MXR Phase 90

I've yet to see the original factory schematic for the Phase 90. Many traced schematics exist. Any excellent analysis with schematic can be found at ElectroSmash.

This type of phase shifter uses op amps and matched JFETs as variable resistances. The signal is subjected to 4 stages of phase shift in series to enhance the effect.

The Phase 90 was MXR's first product. They started to sell them in 1974. Earliest examples use an off the shelf Bud enclosure, followed by enclosures branded MXR in script font. The script logo was used in the beginning, but Ron Wilkerson's wife thought it was too feminine and suggested the more masculine block logo that appears on later versions (from Dunlop's 40th anniversary videos).

Troubleshooting Case Study

Pictures above are of a script logo example. It has a late '75 pot, but mid '76 date codes on the op amps, so this particular one probably left in factory in '76 or '77. Op amps are all LM741. Someone had already removed the decaying foam, but they failed to install another insulator. I like manila file folders for this job.

This particular one was a basket case. On the initial test, it just passed an attenuated signal, no phasing, when the effect was engaged.

That low output problem I traced to the output mixer transistor. The dc voltages there didn't make sense unless the transistor was open, and sure enough it was. Signal was just squeaking through the 150k shunt feedback resistor. Shunt feedback stages, both transistor and op amp, will often do this; the active element is dead/open but the signal squeaks through since there is a path to the output.

There was still no phasing effect. LFO wasn't moving. Checked the pot, and it too was open. This type of pot can be opened up. I did so, and the carbon track was cracked. While you can monkey with this to repair it, I opted for the more solid solution of replacement.

Even with the LFO moving again, I wasn't getting a phase effect. FET bias trim pot was also open! One 200k trim pot later (the original was a 200k), and I could barely get the FETs into conduction. Note there is a 1M resistor in series with the trim wiper. I was getting just a taste of phase when I had the trim pegged, so I added another 1M in parallel, as a test, and this solved the phasing issue. My trim now worked like it should, and I used my scope's XY display to zero in on maximum phasing effect (you can do this by ear too, but I find I get great results quickly with the XY scope method).

After all that, the footswitch was intermittent, so that had to be replaced too.