Phaser Ticking

[Brushthrower]

I recently acquired an old "Reagun" stereo phaser for free as it was no longer working. Being one to never pass up a free pedal I figured it might just need a little tinkering to get it running again. The LED lights up and I can get the dry signal though it whether it is on or off, but no effect. There is only a faint ticking sound which varies when the speed is adjusted. From what I understand from searching the forum, this has something to do with the LFO but it's a little beyond me. Can anyone help me out here?

[Mark Hammer]

Tell us what chips are in it, and we'll help you narrow it down.  For instance, is there, perchance, a TL022 or LM358?

[Brushthrower]

Thanks Mark. There are four chips marked "6552" and "246" just below that, so I'm assuming they're AN6552?

[Mark Hammer]

AN6552 is what they are.  Treat them like 4558s, because that's what Panasonic says to do.  I was sort of hoping there would be a distinctive dual op-amp used for the LFO, which would make for less detective work on both our parts.  No such luck.

Are you able to track where the Rate/Speed pot goes to?  If so, then that would flag which op-amp is used for the LFO.  Our goal here, is to identify it, and decouple it.

[Brushthrower]

I was able track the speed knob to it's respective op amp. If you're looking at the component side of the pcb with the op amp code facing the right way up, the four AN6552s are lined up in a row. The speed/rate op amp is on the left-most side. There is a 0.00047uf (471) capacitor soldered to the top two middle legs of this op amp. Hope this helps.

[MikeH]

A gut shot could really help to, if you can take one.

[Brushthrower]

You bet:








They came out a little blurry on my crappy camera, but you get the idea.

[Brushthrower]

Bump. Anyone?

[Mark Hammer]

Now that I can finally see the pics (no photobucket access at work) my money is on one of the two chips on the left.  Unfortunately, being unable to see the actual part values (and no need to apologize; not your fault they're mounted vertically), it's hard to know which one is the LFO.

At the same time, and reading your post over again, the primary objective is to get a phaser sound out of the thing, and then deal with the ticking.

The phaser is clearly a FET-based unit, givenwhat we see on the board.  I would imagine the transistors that we can't see very well are likely K30A or similar.  Take a gander at the MXR Phase 90 for a vague idea of what does what.  Note that the Phase 90 requires that you bias the JFETs just right to get a decent sound...in fact ANY sound.  I see two trimpots on the board: one at the top near the LED and the other at the lower left.   The one at the top appears to be in the vicinity of some diodes, one of which may be a zener like the P90 has.  The one at the lower left is near an op-amp.  It is a frequent practice to include a trimpot to trim back resonance/regeneration in phasers that have a feedback/regen/resonance/intensity control.  Normally, the panel-mount control is set to max, and the person at the factory adjusts the trimpot to the point just below where the circuit breaks out into howling.  As well, normally maxing out the resonance/feedback will increase noise, so if there is no sweep, but you still have a viable feedback path, you should hear both an increase in sharpness and noise as you adjust the resonance trimpot...assuming that's it.  Try 'em both and see which one is the likely culprit.

Once you've identified which is the resonance trimpot, then the other one should probably be the bias trimpot.  I'm not saying this is definitely your problem, as opposed to a burnt part, a flaw in the switching system (uses a momentary, right?), or a crack in the board.  But, the trimpots don't seem secured in any way by cement or lacquer so they may have been moved from the sweet spot.  Set the speed and depth for max, so that you'll be able to hear sweeping when and if it occurs, and gently move the trimpot from one extreme to the other.  If it is merely an issue of where the trimpot has drifted to, then you should be able to get some audible phasing at some point along the travel of that trimpot.

I'll leave it there for now.  Do what you need to, and report back.

[Brushthrower]

Thanks for that detailed response, Mark. You're correct, the majority of the other transistors are K30As, as well as a couple of C1815s. I believe I've identified the bias trimpot (the one near the LED). I tried tweaking both of these pots with the speed and depth at full and managed to get a bit of effect: almost like a very mild chorusing effect if the speed was on full. However the perplexing thing is that the effect is there whether the pedal is engaged or not. The effect is at it's strongest when the trimpot near the diodes is cranked nearly all the way to the left and the other nearly all the way to the right.

