Help with squealing MXR Phase 90

Gypsey Eyes · 10-25-2009, 02:41 AM

Hi all,

I bought a reissue Phase 90 that wasnt working (the guy sad he botched the r28 mod and couldnt be bothered to fix it).

I thought it'd be a simple reinstallation of the resistor but aparently not.

When I put the resistor back on I get a horrbile sqeal from the pedal when it is switched on. I get the same thing when running a direct connection in lieu of the resistor. The bypass works fine and I get normal clean sound. When I have no resistor or direct connection with the r28 pins, I get a clean sound with a slight volume boost when the pedal is on.

I see no signs of a short and dont know where to look next. Any suggestions?

Gypsey Eyes · 10-25-2009, 07:31 AM

if it helps, I also notice that the original "modder" replaced the R22 2.1m ohm resistor with a 1.2 m ohms one.

here's a schematic: MXR Phase 90.GIF

not sure if this would cause the screeching.

Phostenix · 10-25-2009, 06:00 PM

Have you replaced R22 with its original value?

Gypsey Eyes · 10-25-2009, 06:40 PM

yeah I forgot to mention I tried that and got the squeal. I havent tried reinstalling a 2.1 resistor yet though.

Gypsey Eyes · 10-28-2009, 03:14 AM

okay - tired reinstalling proper R22 resisotr and same problem. Anyone have any ideas or suggestions on what to test with multimeter?

52 Bill · 10-28-2009, 10:30 PM

Quote:

Originally Posted by Gypsey Eyes

okay - tired reinstalling proper R22 resisotr and same problem. Anyone have any ideas or suggestions on what to test with multimeter?

R28 is the feedback/regeneration resistor. R22 limits the osc. control voltage to the fets.

Even if R28 was out of circuit, the phase shift should work at least weakly. There is something else wrong with the circuit.

I'd start by leaving R28 out for now and start checking to see if the low freq. osc. is running. Read the voltage to ground at R22. There should be a rising and falling voltage there. (Set the speed control to a slower setting.)

Gypsey Eyes · 10-28-2009, 10:41 PM

With no R22 resistor in I do get a slightly boosted tone with a very faint swishing effect.

52 Bill · 10-29-2009, 05:44 PM

Quote:

Originally Posted by Gypsey Eyes

With no R22 resistor in I do get a slightly boosted tone with a very faint swishing effect.

Do you mean no R28?

If R22 was missing, there would be no osc. control voltage to the fets and there could be no swishing.

Do you have a voltmeter?

Gypsey Eyes · 10-29-2009, 05:52 PM

my apologies - you are right, I get the slight swishing with no R28 resistor in. The proper R22 is in there.

I have a cheap multimeter but limited skills using it.

52 Bill · 10-30-2009, 11:04 PM

Quote:

Originally Posted by Gypsey Eyes

The proper R22 is in there.

What value R28 have you been using?

Gypsey Eyes · 10-31-2009, 04:57 AM

I've tried a 24k (original) and with a jumper. Both cause the horrible screeching.

52 Bill · 10-31-2009, 05:21 PM

Quote:

Originally Posted by Gypsey Eyes

I've tried a 24k (original) and with a jumper. Both cause the horrible screeching.

Using a jumper is the opposite of what you should have tried, if anything you should have tried a higher value resistor.

In any case, there is something else wrong with the pedal, as the 24K value should have worked. Have you checked to see if there were any other parts changed by the previous owner?

Gypsey Eyes · 10-31-2009, 07:26 PM

Yeah I figure there's a short somewhere or someting fried when he tried mod it. I see no other signs of replaced parts and I'm not skilled enough to test caps, chips and the like.

It'll make a neat paperweight I guess

f13ndus · 11-07-2009, 08:39 PM

You say you hear swishing and a boost when the pedal is turned on - Can you hear a difference in the speed of the swishing when you turn the speed pot?

