1975 MXR Phase 90 - Clock noise?

[clarisso11]

Hi everyone,

I recently got my hands on an original 1975 MXR Phase 90 PCB, from an old repair shop - it had never been installed. I found a script MXR Noise Gate from the 70s, and swapped in the PCB (Schematic and photo attached).
So far so good, everything works, and the effect sounds great. The only issue is that I can hear the clock noise bleeding through, as a ticking that changes frequency with the speed knob.

Things I have tried:

• Checked the tantalums with ESR meter, all looking good
• Checked for bad solder joints
• Ran pedal on its own, as well as after buffered pedal
• Tried battery and isolated power supply
• Checked IC voltages, they seem okay

Questions:

• I've never owned a script phase 90, is the ticking normal?
• Would it be worth tweaking the internal bias trim pot (it is currently locked with solder)?
• Are there any circuit modifications I could try to filter out the clock noise?

Any help would be greatly appreciated!

[Helmholtz]

Can your ESR meter differentiate between low ESR and a shorted cap?
If not I'd also measure capacitance.

Most probably the caps are good, though.

[clarisso11]

Hey Helmholtz,
Yes, it measured 10uF for the color-coded tantalum, and 15uF for the red one (matches the label on it).

[g1]

Since it's a re-housed unit, I'll mention that lead dress can cause such an issue. Have you tried moving wires around?
https://www.diystompboxes.com/smffor...5246#msg885246

[Mark Hammer]

Audible ticking is generally a spike on the power line produced by a sudden "current pull" by the LFO. It's a common enough problem.

There are generally two solutions, which can be tried in isolation OR combination.

1) There weren't many choices of op-amps in 1975. Thankfully, we have many more choices nowadays. Since it is a single op-amp LFO, consider replacing the one in the lower right corner, beside the 15uf tantalum on its side, with a TL061, which draws much less current.

2) Another tried and true strategy is to "decouple" the LFO op-amp from the shared power line by inserting a small-value resistor (100-220R is nice) between V+ and the V+ pin of the chip (pin 7), and a small electro cap (let's say 10uf) from pin 7 to ground. That does a few things. First, since the resistor is in series with V+, it reduces the current drawn by the LFO. Second, the RC pair forms a simple lowpass filter to "slow down" the current draw (a slope, not a spike). Finally, that cap acts like a teensy battery, such that the LFO draws from it as well as the shared power source.

A couple of unrelated things to note.
a) The range of sweep - i.e., where in the spectrum the notches are crated - is actually wider than people think. The trimmer not only sets a suitable bias for the FETs so that they all "row together", but is also used to bias them to make every phaser coming off the line sound the same as each other. BUT, you can play with the trimmer and get other sound characters, from low and gurgly to high and swirly. Feel free to play with it. It won't damage the pedal. But since such trimmers are not really meant to be used as "controls" another way to play with the range is this: There is a 1M fixed resistor coming off the wiper of that trimpot that sets how much current is fed to the gates of the FETs. Assuming it is set optimally, replace it with a 510k fixed resistor and 500k pot, in series, to achieve current-limiting resistances between 510k and 1M. This will move the range of the sweep around. I do this with all my P90 builds.

b) Width of sweep. The current from the trimmer that goes to the FET gates is summed with the current from the LFO. The LFO current is limited/set by the 3M9 resistor coming off the LFO. Happily, it is easy to find because it's the only one of that value on the board. The width of the sweep can be extended by placing other resistors in parallel. I like to use a 3-position toggle to either add 15M or 22M in parallel, for effective values of 3.3M and 3.1M. The 3.3M value was actually used in some issues, yielding a wider sweep.

Important to recognize that the Phase 90 was a compromise design, intended to make everyone happy, and make every speed sound acceptable. Normally, a 2 or 3-knob phaser would allow one to adjust sweep width and resonance/feedback, such that faster speeds could be set for narrower sweep and less feedback, while slow speeds could be made wider, with more feedback so that they sounded more majestic. MXR went for plain vanilla at all speeds. My suggested mods let you get a little more variety out of it. Note that, since the bias current from the trimmer, and LFO current sum together. you'll want to avoid feeding those gates too much current, lest one or more of the FETs freeze and refuse to change resistance any more. One wants to have all FETs varying resistance all the time. So if you increase the LFO current for wider sweep, try not to maximize the bias current from the trimmer. Nothing will break, if you don't, but it may not sound as good.

[clarisso11]

Hey Mark, thank you for this extensive reply! I will start by swapping in the TL061, and if that doesn't get rid of the clock noise, move on to adding the resistor and capacitor. Will report back with my findings! The mods sound great, too, might give those a try.

Thanks also g1. I spent some time separating wires and even swapping the input wire for a shielded one, but it made little to no difference. I then moved wires around while listening to the output, again with very little effect.

Hopefully the upgraded op-amp will do the trick!

[clarisso11]

Since it will take a few days for the TL061 to arrive, I decided to try decoupling the LFO first. Cut the track between V+ and pin 7 and inserted a 150R resistor, plus 10uF cap to ground. It helped reducing the ticking a little bit (I think), but it's definitely still there. Hopefully the new op-amp will do the trick!

[clarisso11]

Hi everyone,

The TL061 finally arrived, and I installed it in place of the original LFO op-amp. The clock noise did not improve in any way, so I'm wondering if I can exclude the LFO as the issue, at this point.

I also found videos of 1970s Phase 90s, and didn't notice the clock noise on those, so it doesn't seem to be normal of these units. Rather than a flaw in the design, I'm starting to suspect a defective part.

I noticed that the clock noise persists even when the effect is turned off, although it is less noticeable. I converted the pedal to DPDT true bypass (no LED), maybe the fact that I can still hear the clock noise points to the defect?

I will start gradually replacing parts to see if I can narrow the fault down further.

[clarisso11]

Going back to the idea of clock bleed through the wires: I found this imagine of a 1974 script Phase 90, with all of the wires closely twisted together. Absolutely no clock noise audible, according to the owner.

[Helmholtz]

Do you still get the ticking with the board lifted as shown in the picture?

[clarisso11]

Do you still get the ticking with the board lifted as shown in the picture?

I just tried again, and I still get ticking, with very little to no change when moving wires around (moving them apart etc.).

I'm starting to wonder if this could have something to do with the true bypass? Although that might just be my desperation speaking, it's not grounded in any theory.
I used this wiring scheme:

[clarisso11]

Some more observations:
1. With the 5.1V zener, I'm guessing that the voltage at pin 2 & 3 of the op-amps should be 5.1V. With 8.79V supply voltage, I measure 4.98V at pin 2&3 of the op-amps, I'm not sure if that is low enough to be suspect
2. Pin 3 voltage at one of the op-amps (the one above the LFO) is slightly lower, 4.67V vs. 4.98V

[g1]

If you suspect the true bypass, why not just temporarily wire it up as stock. That will tell you for sure.

[Helmholtz]

Is C8 good and well connected?

Does the unit apart from ticking work as it should?

[clarisso11]

If you suspect the true bypass, why not just temporarily wire it up as stock. That will tell you for sure.

Good point. I've just wired it back to stock, shortening some wires while I was at it. No change at all to the ticking noise, will convert it back to true bypass again.