Modding a Phaser with no schematic? Volume drop issue. Where to start?

[PurpleStrat]

I have a Cortez Phaser that I hcvae ehoused and true bypassed. It's a 2 stage Phaser but is not like the Phase 45 or DOD 2 stager. It has a slight volume drop which drives me nuts! I can't find a schem and probably won't be able to.

Can I get some advice on what I might want to try? If it was drive pedal I could figure it out quick but I have never worked on a modulation pedal. I can get a gut shot if that will help!

[mdh]

Can you give us an idea of what kind of phaser it is?  E.g., you say it's not like a Phase 45... does that mean it's not a JFET phaser, or what?  A good close-up of the component side of the board or even just a list of the active components would be a good start.

[PurpleStrat]

Had to find some batteries for the camera.

[mdh]

Wow, that's different.  I expected to see FETs or optocouplers or OTAs, but it looks like the active components in the audio path are just op amps and possibly a transistor of some kind.  And presumably one or two of the 6 available op amps make up the LFO.  I was hoping to identify a similar circuit whose schematic could guide you, but I've never seen a phaser like this.

If a slight volume drop is the only problem, though, maybe the solution would be to add a little gain stage just before the output.  Maybe you could fit a single transistor or op amp booster on a little piece of perf and shoehorn it in there somewhere?

[PurpleStrat]

Wow, that's different.  I expected to see FETs or optocouplers or OTAs, but it looks like the active components in the audio path are just op amps and possibly a transistor of some kind.  And presumably one or two of the 6 available op amps make up the LFO.  I was hoping to identify a similar circuit whose schematic could guide you, but I've never seen a phaser like this.

If a slight volume drop is the only problem, though, maybe the solution would be to add a little gain stage just before the output.  Maybe you could fit a single transistor or op amp booster on a little piece of perf and shoehorn it in there somewhere?

Told ya!

The transistor is an AK19?

I have a little preamp device I got from Ryan at ToadWorks and plan to use it if all else fails. That would mean a bigger enclosure though.

[Mark Hammer]

1) My eyes start to glaze looking at the datasheet, but the 4011 is, after all just a bunch of MosFet devices in a chip.  If the MosFets in 4049 invertor sections can be used as voltage-controlled resistors in some phasers, then I suppose so could a 4011.  After all, if it is a 2-stage phaser, then 6 op-amp sections is enough to provide an LFO, input stage, 2 phase-shift stages and an output mixer, leaving the 4011 free for other duties.  If only a couple of pins on the 4011 are used, and if they have traces that run to the 3403, that cinches it.

2) If you can trace the wires from the bypass switch or jacks (since that AK19 may simply be a FET used to disable the wet side, while leaving the input and output stages always connected to the jacks and in-circuit, as is done in a LOT ofphasers), it may be possible to identify the op-amp section used as the output wet+dry mixer.  This would often be the logical place to mess with level since it may only involve a change in a single feedback resistor.

3) If 1 and 2 are correct, it is still not clear which chips to concentrate on.  Clearly the two phase-shift stages are in the 3403 given the bunch of 10k resistors and two .047uf caps (a classic configuration).  Good practice would situate the LFO in a separate chip from the audio portion since you could decouple it to reduce audible ticking.  But the components surrounding the 4558 and at the left side of the 3403 suggest that the LFO is made of two sections from the 3403, with the 4558 used for the input and output stages.  That's a guess, though, based on the component side.  The copper side is free to tell a very different story.

[mdh]

Interesting ideas, Mark.  I had assumed that the 4011 was being used to make a flip-flop for bypass.

[George Giblet]

If you can post a shot of the unit and the underside of the board it would be easy to trace and there would no doubt where to start.

There's two options try to understand the circuit, or trace a small part of it and forget the rest - this is quicker but not guaranteed.

How is the effect switching done?  Do the input and or output audio signals goes through the switch, in which case the switch would have many wires and would be fairly large?  or, is it a small simple push button with two wires?  In the later case that would imply the unit uses solid state switch and I would then guess the 4011 would be used for switching and perhaps the transistor is a JFET for the switching.   However there doesn't seem to be enough circuity for that so I'm guessing it's the first case.

The problem is it is hard to make the best suggestion without knowing how the switching is done is known - because that constrains where mods can be made.   Doing the wrong mod will affect the dry signal level.

The quickest path do the mod is to start at the output socket and work out how the mixing is done:

1) Resistive mixing:
http://www.geocities.com/CollegePark/Library/1355/mxrdodphase.gif

2) Active mixing with an opamp
http://filters.muziq.be/model/ibanez/808/pt909

3) Active mixing using a transistor
http://www.geofex.com/FX_images/p180.gif

The suggested mods for a unit which switches the audio signal with the switch are:

case 1:   Trace input circuit from the input wire.  Determine of the first stage opamp has gain.  If so adjust gain by changing resistors.

case 2:   Decrease the two mixing resistors on the last stage mixer

case 3:   Increase the feedback resistor on the last stage mixer (or decrease the two mixng resistors as per case 2).

[PurpleStrat]

If you can post a shot of the unit and the underside of the board it would be easy to trace and there would no doubt where to start.

There's two options try to understand the circuit, or trace a small part of it and forget the rest - this is quicker but not guaranteed.

How is the effect switching done?  Do the input and or output audio signals goes through the switch, in which case the switch would have many wires and would be fairly large?  or, is it a small simple push button with two wires?  In the later case that would imply the unit uses solid state switch and I would then guess the 4011 would be used for switching and perhaps the transistor is a JFET for the switching.   However there doesn't seem to be enough circuity for that so I'm guessing it's the first case.

The problem is it is hard to make the best suggestion without knowing how the switching is done is known - because that constrains where mods can be made.   Doing the wrong mod will affect the dry signal level.

The quickest path do the mod is to start at the output socket and work out how the mixing is done:

1) Resistive mixing:
http://www.geocities.com/CollegePark/Library/1355/mxrdodphase.gif

2) Active mixing with an opamp
http://filters.muziq.be/model/ibanez/808/pt909

3) Active mixing using a transistor
http://www.geofex.com/FX_images/p180.gif

The suggested mods for a unit which switches the audio signal with the switch are:

case 1:   Trace input circuit from the input wire.  Determine of the first stage opamp has gain.  If so adjust gain by changing resistors.

case 2:   Decrease the two mixing resistors on the last stage mixer

case 3:   Increase the feedback resistor on the last stage mixer (or decrease the two mixng resistors as per case 2).

I am working on tracing the whole circuit. The switch is a 3PDT. It came with another board that had a Boss style switch and buffer. I cut all the leads wired it to the new switch.

[mdh]

Keep us posted on the trace!  I'd be interested in seeing how this thing works.

[Shepherd]

Has the bottom of the board been posted anywhere?

[Brian Marshall]

if it is a 2 stage phaser the 4011 almost has to be being used as a couple of variable resistors.

I've never even heard of an AK19.  judging by the resistors around the transistor, i almost think it may have been used to turn an LED on and off, but you would think that would have been on the switching board.

I also don't see an area that has an obvious high impedance input for audio, which makes me think that may have also been on the other board, which could explain the volume drop problem... if your guitar signal is seeing a 10K input impedance that would lower volume significantly.

[PurpleStrat]

Yep the buffer was on the small board. Maybe I will just build a small buffer to put in it . I still have to work on the trace and post it when I am done since some of you guys are curious are curious. It really is a sweet sounding Phaser. After the trace maybe I'll build one from scratch.