If so, there is a trim pot mounted on the top of the board (R27 on the schematic) that adjusts the transistor bias - It should be glued into position, but it's entirely possible the previous owner tweaked it for some reason. You could mark the current position and then try different positions to see if that solves your problem. If I wasn't at work, I'd try it on my own pedal to see if I could reproduce the symptoms of yours.

By the way, I've done the R28 mod to my pedal, I just desoldered one side of R28 and lifted it out of the board, same as removing it entirely. Sounds a lot better IMHO!

Gypsey Eyes · 11-07-2009, 10:18 PM

hey thanks for the tip. I tried what you suggest as turning the rate knob does change the sound of the squeal slightly.

Rotating that r27 pot glued to the board with an allen key, however, only further changed the rate of the squeal. I dont think that is the problem.

any other ideas?

52 Bill · 11-10-2009, 12:07 AM

Quote:

Originally Posted by Gypsey Eyes

I tried what you suggest as turning the rate knob does change the sound of the squeal slightly.

Are you saying the rate control does work? That is, it changes the speed of the control voltage oscillator.

Gypsey Eyes · 11-10-2009, 02:52 AM

yeah it definately does something - but the squeal is still horrible.

52 Bill · 11-10-2009, 05:52 PM

Try this.

First you don't need to plug the pedal into the amp for this test, so don't worry about the screech. Plug a cord into the input jack to turn on the battery. Take your voltmeter and set it to a range that will handle 9 volts dc. Ground the black meter lead to any convenient ground point like the metal case. I will usually stick the probe into one of the four screw holes for the bottom plate. To test the ground connection of your meter, touch the red lead to the battery terminals and see if you can read 9 volts dc.

Now take the red meter lead and touch it to one of the terminals of the rate pot and see if there is a voltage there. If the oscillator is working, there should be a voltage that rises and lowers at a rate set by the pot. If this voltage is there, then you know that the oscillator is working.

If the oscillator is working, and the pedal will pass signal when it is on, then you may assume that the TL062 dual op amp is ok, as this chip controls the input buffer and the oscillator circuit.

If the oscillator is not working, then you will need to troubleshoot the circuit around the TL062 chip.

Whenever I have to work on anything that has been worked on by a novice, I will inspect every possible thing that the hacker could have worked on. One problem with modern pc boards is that they tend to be fragile. Often the pad where the part is inserted will break off from the board and the part will no longer connect to the circuit. Then of course there are always bridged solder traces and broken part leads. Be sure to check everything on the board, even though the original owner said he only did one mod he may have done other things as well.

Gypsey Eyes · 11-14-2009, 05:46 AM

Hey thanks for the tip - I checked the voltage and it looks like the oscillator and TL062 chip are okay. It must be a short or bad part somewhere further down the circuit.

52 Bill · 11-14-2009, 05:42 PM

The only other things are the FETs, a transistor and the quad opamp (TL064).

Based on the symptoms, the TL064 would be my first guess. Using the meter the same way as before, read the voltages on the pins of the TL064. Because of the single voltage power supply, there will be voltages on all of the outputs. Look for odd or missing voltages. Or just post your findings here and we can help you figure if there is a problem.

I guess the easiest thing to do would be to just replace the chip and see what happens.

In my experience, the newer Dunlop boards are very fragile with tiny traces that can be damaged easily. Carefully inspect the previous owners work to see if there are any signs of damage to the traces, pads, etc.

Gypsey Eyes · 11-14-2009, 07:07 PM

ok here's what I found but one question first -

should the voltage of all three prongs for the rate pot change? I notice that only two of the three seem to change - one stays the same.

now onto the TL604:

these are the readings
14 6.74
13 6.72
12 6.57
11 8.98
10 6.48
9 6.74
8 6.72
7 6.74
6 6.72
5 6.58
4 0
3 6.47
2 6.73
1 6.72

Some of the readings vary every testing by up to .10v

Gypsey Eyes · 11-14-2009, 07:17 PM

I've also checked the board all over for other damage. the only thing I keep wondering about is a mark where the orig 'modder' snipped the R28. Looks like his wire cutters cut into the board when he did it but upon magnification it looks only like surface damage to a grounded part.

