Help with squealing MXR Phase 90

[Gypsey Eyes]

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]

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]

Have you replaced R22 with its original value?

[Gypsey Eyes]

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

[Gypsey Eyes]

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

[52 Bill]

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]

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

[52 Bill]

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]

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]

The proper R22 is in there.

What value R28 have you been using?

[Gypsey Eyes]

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

[52 Bill]

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]

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]

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]

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?