My nemesis the Orange Squeezer-trying to debug!!! Arggghhhh

[theundeadelvis]

OK, I am trying to overcome my nemesis the O.S. (using GGG's layout). I am using an audio probe and I get no sound (starting at the input) after R2(82k). I checked and replaced the resistor and nothing changed. What on earth am I doing wrong (w/pedal not life in general)? Thanks!

[Mark Hammer]

For the time being, just lift the FET out so that you have basically an op-amp gain stage with that weird little asymmetrical bias circuit (the two resistors at the input of the op-amp.  Power it up and see if you get audio output.

If not, there are a number of possibilities:

• there is a fracture in one of the connecting wires
• there is a crack on the board
• there is a bad solder joint (note that simply reflowing the joint will do very litle if you have, say, re-used a component that has tarnish around the lead and doesn't conduct well)
• a pin on the IC has gotten bent under the chip and isn't really inserted into the socket (assuming you are using one)
• you have, like so many before you (myself included) soldered the wrong lead to the wrong lug on the jack and your input is grounded without you even being aware of it
• something is shorting out the signal by touching something it shouldn't (like the chassis or the back of a pot))

trust me, you WILL get it working, and in the process you will likely learn about 2 or 3 things you vow never to do again.

[theundeadelvis]

OK, I took out the fets and nothing is making it to the output, BUT even when the audio probe is just hooked to the ground and the other lead is touching nothing sounds is leaking through. Its faint and buzzing and depending how I move the board it may get louder and clearer or cut out. Does this sound like maybe something is touching the ground that isn't supposed to be? Also, now I can make it to R4 and get sound to one side of it but not past. Thanks again!

[AdamB]

Ha, my personal nemesis is the Axis Face - I've tried to build it 3 times now, and each one has failed and I can't figure out why. Grrr.

-Adam

[Mark Hammer]

When yousay "no sound", I gather you mean that there is no reading on the meter at that point when your guitar is plugged in?  Normally there would likely be something like 30-50millivolts AC.

[theundeadelvis]

I have yet to touch the circuit with my DMM. I will do that ASAP and take a few other measurements as well. Thanks!

[theundeadelvis]

Yep Mark 0 volts at the output.

[theundeadelvis]

OK here are some measurements I just took 34" 24" 32"... ...wait, wait thats not right!

Seriously here they are:

Q1-MPF102
D=8.04 V
S=5.04 V
G=3.08 V

Q2-MPF102
D=3.08 V
S=3.08 V
G=1.1 mV

IC1-RC4558P
P1=7.46 V
P2=7.46 V
P3=.1 mV
P4=.1 mV
P5=106 mV
P6=7.25 V
P7=7.45 V
P8=8.02 V

D1-1N34A
C=-1.3
  K=0.0

[theundeadelvis]

OK, now I've tested my tantalum caps and they all seem to be fine. I've tested a few resistors and they also check out fine. I've replaced the transistors and double checked the pinouts. I've tried multiple IC's. I am dumbfounded!

[theundeadelvis]

Does anyone have a list of what the ideal voltages are? Or can you check mine and see if any look suspicious? If so what may be causing it? Thanks as always!

[johngreene]

Check R4, Pin 5 of the opamp shouldn't be that close to ground....

--john

[Mark Hammer]

Something is clearly wacky.  You're getting huge voltages on pins 1/2 and 6/7 of the IC (the output and inverting pins of each half).  That should not be the case.  Um, is the chip oriented properly?  Have you used the right values of resistors for biasing?

Incidentally, never discount misreading resistors.  I was trying to debug something the other night, and realized I had mistakenly installed 750k resistors (purple - green - yellow) rather than the 4M7 units (yellow - purple - green) called for.  Of course, installing the right values brought the thing to life.  The source of the error was that I have one drawer in my parts bin for all resistors above 500k, and was so happy to find the right colours that I neglected the subtle differences in their order.

It happens....

[alextdel]

I always use the 'split half technique'. For example, check if things seem right about half way through the signal chain (eg biasing of the op amp and the chip itself) then investigate half way through the half you suspect to be faulty.

So I would check up to the op amp, then if the fault is before I would check the FET stage and signal chain, if the fault is after the op amp I would check the output stage  with the feedback section isolated, then the feedback stage or output chain as indicated.

In this case, if you are using a chip socket make sure the socket is oriented correctly. If you can unplug the socket (or desolder) you should be able to check the obvious biasing of the op amps especially pin 5 [9Vx(470/(470+390)] just under 5V. If this is correct and you plug the op amp in and the d.c voltage changes more than a couple of decimal points, then there is something wrong with the chip. If the biasing is wrong with the chip out then desolder C4 and re check. If the d.c is still not approx 5v on pin5 then Vcc or R4 or R8 are faulty or the wrong value or solder or tracks are a problem. if the biasing comes right then suspect C4 as this should not affect d.c levels unless it is faulty.

If I have found the biasing to be correct at pin 5 and the chip is oriented correctly and has little or no effect on the biasing whether plugged in or out then I would be following Mark's suggestion with the FET and checking this circuit in minute detail. The voltages given previously will help here.

Don't expect to measure much a.c at pin 5 it will be approximately half of 'not much'. Op amps operate with a 'virtual' signal earth on their inputs due to their large native gain.

Anyway if, in the first step, everything seemed right up to the output of the op amp I would disconnect the leg of R11 and see if you get a signal (here you should be able to measure some a.c) This is basically checking the operation of the bypass, if fitted, and the C6 R13 chain. If the fault is here chech the orientation of C6 if bipolar cap and that R13 is wired correctly. (Good idea to isolate the bypass switch as incorrect wiring here can easly earth input signals -more especially with 3 pole switches)

In my OS I found that the wires from the board to the external components such as jacks and pots were susceptible to breaking off inside the insulation just next to the edge of the solder region. In my case the signal got through but the pedal was very noisey.

None of this will probably help - but I felt like contributing as I had a 'learning experience' with the OS which Mark and others helped me through (I did the version with Mark's added output stage)

Good luck!
Here endeth  .....

[theundeadelvis]

Thank you Mark and Alex. I will try all the suggestions and report back!