52 Bill · 11-16-2009, 06:02 PM

Two of the three terminals of the rate pot are connected together, and it really doesn't matter, as we were only trying to see if the oscillator was working.

I'm guessing that you have counted the pins wrong on the TL064, as pin #4 should have 9vdc and pin#11 should be at 0. But in any case, none of the voltages seem to be abnormal.

You should test the board with your meter and not just your eyes. If your meter has a continuity setting use it to follow the traces around r28. Sometimes something might look ok, but in fact isn't. Use the ohm reading setting if you don't have a continuity setting.

Has the trim pot r27, been messed with? Older versions of this pedal would have the trim pot either soldered or painted to lock in the adjustment spot.

Gypsey Eyes · 11-16-2009, 09:00 PM

yes, I was looking at the chip from the backside rather than the front so all my numbers are reversed.

All the connections coming from r28 appear to test fine with my meter - I also see no sign of the trim pot being replaced or any setting mark on it.

52 Bill · 11-16-2009, 10:55 PM

What are the values of C11 and C12? Have these been changed? Apparently these are a couple of other parts that are often messed with.

When the pedal is screeching, is the screech a solid tone or does it change with the low freq. osc.?

Gypsey Eyes · 11-17-2009, 01:41 AM

both caps look original - c11 says 103; c12 is hard to see but it looks like 681.

I uploaded a video that shows what happens when I turn the oscillator pot and then the rate pot.

YouTube - bkplant's Channel

52 Bill · 11-17-2009, 05:44 PM

You know, this video thing might just catch on.
The sound is really cool, maybe someone would want it as a signal generator.

I sounds like too much regeneration. Again I assume that this video was shot with the original 24K value for R28 in circuit.

For lack of anything else to try, tweak the bias pot R27. Mark it so you know where it was originally set before you start turning it. You may be able to reduce the effect enough to kill the feedback.

10-25-2009, 06:00 PM	#3
Phostenix Senior Member Join Date: Jul 2009 Location: Phoenix, AZ Posts: 291	Have you replaced R22 with its original value? __________________ ST in Phoenix

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10-25-2009, 02:41 AM	#1
Gypsey Eyes Junior Member Join Date: Oct 2009 Posts: 16	Help with squealing MXR Phase 90 Hi all, I bought a reissue Phase 90 that wasnt working (the guy sad he botched the r28 mod and couldnt be bothered to fix it). I thought it'd be a simple reinstallation of the resistor but aparently not. When I put the resistor back on I get a horrbile sqeal from the pedal when it is switched on. I get the same thing when running a direct connection in lieu of the resistor. The bypass works fine and I get normal clean sound. When I have no resistor or direct connection with the r28 pins, I get a clean sound with a slight volume boost when the pedal is on. I see no signs of a short and dont know where to look next. Any suggestions?

10-25-2009, 07:31 AM	#2
Gypsey Eyes Junior Member Join Date: Oct 2009 Posts: 16	if it helps, I also notice that the original "modder" replaced the R22 2.1m ohm resistor with a 1.2 m ohms one. here's a schematic: MXR Phase 90.GIF not sure if this would cause the screeching.

10-25-2009, 06:40 PM	#4
Gypsey Eyes Junior Member Join Date: Oct 2009 Posts: 16	yeah I forgot to mention I tried that and got the squeal. I havent tried reinstalling a 2.1 resistor yet though.

10-28-2009, 03:14 AM	#5
Gypsey Eyes Junior Member Join Date: Oct 2009 Posts: 16	okay - tired reinstalling proper R22 resisotr and same problem. Anyone have any ideas or suggestions on what to test with multimeter?

