How to debug a phase 90 clone?

[ItsGiusto]

I've built a Phase 90 clone based mostly on this layout: http://tagboardeffects.blogspot.com/2014/05/mxr-phase-90-with-script-switch.html

but I'm having issues with it, and I'm not sure of how to track down the problems. Basically, when it's on, it sounds mostly similar to when it's in bypass mode. It doesn't seem to be affecting the guitar tone very much at all, and I certainly don't hear the oscillating quality that it's supposed to have.

I have an oscilloscope, if that would help me track down the problem. Should I run a sine wave through the pedal and trace it along the signal path? I'm not entirely sure what I'd be looking for if I did this, what a phaser is supposed to do to a sine wave. Maybe it'd help if I probe around the LFO area so I can see why I'm not getting the oscillations?

[jimilee]

Did you turn the trim pot? I'm assuming you did. Got pictures? Look closely at your solder joints to see if you've missed something. It's easy to do, the silver will play tricks on you. Have you checked your trannies with your DMM to see if you have fluctuation / oscillating?

[ItsGiusto]

Did you turn the trim pot? I'm assuming you did. Got pictures? Look closely at your solder joints to see if you've missed something. It's easy to do, the silver will play tricks on you. Have you checked your trannies with your DMM to see if you have fluctuation / oscillating?

I should clarify, I had this build formerly working, but I used poor quality pots and switches that kept crapping out on me, so I ended up replacing them. Now after replacing the parts, I'm in this place where it doesn't seem to do anything for some reason. I guess I broke something else along the way, while replacing them.

How do I check the trannies with the DMM to see if it's oscillating?

[ItsGiusto]

It's interesting, if I touch the back of the board, I can get it to phase. It's usually just temporarily (probably because my skin is carrying a current and causing the transistors to activate). And then other times, if I shake it, it will start phasing too. This is so strange. I have a feeling that the oscillation isn't working, but I don't know how to tell.

[ItsGiusto]

So, I've checked with an oscilloscope, and the LFO portion of the circuit does seem to be working. It responds appropriately to the rate knob, getting faster or slower when turned. When I measure the voltage on the gates of the phasing transistors, it seems to oscillate between about 3.5 and 4.1 volts. Meanwhile, the source of the phasing transistors is constant at around 5 volts.

Given that the oscillation is working, and given that I can get the phasing to work if I manually touch the transistors (see my previous post on this thread), I'm baffled as to why the phasing isn't working. Does anyone have any suggestions? Do the above voltages seem okay?

I'm thinking that maybe the voltages just aren't right, and it's just not getting the gates of the phasing transistors into the right range that they need to be. I'm guessing that the source voltage needs to be lower. However, no amount of tweaking the trim pot seems to alter the voltage that's being presented on the transistor's source. Maybe there's some missing connection somewhere?

[ItsGiusto]

Okay, I figured it out. Two pots I was using had gone bad for some reason, both the pot for the depth control, and for an added phase-range-offset control. Both had lost connection with their legs for some reason. I've replaced them and it sounds good, just like the original MXR phase 90 now!

[anotherjim]

You seem to be having a lot of pot trouble. Are you soldering wires into the little rivets on the pot lugs by any chance?

[ItsGiusto]

You seem to be having a lot of pot trouble. Are you soldering wires into the little rivets on the pot lugs by any chance?

You mean those little holes that are closer to the potentiometer body than the legs? If you're talking about what I think you are, then yes, I do on occasion solder into those, or use them as an additional soldering point. Should I not?

[anotherjim]

You should not. Those rivets are the only thing holding the leg in electrical contact with the printed-on resistance track. Heat and flux can disturb the contact and partially or totally insulate it. It also lets the flux (usually rosin) contaminate the resistance track since it's in close proximity. The flux sputters when hot throwing little globules around. Keep the solder as far away as possible.

[ElectricDruid]

....aaannnndd Jim gets twenty five debugging points!!! 

Nice work!