No phase from Phase 100

[msclford]

I built a P100 from General Guitar Gadgets a while back, and it worked fine, but I wanted to expand its tonal possibilities. I decided to modify a couple of things (added a switch to select between using the R18 6k2 resistor and removing it, added a pot to give a little tweakability to the R1 resistor.) After dealing with some howling problems with the pot, I decided to remove it and put a fixed 15k resistor back in. Somewhere between having the pedal working with the mods in, and going back to just having a fixed resistor for R1, the pedal lost its phase sound. All I get is a little bit of a midrange emphasis with the R18 switch. That switch used to switch between phase and vibrato. I've checked the board over for solder shorts, and I can't find anything. I'm a little worried that the pedal will never be back to working I've attached photos of the board. Here is the link for the schematic. And here is the link for the board layout.

I wish this would have happened with a simpler circuit ... I could have probably figured it out myself!

Also - I made an audio probe, but the phase sound really doesn't come through anywhere in the circuit, and I checked the switch to see if the input was just shorting to the output, and it isn't.

[msclford]

Should I just use the shotgun approach, as referenced here? In other words, should I just go ahead and replace the opamps (all 6)? It looks like it is an older version of the pedal than the one currently offered at GGG, and uses TL072's instead of TL022's or LM1458's. Should I replace the 072's with 1458's, as I have seen here that this pedal is a bad application of the TL072, and the TL022 is not currently available (I think).

Is it possible that, since the TL072 is not recommended for this circuit by some, one or more of the photoconductors got fried? I'm really at a loss on this one ... any thoughts on this would be greatly appreciated.

[Mark Hammer]

First, congrats on your courage! Big circuit for a beginner.

The T072 will work in the circuit, and nothing should go wrong. The noise levels may not be optimal, and the current draw will likely be more than a battery wants, but replacement w/1458s will not make the difference between working and not working.

The first thing to check is whether your LFO is O-ing. Set the rate control for a medium speed, and measure the voltage at the junction of the Q1 and Q2 collectors. In theory, you should see something going up and down there. If not then the absence of effect is likely because the LED/LDR units are not being swept.

[msclford]

Alright, I think I'm getting somewhere here. I checked the voltage at Q1 and Q2, and it's sweeping between .40v and .45v. I then checked the voltage at the LED's for the photoconductor units, and the + side of VR1 read +9v. The - side read 0v, the same for the rest until the - side of VR3 (connected to Q1 and Q2). I've attached the schematic showing this. There was conductivity between the - lead of VR1 and the + lead of VR2, so I ruled out bad joints on those leads. I jumpered the + lead of VR1 to the + lead of VR2, and lo and behold! I got the effect to come through, albeit a little weak. It seems I have a bad photoresistor unit (VR1).

I don't know what caused it to go bad, but it doesn't work. The rest do. Should there be a resistor between + on VR1 and +9v? I don't know totally how the units work, but I'm assuming they're just and led and two photoconductor units housed in a light-proof enclosure. Any other thoughts are appreciated - I think I've got it narrowed down now, so I just have to find a VTL5C2/2 ... they don't carry those at Radio Shack

Thanks for the help, Mark. I appreciate it!

EDIT: I was a little confused earlier - VR1 should be VR3 above, and vice versa. It seemed logical to me that the optoisolator toward the input of the circuit would be VR1, but I was wrong.

[msclford]

SBE carries the VTL5C3/2, but my pedal now has the VTL5C2/2. Is there going to be a problem if I use only one VTL5C3/2, or am I going to want to replace all of them? Datasheet here. It seems that, according to the datasheet, the 3/2 behaves a little differently. See what you think - page 52.

[Mark Hammer]

Based on the measurements you posted in the annotated schematic (VERY useful, BTW), it may be that one or more of the LEDs in the optoisolators are blown. Like any LED, they can get overheated during installation and burnt out.

Of course, since the LED sides of the optoisolators are wired in series, fry one and they all stop working. Given their price, you'd want to see if only one needs replacing, or more than one. One thing you can do in the meantime, is replace the optoisolators with actual visible LEDs, just to see if the sweep generation is doing its thing.

To do this, you'd want three same-type LEDs (so that they all glow about the same amount). The LED side is the end with two leads, not three. You can simply desolder (gently) the LED side of all 3 vactrols, and solder in LEDs instead, making sure the orientation is correct.

How can you tell which LED end is which? Simple. You likely have a near-dead 9v battery hanging around. Using something that appears to be capable of only the mildest tongue-tickle, touch the LED to the battery and identify which lead needs to go to "-" to result in glow. That end will go to Q1/Q2 collectors. Orient the remaining LEDs the same way, and fire it up.

Note that you will NOT hear any phasing when you do this, since the LDRs are sealed up inside the Vactrols and not exposed to any of that light. But, you WILL be able to confirm that the sweep generator is working as intended. Once you have that confirmed, then you can begin the process of removing the LEDs, one at a time, and re-soldering the LED side of one of the Vactrols to determine if it is functional. For each Vactrol that is still functional, you should be able to hear some phasing, once it is reconnected. With any luck, it will turn out that only one of these pricey items is fried, and you will have correctly identified which one.