I've examined the pcb carefully with a lupe and there doesn't appear to be any cracks. I also tried using an audio probe, but not having a schematic and the cramped placement of the components made it difficult to follow the audio path. There did seem to be less audio/more pronounced ticking on the lower part of the circuit. If you look at the solder-side pic of the PCB you'll see there is a trace at the top that runs nearly the width of the board. Most of the sound above this trace seem very weak or nonexistent, the ticking louder.

[Mark Hammer]

Okay, at least we have some evidence of phasing.  Though it would be nice to have more evidence of phasing than of ticking!

The ticking occurs as a result of the op-amp used for the LFO suddenly drawing lots of current.  This sudden draw is felt around the remainder of the circuit, much like the way refrigerator motors/compressors would create noise in the stereo elsewhere in the house in the old days.  The cure takes many forms.  One is to use a lower-current op-amp instead of what's there.  Given how cramped the board is, we'll leave that one alone.  The second strategy is too complicated to explain here so I'll refer you to this document instead: http://moosapotamus.net/IDEAS/stompboxology/MoTremLo.htm

The third strategy is to create a tiny reservoir of current for the LFO so that it doesn't have to rob the other chips when it needs to produce a square wave (a necessary aspect of the LFO circuit).  The traditional means for doing that is to break the trace leading to pin 8 of that chip (the V+ line) and connect the two sides of the break with a small value resistor such as 100 ohms.  Now connect the + side of a medium-value electrolytic capacitor, like 10-22uf (15V rating or better), to pin 8, and the other end to ground.  That cap becomes your little current reservoir, and is said to "decouple" the LFO from the rest of the circuit.

Now, about the low-level phase signal.  It is quite possible that the switching is amiss.  I'm guessing here, but it is VERY common for phasers like this to use a Boss-style flip-flop circuit to turn a FET on and off.  When the phase-shift signal is blocked, the pedal is not "bypassed" as such, but the phasing effect is cancelled.  If you could find the relevant FET, you could put the pedal permanently "on" by simply tacking on a wire shunt between the drain and source pads of the JFET.  If the phasing re-emerges, then you know it's the switching circuit.  unfortunately, one of the most common JFETs for doing that is, you guessed it, a K30A.  Happily, it can be identified by a diode connected to its gate pin, something none of the other K30As on the board share.

[Brushthrower]

Thanks again Mark. I hope I'm not wasting too much of your time. I was able to locate the correct JFET from your instructions ans tried your shunting suggestion (please excuse my lack of terminology, but I assumed you were using "shunt" in this case as being synonymous with "jumper" - either way, I just connected the drain and source legs with a piece of wire) and the pedal was switched to permanently "on", however the phasing did not re-emerge, except for the faint effect that I managed to squeeze out of it from tweaking the trimpots.

[Mark Hammer]

You inferred correctly.

Okay, we have successfully determined that it isn't the switching circuitry, so you can remove the shunt/short/jumper. (Or at least I'm naively assuming we've determined that.  The LED does go on and off under normal circumstances?)  Indicentally, this is often a handy shortcut for deciding which general portion of the circuit warrants one's attention. 

So now the question is "Why is the phasing so faint?"  One way to determine that is to use your audio probe to check the signal at each of the successive phase-shift stages.  There are 4 successive op-amp stages the signal has to successfully make it throgh to get where it's going, which means there are multiple points where signal might go astray. 

The 6552 has the same pinout as a conventional 4558 so you'll want to stick your audio probe on each of the output pins - pins 1 and 7.  Note that since that path carries only phase-shift signal, you won't hear an effect - it takes dry+wet mixing to accomplish that - so let's do a two-fer-one and verify both the LFO and wet audio path.  Turn the rate knob up about 3/4 so it's an obvious sweep.  If the audio signal is successfully making it to that output pin from previous stages, then you'll hear a pitch wobble or vibrato sound.  Each additional phase-shift stage will add more wobble so they won't all sound the same.

Incidentally, what is the voltage at the wiper of the trimpot that we believe is the bias trimpot?

[Brushthrower]

With the pcb orientated as shown in the pic above I made readins from left to right as follows:
1
1: loud low ticking
7: effect quite strong with ticking

2
1: effect with ticking
7: effect with ticking

3
1:distortion with ticking
7: distortion with ticking

4
1:more distortion with a higher frequency ticking
7:less sound with more ticking

The phasing seemed strongest in the leftmost op amp, so I'm assuming this is the last of the four.