Help Troubleshooting a Phaser-90 Style Circuit

[DrT]

I decided to try to learn a bit more about how pedals work and what better way to learn than to do?  So I picked up a nonfunctional Retro-Sonics phaser to see if I could figure out how to repair it.  I have a minimal background in circuit analysis (two engineering courses a decade ago), so I apologize if any of my questions are a bit novice.

Here is what I know. The Retro-Sonics phaser is based on a Script P90 with some extra flexibility through a depth and level control.  The pedal turns on correctly, the bipass works and signal comes through when the effect is engaged.  The signal is definitely modified by the effect but there is no sweeping evident.  The depth knob affects the tone and sweeping it manually gives a bit of the phaser effect.  The level knob works as expected.  There is no audible effect when moving the speed knob. 

At first, I thought it might be that the variable resistor was not set correctly.  After messing with this for about an hour, I don't think this was the underlying issue but I will need to reset it once I get the rest of the circuit working.  I could find spots where the tone was more sensitive to adjusting the resistance but it depended on the depth setting and I never got the sweeping effect.

I then focused my attention on the LFO section.  I think the issue is with the capacitor in this circuit (labeled C7 in the schematic https://www.electrosmash.com/mxr-phase90).  My multimeter doesn't have a capacitor function, it does read DC voltage, resistance and current.  I have tried measuring the voltage across the capacitor with different speed settings and get a constant value.  I have tested the speed pot and get a large range in resistance.  I expect to get a variable voltage across the capacitor.  Is this the correct way to test the circuit?  Does this mean the capacitor is bad?  Are there any other diagnostics I should do before replacing it?

[ElectricDruid]

It sounds like you're pretty close, and it definitely sounds like the LFO isn't oscillating.That could be the cap, but it could be several other things too. Check the voltage on the positive input of the LFO op-amp, and make sure it's the correct ratio of whatever the output voltage is (even if the LFO isn't running, the output must be at some voltage or other). If the LFO is running, you should see the voltage jumping about abruptly at the output pin of the LFO op-amp. If that isn't happening, the LFO isn't running and no amount of twiddling the bias trimmer is going to help. If it's not running, then we have to start thinking about what might stop it from working. The Pot going open circuit is one possibility. I always look at mechanical failures first, since that tends to be most likely. An old electrolytic cap is also going to be fairly suspect, so your guess on the 15uF cap would be the second place I'd look. After that, I dunno - dead op-amp? cracked track somewhere? end cap broken off a resistor (sometimes this can be really hard to spot).

HTH,
Tom

[DrT]

Hi Tom,

I already checked the pot.  It reads anywhere from a very low resistance up to 30 something kOhms.  The capacitor in question is ceramic and a local guru said that was unlikely.  He did think it could be the op amp.  I don't measure a varying voltage anywhere, though I am not 100% sure I am checking the correct junctions.  I just tried between the + terminal of the op amp and the leg of the capacitor that goes to ground. 

The op amp is a on a dual chip labeled 14558D.  I checked voltage between pins 3 and 1 which gave 17 mV.  I check pins 5 and 7, part of the depth circuit, and got 410 mV.  I am not sure exactly where I should be measuring and what I should expect.  I'd like to have some confidence that it is the op amp before I remove and replace 8 solder joints.  :-p

[ElectricDruid]

When I say "the cap" I'm talking about the 15uF electrolytic, C10, not the little ceramic C7. C7 is probably fine. C10 is the one to worry about.

0 to 30K doesn't sound right for a 470K pot! But if you're testing it in-circuit, all bets are off. To be sure, you'd have to desolder it, but you might learn something by testing the resistance between the outside lugs (to have an idea of what the max resistance looks like in circuit) and then testing the resistance between each of the outside lugs and the centre. One pair should give you zero wherever the pot is set. The other ought to vary between 0 and whatever your maximum was.

To test the op-amp, you could look for the voltage between ground and the output pin. It should jump up and down. The output pin is either pin 1 or pin 7 for the '4558, depending on which one of the two amps you used for the LFO.

Finally, IC sockets are your friend. Imagine being able to just pull the op-amp out and try a different one now. Wouldn't that be nice?

HTH,
Tom

[Transmogrifox]

One thing you can do to test the op amp is alternately short pin 3 between 9V and 0V using a 1k resistor.

If the op amp is working correctly you can expect the output (pin 1) to follow the input.  For example, if you pull Pin 3 up to 9V, you would expect the output Pin 1 to go up to something between 7.5V to 8.5V (wherever the op amp output saturates high).  Likewise, when you pull it down, you will see the output go down to somewhere between 0.5V to 1.5V (wherever the op amp output saturates low). 

Also when you are doing this if you have the phaser hooked up to an audio source and speaker you will hear the phaser swoosh each time you reverse it.  If you just hear a click or a pop, it's probably the electrolytic capacitor.  If nothing at all audible happens then it's probably some other component.

If the op amp output doesn't change when you force it at pin 3, then it is probably the op amp.

[DrT]

Thanks for the advice guys.  With you advice, I am pretty confident that the main issue is indeed the op amp.  I will send an update when the parts come in and it get it replaced.  I will probably have to replace the speed pot also but I'd to get the oscillation working first.  Thanks!

[DrT]

I finally got around to replacing the op amp and that fixed it.  Thanks for the advice.  A demo can be found https://www.youtube.com/watch?v=0Qmi0abALys.

[Mark Hammer]

Congrats on fixing it and thanks for the Youtube.  Tim Larwill, the fellow who makes them, is local, and I'm happy to say, a buddy of mine.  One of the more bizarre anecdotes he told me about the pedal is that he received an order from a Japanese retailer some years back, who was willing to place an order for 50 pedals IF Tim was willing to rename the pedal the "Shattered" Phaser (after the Rolling Stones song "Shattered" - https://www.youtube.com/watch?v=6nULwgHsVqw ), accompanied by suitable graphics.  Tim complied and got the order.  It was unique to Japan.

[ElectricDruid]

Excellent work. Well done!

+1 thanks for the video, nice to hear it going again.

T.