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Vintage threads from the first ten years

MXR Phase 100 problems

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11/21/1997 1:22 AM
Anders Westerberg	MXR Phase 100 problems I'm looking for the schematics for MXR Phase 100. Can anyone help me or does anyone now if there is any common problems with any components in these? The only sound coming out is a high pitch tone following the sweep generator. It has six TL022 and three optocouplers. Anders

11/21/1997 7:27 AM

For all the classical phasers (MXR 100 included), you can debug the thing by disconnecting the sweep circuitry so that there is NO drive to the sweep elements (in this case the LED's in the LED/LDR modules) and then getting audio to go through the audio path, then reconnecting and debugging the sweep circuit.

In the MXR 100, lift one pin of each opto coupler or trace the + side of the LED's back to the drive opamp and disconnect all of them there. Then get clean audio to come through them.

Do this by first getting the DC levels right. Measure the power pins on the opamps, usually pin 8 (+) and pin 4 (-) on duals like the 022. If pin 8 is positive and pin 4 is negative relative to ground, then you have a bipolar supply and the thing should bias with the + and - inputs to the opamps and the output all at very close to 0V respect to ground. If pin 4 is at 0V, then the signal pins should all be at about half of the 8in 8 voltage. Check all of the power pin and in/out pin voltages and find out why they aren't right if they aren't. In your case, one of the opamp stages is oscillating, could be any one. One "shotgun" approach is to simply replace all of the opamps. This costs about US$0.50 per opamp and may be cheaper than the debug time.

11/21/1997 4:30 PM
Anders Westerberg	Thanks for the tip, I'll try it as soon as I can. The supply is not bipolar, i.e. pin 8 is at 9V and pin 4 is at 0V. I've already considered using the "shotgun" method (even if it feels a little unsatisfying never knowing which part was faulty) but since I didn't know if the optocouplers were likely to break down I didn't want to waste time on it. Anders

11/24/1997 4:42 AM
Anders Westerberg	Unfortunately there was no obvious fault in the input/output voltages, so my next step is to follow the signal with a scope. As I mentioned in my first question, the box is equipped with TL022's, but since they're no longer is avaliable I now wonder what the best substitute will be. Probably a TL072, but since I cant find any data about the TL022 I don't know how big the difference is. Anders

11/24/1997 6:11 AM
Preben Hansen	Hello Anders. TL022 is a very low power IC-OPAMP. I think you could use TL072, but it depends on the housing that TL022 has. TL072 will eat batteryes mor quickly than 022. try http://www.ti.com I got the data-sheet for TL022 from there this morning. Data-sheets are in pdf-format Preben

11/24/1997 6:35 AM
Anders Westerberg	Hi! Thanks a lot for the tip! I compared the two IC's, the TL022 draws 130uA and the TL072 draws 1.4mA, so I'm afraid it's not a good substitute. Any suggestions for substitutes? TL022 seems hard to find (at least in Sweden). Anders

11/24/1997 7:02 AM
Preben Hansen	Hi again Anders. If you deside ONLY to to run this pedal from the mains, via a power-supply, 9 or 18 volts, then i think it should be ok to to use tl072. By the way: DOD FX75B and BOSS BF-2 both uses TL022. Preben.

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