Help Request - Low Volume on Phase45

[Csquare4]

Hi folks,

New here, I have been lurking for a while and have built several pedals, I just seem to be having problems so I thought I would register and ask you experts.

I am building the MXR Phase45 clone using the schematic/PCB from General Guitar Gadgets located here:
http://www.generalguitargadgets.com/pdf/ggg_p45_sc.pdf 
and here
http://www.generalguitargadgets.com/index.php?option=com_content&task=view&id=156&Itemid=26

I used TL072 for the op amps and 2N5952 for the JFETs.  I checked the pin configs on the JFET and as all who use these, the layout on the board has to be reversed.

Troubleshooting:
Everything seems to be functioning fine, the phasing works, only it is just much lower in volume than the bypass signal.  Switch on, volume goes down significantly, not out, just much lower, but the phasing seems fine.  I used an audio probe and checked around to verify that I get volume throughout the signal path. 

Battery = good (~8.1 VDC), and checked with wall-wart as well at 8.0 VDC.

The biggest drop is across the 10k R1, everything after that is pretty much the same volume, with obvious bumps after the outputs of the op-amps.  I assumed that R1 was probably causing some issue, so I have changed it out to be safe, with no luck.  Still significant drop across this resistor.  I have traced it out and have found no shorts or faulty solder joints.

The JFETs themselves have very little signal level, but I assume that is because of the 470k resistors (R17, R18) across the source. 

Both IC's have power.  Again, the op amps seem to be functioning and signal flows through the appropriate pins.

Pins 5, 6 & 7 on the IC2 (IC2b) have no real detectable signal across any of them, again, I assume this is because of the 3.9M resistor...I don't really get this part of the circuit.

I don't really know where else to look.  Is there any way to truly verify the op amp and transistor performance while in the circuit and no Oscilloscope?  I could go and start swapping out the Si but I hate to do that if it is likely somewhere else.  I don't like the shotgun approach unless I really have to.

Thanks for any help.
Craig

[petemoore]

  Welcome to the forum ! Phase 90 can be a bugger sometimes.
  Read-re the debugging thread, and try it again.
  It can bring to life a whole new understanding of what matters, and what is needed in the content of a question in order to make it ever increasingly answerable.
  "Pin Voltages" is a first line of bug detection.
  R1 can be measured with a DMM, 10k is of course a signal impedance in this application.
 
 

[frequencycentral]

First thing I'd do is double check the values of R3, R11 and R28. If any of those resistors are mistakenly lower than they should be, so will be the volume.

[R.G.]

I know it hurts to do it, but read and follow "what to do when it doesn't work".

That may or may not turn up the complete solution, but it will sure filter out the vast majority of things that could be wrong.

[jkokura]

On a tubescreamer and a small stone I built I used a 100k resistor instead of a 100r resistor. It's easy to use the wrong parts.

Also, R.G. hit the nail on the head. Give us a complete set of data and we'll be able to help you out.

Jacob

[Csquare4]

First thing I'd do is double check the values of R3, R11 and R28. If any of those resistors are mistakenly lower than they should be, so will be the volume.

Wow...thanks for all the quick assistance.  I appreciate the replies, obviously a lot of knowledge floating around here.  So I was getting ready to run through the checklist to write down the pin voltages on the op amps and JFETs and thought I would check my resistors once more.  R3 was incorrect!  470 ohm, not 470k.  Also, there were two other that were the same values, R17 and R18.  Replaced those, and yes, it works like a champ.

So Rick -- kudos to you kind sir!  And Jacob, you too.  Thanks again all.

Craig

[R.G.]

All Jacob's doing.

I'm mildly red-green colorblind, so shades of red are sometimes hard for me to tell apart. I'm forever confusing 4.7K with 47K. The difference between red and orange in different resistor lots gets tricky. And frankly, many "browns" are pretty red.

[Csquare4]

It was a good learning experience for my son and myself.  I let my 13 year old son populate the board.  His first pedal, though I was supervising.  Even though I thought I double checked everything, I missed that. He actually did a better job soldering than his old man, ha ha.  He can still see close up.

Fantastic phasor btw.  Nice and lush, but not over the top.  Just what I was looking for.  Thanks again all.

Craig

[jkokura]

What a great idea to do a joint build with your son! I hope you don't mind if I scam that same idea in 12 years or so?!?

Glad to hear it worked out for you, but I think RG and Rick offered better help than I ever did... I learned all my debugging from guys like them...

Jacob