10-28-2009, 10:41 PM	#7
Gypsey Eyes Junior Member Join Date: Oct 2009 Posts: 16	With no R22 resistor in I do get a slightly boosted tone with a very faint swishing effect.

10-29-2009, 05:52 PM	#9
Gypsey Eyes Junior Member Join Date: Oct 2009 Posts: 16	my apologies - you are right, I get the slight swishing with no R28 resistor in. The proper R22 is in there. I have a cheap multimeter but limited skills using it.

10-31-2009, 04:57 AM	#11
Gypsey Eyes Junior Member Join Date: Oct 2009 Posts: 16	I've tried a 24k (original) and with a jumper. Both cause the horrible screeching.

10-31-2009, 07:26 PM	#13
Gypsey Eyes Junior Member Join Date: Oct 2009 Posts: 16	Yeah I figure there's a short somewhere or someting fried when he tried mod it. I see no other signs of replaced parts and I'm not skilled enough to test caps, chips and the like. It'll make a neat paperweight I guess

11-07-2009, 08:39 PM	#14
f13ndus Member Join Date: Oct 2008 Posts: 32	You say you hear swishing and a boost when the pedal is turned on - Can you hear a difference in the speed of the swishing when you turn the speed pot? If so, there is a trim pot mounted on the top of the board (R27 on the schematic) that adjusts the transistor bias - It should be glued into position, but it's entirely possible the previous owner tweaked it for some reason. You could mark the current position and then try different positions to see if that solves your problem. If I wasn't at work, I'd try it on my own pedal to see if I could reproduce the symptoms of yours. By the way, I've done the R28 mod to my pedal, I just desoldered one side of R28 and lifted it out of the board, same as removing it entirely. Sounds a lot better IMHO!

11-07-2009, 10:18 PM	#15
Gypsey Eyes Junior Member Join Date: Oct 2009 Posts: 16	hey thanks for the tip. I tried what you suggest as turning the rate knob does change the sound of the squeal slightly. Rotating that r27 pot glued to the board with an allen key, however, only further changed the rate of the squeal. I dont think that is the problem. any other ideas?

11-10-2009, 02:52 AM	#17
Gypsey Eyes Junior Member Join Date: Oct 2009 Posts: 16	yeah it definately does something - but the squeal is still horrible.

11-10-2009, 05:52 PM	#18
52 Bill Senior Member Join Date: Jun 2007 Posts: 1,125	Try this. First you don't need to plug the pedal into the amp for this test, so don't worry about the screech. Plug a cord into the input jack to turn on the battery. Take your voltmeter and set it to a range that will handle 9 volts dc. Ground the black meter lead to any convenient ground point like the metal case. I will usually stick the probe into one of the four screw holes for the bottom plate. To test the ground connection of your meter, touch the red lead to the battery terminals and see if you can read 9 volts dc. Now take the red meter lead and touch it to one of the terminals of the rate pot and see if there is a voltage there. If the oscillator is working, there should be a voltage that rises and lowers at a rate set by the pot. If this voltage is there, then you know that the oscillator is working. If the oscillator is working, and the pedal will pass signal when it is on, then you may assume that the TL062 dual op amp is ok, as this chip controls the input buffer and the oscillator circuit. If the oscillator is not working, then you will need to troubleshoot the circuit around the TL062 chip. Whenever I have to work on anything that has been worked on by a novice, I will inspect every possible thing that the hacker could have worked on. One problem with modern pc boards is that they tend to be fragile. Often the pad where the part is inserted will break off from the board and the part will no longer connect to the circuit. Then of course there are always bridged solder traces and broken part leads. Be sure to check everything on the board, even though the original owner said he only did one mod he may have done other things as well.

11-14-2009, 05:46 AM	#19
Gypsey Eyes Junior Member Join Date: Oct 2009 Posts: 16	Hey thanks for the tip - I checked the voltage and it looks like the oscillator and TL062 chip are okay. It must be a short or bad part somewhere further down the circuit.

