VIDEO: HELP with Vintage MXR Phase 100 Rebuild / Repair - Strange Sounds!

[ammalato]

Hi All:

I received an MXR 1977(ish) Phase 100 circuit which had been removed from the original enclosure, pots, jacks and switches. I rehoused the circuit and wired it to all new components of the same value as per spec (Switchcraft Jacks, "2 Pole / 4 Way" Rotary Switch, C500k pot.

The Pedal is giving off some very strange sounds and I'm unsure what's wrong. Could be my wiring (I looked at several pictures of similar era pedals as well as the general guitar gadgets wiring diagram) or perhaps something shot on the board. See video below.  Appreciate any help or suggestions you may have.

http://youtu.be/k90L6rs5hWE

[ammalato]

Bump?

[Govmnt_Lacky]

Im gonna take a rough guess and say that some of your wiring may be wrong. Particularly with the rotary switch. Also, possibly one or more of the vactrols may be bad.

Can you post some good pics of the wiring?

[ammalato]

Photos added to my original post above.  Thanks!

[Govmnt_Lacky]

Are you certain that your SPST footswitch is wired correctly?

[ammalato]

I'll give it another look when I get home.  Initially I had the SPDT wired up as per my other MXR's wiring and was having the same issues so I moved a few things around.  It's possible both were incorrect.  I'd be lucky (and embarrassed) if that's all that's wrong.  Thanks.

[jimilee]

GGG has a phase 100 clone with great wiring schematics and layouts. The board seems to be an exact clone.

[ammalato]

So below is the current wiring of the pedal, everything seems to be correct based on the pictures of seen of other units from the era and GGG's layout.  Anything look funny?

The only other thing I can say is that when testing it earlier I noticed something strange.  I had a guitar cable plugged into my amp but held the other end of the cable in my hand (hadn't plugged it into the pedals' output yet).  As I moved the cable end toward the pedal to plug it in I noticed that a whine was coming through the amplifier.  It was almost as if the pedal itself was giving off some sort of radio frequency and the exposed guitar cable end was acting as an antenna and picking it up.  The closer to the circuit board I moved the cable end, the louder it got.  



http://i.imgur.com/KwaZroR.jpg  (See it here in Full Size)

[Govmnt_Lacky]

2 recommendations:

1) Measure your ENCLOSURE to see if there is any DC voltage (+9V?) being put onto the enclosure when you have the pedal all buttoned up. Also, if/when you check that there is NO voltage on the enclosure.... check to make sure that your PCB ground, Input jack Sleeve, Output jack sleeve, AND enclosure ALL read to your DC jack's Ground lug. In other words, make sure you have good grounds where they are supposed to be.

2) Double check that your SPDT stomp is wired correctly. Make sure that you have the right wires on the right lugs. I know I said this before but, for some reason it just does not look right  For some reason I think your Output jack and PCB output need to be swapped!

[ammalato]

A little update here.

I've since rewired the SPDT switch, so I'm not getting the strange feedback, squealing as shown in the video.  What's happening now though is that the pedal isn't really phasing.  There is a difference in tone when the pedal is engaged however no phase.  When I twist the knob into different settings, particularly one of the knobs settings the pedal gives a 1x sweep of phase and then the guitar signal returns to an unaffected sound.  I'm convinced that the pedal is wired correctly but there is something fried on the board.  I've read other threads where some folks say the LED's on theirs were shot and others said the transistors were shot.  Curious if anyone has suggestions on test points to troubleshoot the failing component.

[armdnrdy]

You can test the LFO and the Vactrols in one shot by checking resistance between the center "leg" and the outer legs of the Vactrols.

The LFO causes the Vactrol LEDs to ramp up and down, which in turn causes the resistance of the photo resistors to move up and down.

If any of the transistors are bad (Q1- 4), you won't read a resistance change, If the Vactrol LED(s) are bad....same thing...no change in resistance.

It is my opinion that you have something wired incorrectly.
I hear phasing in the video sound sample.

[Mark Hammer]

The 20k trimpot on the board is for adjusting the sweep so that it sits in a favorable zone where the full upward and downward sweep presents a smooth change.  If the trimpot is mis-set, one might only hear a very small sweep.  So mark off where the trimpot is currently set to (in case the trimpot is not the problem and you need to get it back to where it was), and play with its setting a bit to see if that makes a difference.

[ammalato]

So in testing the Vactrols I notice a alternating/varying resistance for 2 out of the 3 Vactrols.  With the pedal back open and the switch closest to you, when looking at the circuit the Vactrol to the right is showing a consistent resistance when measuring from the center leg of the Cell and either outer leg of the LED.  The other two show resistance that varies in a pretty repetitive fashion (leading me to believe they are in direct correlation with the sweep of the phase).  Not sure if this indicates a problem with the vacrol, the IC's or one of the Transistors.

[Govmnt_Lacky]

Continuing on the debugging....

On the particular Vactrol that is suspected, did you try checking resistances with the rotary switch in ALL of the positions?

If so, then the next step would be to check the other side of the Vactrol to see if the LED's voltage is alternating or even getting voltage at all. If you are seeing voltage but no changes then the problem could be in the vactrol's control system. If you are seeing voltage changes then the Vactrol is most likely a goner.

[armdnrdy]

I spent too long typing and looking at drawings...Greg slid his post under the door. 

I'm not sure what you mean by, " With the pedal back open and the switch closest to you."

How about using the off board wire pad side of the board as the "top"

Looking at the board oriented in this fashion, we have a left (VR-1), center (VR-2) and right Vactrol. (VR-3)

The Vactrol LEDs are connected in series with the LFO output from Q3 and Q4 feeding the first LED cathode (VR-1) and +9 volt connected to the anode of the last LED. (VR-3)

It seems that if you are truly not getting any resistance movement from one of the Vactrol....the photo resistor has a fault..which is not that common. The way that the LEDs are connected....if one fails open...the other LEDs won't work either.

Double check the "bad" one.

If you have an LED handy...put the leads across the LED side (two lead side) of the Vactrol. If it doesn't light...turn it around.

[ammalato]

So a bit of a discovery.  I noticed that if i just touch/press down on the trimpot with my bare finger while playing through the pedal the phasing comes to life.  the second I let go of the trimpot the phasing dies out.  I noticed a blob of solder on the trimpot almost as if to lock the pot in the proper place (not sure if this was factory done or someone else's handy work) so i bike the joint and can freely adjust the pot now.  That said, it still only seems to work when I press down on the trimpot.  ANy idea why this might be before I just go replace the thing?

[armdnrdy]

Dirty trimpot, bad solder joint on one of the trimpot pins.

[Mark Hammer]

The wiper may simply be deformed, too oxidized, or some other basis for not making contact.

It's a sweeping generalization, but my experience is that the smaller the diameter of the pot, the flimsier and poorer quality the wiper.  And trimpots are about the smallest diameter pot you can find.

[ammalato]

Eureka!!!  Low and behold, after armdnrdy's suggestion I checked the solder traces on and around the trim pot and found not only some poor looking joints, but also a partially lifted trace to the resistor immediately next to it.  A quick redo with the soldering iron and the pedal is alive.  Thank god, because I've been sick of having this thing gnaw away at my conscience.   Thank you everyone for your help.  Truly a great forum full of even greater people. 

[Mark Hammer]

Well, ALRIGHT!    Phase on!