Great Cheddar = Great Problems - need debugging help please

[hendrix2489]

Less than a year ago i started building a pedal and with the help of this forum i completed that project (ts808) and is still on my board today.  Since then i have built a good 25 pedals, and in that period i have only had trouble  with one pedal, a small stone which was an easy fix(copper traces were too close). 

I really hope this is just another stupid mistake.  I started building this distortion pedal                                       ( http://www.generalguitargadgets.com/diagrams/geo_great_cheddar_v6.pdf) . and when i finished it did not work . 

I get bypass but no effect, i have wired exactly like pdf.  using dpdt, no led. 

What is the control line for on the stomp switch?

It also might be how i wired my rotary, some help would be much appreciated.

here are my voltages:

                          fyi ( i am using an analog multi meter so 4.5 might actually be 4.59 or 4.43 etc.....)
      U1
1  .1
2  4.6
3  2.3
4  0
5  .1
6  4.65
7  9
8  0

      U2
1  .1
2  4.7
3  2.25
4  0
5  .1
6  4.7
7  9
8  0

      Q1
c  1.5
b  .7
e  0- 0.1


      Q2
c  1.6
b  1.5
e  .7


      Q3
c  .43
b  0
e  .3

[Processaurus]

Hi, for starters, the bias on the non inverting input of each opamp (pin 3) is wrong, it should be 4.5v.  Something is pulling it down...  the fact both of them are doing the same thing says the same thing is wrong with both, check that you have TL071's (or whichever single opamp you want) rather than TL072 (or any other dual opamp).  Also check that they are oriented correctly.  I bet if you take the chip out of the socket (you used a socket, right  ) the bias will go back up to 4.5v.

[hendrix2489]

i checked and i do have tl071s and they are not in sockets so i will unsolder and resolder them snd retake voltages

[hendrix2489]

i  resoldered ics and trans but to no change in voltage.  Could this be because of the rotary switch, i could not tell how to wire it a b c

[Processaurus]

Oh, I meant if you leave the chips out and measure the DC on the empty pins, as the bias might return to the 4.5v (V1/2) which would mean something is wrong with the chips themselves. 

You can plug the signal at the connection between d1 and r11 into your amp (it'll be loud, but is a safe level, and it has no DC bias), you should hear your fuzz, but before the tone control and output opamp.  Do that to see if your instrument signal is even getting that far.  Something is fishy with your opamps though.  You sure they are in the right direction?  Also are your r3, r4, and r16, r17 indeed 680K apiece, and hooked up to power and ground correctly as shown on the schematic (measure the sides that aren't connected to the non inverting input).

That's amazing this is the 3rd problem you've had after making 25 pedals... none of my stuff works the first time.

[R.G.]

What is the control line for on the stomp switch?

It's for attaching a Millenium Bypass indicator.

It also might be how i wired my rotary, some help would be much appreciated.

If you didn't use the specified rotary switch, you'll have to map out your switch. You can use the pin diagram on the GEO schematics to do this. First, look up the pin numbering for the specified switch on Mouser.com. This tells you which holes in the PCB are A, B, C and 1...12 so you know which holes are which number.

Take your ohmmeter and the switch you have and locate the poles - these are the pins that connect to the throws as the shaft is turned. Poles will connect to multiple throws; throws only connect to one pole in one position.

LABEL the pole pins A, B, C like the schematic. Then label the throws. Set the shaft position fully counter clockwise. Now see which throws A, B, and C poles connect to. Label these pins 1, 5, and 9 respectively. Turn the shaft one click clockwise. Now label the pins which connect to A, B, and C as 2, 6, and 10. Keep going until they're all labeled. If you do this carefully, you can then wire up your switch to the PCB pads correctly no matter what pinout your switch has.

here are my voltages:
fyi ( i am using an analog multi meter so 4.5 might actually be 4.59 or 4.43 etc.....)

This is a good lesson in the difficulties you can get into with meters. It's likely that the low reading on the + inputs is actually just fine and you can't read it correctly because your meter loads it down. Most analog meters are around 20K ohms/volt, meaning that if you put it on a 20V scale, the meter itself is a resistance of 400K; on a 20V scale, 40K. Modern DMMs have input impedances of 1M to 10M, and are much less loading; still, if you have a 1M version, and there is a 1M bias resistance, the meter itself will pull down the voltage by half.

This last is a tidbit they don't teach in schools of any kind any more, I don't think. I had a 2-credit college sophomore course in the foibles of instrumentation - effectively, when NOT to trust your meters.

The key to recognizing this situation in your case is that the output of the opamps and the (-) inputs are sitting at the correct voltage, 4.7V. These are both low impedance points (which you have to know about opamps... ) and so the low reading on the (+) input must be a problem with the meter.

I think your problem is with your switch wiring, not the rest of the circuit. At least until you get the switch right, we can't tell.

[hendrix2489]

thanks for the help R.G. and Processaurus, i think  the switch is wrong too, i have used a 1p-6t on a wah before but never a rotarty like this.  I found out the pinout of the switch but how can i tell where a b c is on the board, it does not show, thanks in advance

[hendrix2489]

i used the schematic and the layout and rewired it, but to still no avail, it still wont work.   

[hendrix2489]

new voltages;   i have no idea what is wrong

U1
1  .1
2  6.6
3  2.3
4  0
5  .1
6  6.7
7  9
8  0

      U2
1  .1
2  4.7
3  2.25
4  0
5  .1
6  4.7
7  9
8  0

[hendrix2489]

i am just about ready to resolder everything, i can not think of anything to look at

[Processaurus]

Hi, do you get fuzz at the place (D1 and R11) I mentioned? That will let you know if the first half of the circuit is working.  You'd hook a jack up with alligator clips, tip to the spot mentioned, and sleeve to ground somewhere.  Make sense?

[hendrix2489]

i will try that tomorrow, if it does not have fuzz then there is a problem in the first half , if i understand correctly

[Processaurus]

Yes. Or both halves.  Divide and conquer.  Make sure the switch that connects the trim pot to ground (off the emitter of q2, using section A of the rotary switch) isn't on, that makes the gated, splatty, battery dying sounds by intentionally misbiasing things.

[hendrix2489]

still nothing,  i should audio probe next