11-14-2009, 05:42 PM	#20
52 Bill Senior Member Join Date: Jun 2007 Posts: 1,125	The only other things are the FETs, a transistor and the quad opamp (TL064). Based on the symptoms, the TL064 would be my first guess. Using the meter the same way as before, read the voltages on the pins of the TL064. Because of the single voltage power supply, there will be voltages on all of the outputs. Look for odd or missing voltages. Or just post your findings here and we can help you figure if there is a problem. I guess the easiest thing to do would be to just replace the chip and see what happens. In my experience, the newer Dunlop boards are very fragile with tiny traces that can be damaged easily. Carefully inspect the previous owners work to see if there are any signs of damage to the traces, pads, etc.

11-14-2009, 07:07 PM	#21
Gypsey Eyes Junior Member Join Date: Oct 2009 Posts: 16	ok here's what I found but one question first - should the voltage of all three prongs for the rate pot change? I notice that only two of the three seem to change - one stays the same. now onto the TL604: these are the readings 14 6.74 13 6.72 12 6.57 11 8.98 10 6.48 9 6.74 8 6.72 7 6.74 6 6.72 5 6.58 4 0 3 6.47 2 6.73 1 6.72 Some of the readings vary every testing by up to .10v

11-14-2009, 07:17 PM	#22
Gypsey Eyes Junior Member Join Date: Oct 2009 Posts: 16	I've also checked the board all over for other damage. the only thing I keep wondering about is a mark where the orig 'modder' snipped the R28. Looks like his wire cutters cut into the board when he did it but upon magnification it looks only like surface damage to a grounded part.

11-16-2009, 06:02 PM	#23
52 Bill Senior Member Join Date: Jun 2007 Posts: 1,125	Two of the three terminals of the rate pot are connected together, and it really doesn't matter, as we were only trying to see if the oscillator was working. I'm guessing that you have counted the pins wrong on the TL064, as pin #4 should have 9vdc and pin#11 should be at 0. But in any case, none of the voltages seem to be abnormal. You should test the board with your meter and not just your eyes. If your meter has a continuity setting use it to follow the traces around r28. Sometimes something might look ok, but in fact isn't. Use the ohm reading setting if you don't have a continuity setting. Has the trim pot r27, been messed with? Older versions of this pedal would have the trim pot either soldered or painted to lock in the adjustment spot.

11-16-2009, 09:00 PM	#24
Gypsey Eyes Junior Member Join Date: Oct 2009 Posts: 16	yes, I was looking at the chip from the backside rather than the front so all my numbers are reversed. All the connections coming from r28 appear to test fine with my meter - I also see no sign of the trim pot being replaced or any setting mark on it.

11-16-2009, 10:55 PM	#25
52 Bill Senior Member Join Date: Jun 2007 Posts: 1,125	What are the values of C11 and C12? Have these been changed? Apparently these are a couple of other parts that are often messed with. When the pedal is screeching, is the screech a solid tone or does it change with the low freq. osc.?

11-17-2009, 01:41 AM	#26
Gypsey Eyes Junior Member Join Date: Oct 2009 Posts: 16	both caps look original - c11 says 103; c12 is hard to see but it looks like 681. I uploaded a video that shows what happens when I turn the oscillator pot and then the rate pot. YouTube - bkplant's Channel

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11-17-2009, 05:44 PM	#27
52 Bill Senior Member Join Date: Jun 2007 Posts: 1,125	You know, this video thing might just catch on. The sound is really cool, maybe someone would want it as a signal generator. I sounds like too much regeneration. Again I assume that this video was shot with the original 24K value for R28 in circuit. For lack of anything else to try, tweak the bias pot R27. Mark it so you know where it was originally set before you start turning it. You may be able to reduce the effect enough to kill the feedback.