 should be off and Q2 base should be 0v. Nick has voltage there, which appears to be due to a fault.  Q2 should be off and so should Q1. Both collectors will be at the 5v regulators output voltage. While off, Q1 emitter is whatever voltage happens to leak back via R31 from the bias control setting.
There will need to be a signal present into the envelope circuit before you can fully test Q1 & Q2 operation.

[nickbungus]

Wow - I've been away for the weekend, come back and there's loads to digest.

Thanks guys, the help on here is amazing.

I'll get back on it tonight after work.  Looks like the first thing I need to do after reading everything again is to is double check my voltages.

[nickbungus]

I just spent awhile verifying the traces on the PCB to the schematic, and it looks like the PCB follows the schematic except for the following labelling differences:
1) All 4 pots are labeled backwards according to the convention we use
2) Transistors Q1 and Q2 are labeled Q2 and Q1

Can you post clear photos front and back of your build? There may be something one of us can spot and hopefully help you resolve this.

I'll take some pics tonight.  Thanks for verifying the pcb and schematic.

I hope no-one thought I was insinuating there were issues with any of the pcb layouts.  I'm sure what ever the fault is with my board, its definitely my error.

[Yazoo]

I've built a Snow White which is working. Let me know if you would like the voltages posting and if you want the pots set in a particular way.

[nickbungus]

Hi Yazoo

The working voltages would be an amazing help.  I've been setting my pots to an approx 12 o'clock setting (but it is an approximation on feel of the pots).

Thank you.

[Yazoo]

Here are the voltages using a 9 volt power source. I used the pcb layout available via the Revolution Deux site, created by DimebuGG:
http://revolutiondeux.blogspot.co.uk/2012/07/mad-professor-snow-white-auto-wah.html

I used MPSA18 transistors for the two CBE transistors. I originally tried BC550s but I didn't get enough wah. I must have found this suggestion about the MPSA18s somewhere but I can't remember where.

LM3234
1:   4.04
2:   4.04
3:   4.02
4:   8.1
5:   4.04
6:   4.03
7:   0.22
8:   3.96
9:   4.04
10: 4.04
11: 0
12: 4.04
13: 4.04
14: 4.04

LM13700
1:   1.2
2:   0.44
3:   4.04
4:   4.04
5:   5.2
6:   0
7:   5.21
8:   4.03
9:   4.05
10:  5.23
11:  8.1
12:  5.23
13:  4.04
14:  4.04
15:  0.4
16:  1.2

2N5457
D: 8.1
S: 5
G: 3.5

MPSA18 (Q1)
C: 5.0
B: 0.6
E: 1.2

MPSA18 (Q2)
C: 5.0
B: 180 mV
E: 0.48

I hope this helps.

[nickbungus]

Looks like a made a couple of errors with my voltage readings.  Because there's a lot of resistors and diodes standing up, I think I was touching other components.   Lesson learnt here, I think next time, I'll measure them and then come back and measure again.

Heres the voltages of Yazoo (left), compared to mine (right).

LM324   LM324
1:   4.04   3.99
2:   4.04   4.00
3:   4.02   4.00
4:   8.1   8.01
5:   4.04   4.02
6:   4.03   3.99
7:   0.22   0.60
8:   3.96   4.00
9:   4.04   4.03
10: 4.04   4.04
11: 0   0.00
12: 4.04   4.03
13: 4.04   4.03
14: 4.04   4.03
   
LM13700   LM13700
1:   1.2   1.20
2:   0.44   64mv
3:   4.04   3.97
4:   4.04   4.01
5:   5.2   5.20
6:   0   0.00
7:   5.21   5.39
8:   4.03   4.15
9:   4.05   4.17
10:  5.23   5.38
11:  8.1   8.17
12:  5.23   5.19
13:  4.04   3.99
14:  4.04   3.97
15:  0.4   49mv
16:  1.2   1.20
   
2N5457   J201
D: 8.1   7.96
S: 5           4.74
G: 3.5   3.56
   
MPSA18 (Q1)   BC550C
C: 5.0              5.05
B: 0.6              362mv
E: 1.2              266mv
   
MPSA18 (Q2)   BC550C
C: 5.0             5.05
B: 180 mV     215mv
E: 0.48              1.21

And heres the top and bottom shots.  The pink oval shows where I nearly have a bridge although I tested this thoroughly and its ok.

[anotherjim]

So far I can't see much wrong with the new readings. There is still a difference between Q2 emitter and Q1 base. They are joined by a short dog-leg track. Is it that the voltages are wobbling about between readings or is there a hairline crack in that short track? Turn the sensitivity pot down so noise isn't wobbling the envelope.
There is a difference with your readings & Yazoo for the 13700 pins 2 & 15, which are unused. Hmmm... maybe just variation between chips?

[nickbungus]

I have autowah and the high squealing has gone (ish).

Jim, as predicted, once I turned the sensitivity down, I was getting closer matches between Q2 emitter and Q1 base.

Now for the weird bits:

Firstly, the only thing that has changed since I last sound tested it was to swap the BC550Cs so I had longer legs in the sockets for easier voltage readings.  The previous trannies had had their legs trimmed which was making it difficult to get the probes in, which may have lead to inaccurate readings.

Secondly, the bias pot/wiring seems to have a fault, and I can kill the signal or bring it back by giving it a slight twist.

Thirdly, I could get it squealing again by adjusting the Resonance.

Lastly, when I had it working well, the wahability would seem to gradually fade, but if I touched the Sensitivity, it would immediately come back to life, then begin to fade again.

So my plan of attack is to take off all the pots, remove all solder and wire.  Clean, and then re-wire and solder.  The pad for pin 1 on the bias is so close to the ground I scratched a groove to be sure of no bridge but I may just lift R26 and wire pin 1 directly.

I've had it a few times on builds where my pots can cut out and come back to life by just movement.  Once or twice I've replaced the pot and the problem goes away.  Is this to do with the pot it self or is it the wire breaking in its sleeve?  I like to use single core exclusively.