MXR Phase 90 Mods
Within the last couple of years, or at least since the EVH Phase 90 was brought to market, the orange reissue block Phase 90′s (with the exception of the Script logo models) use SMT components. This makes modifications a little tricky. From what I have seen, the component numbers are not the same as earlier conventional component models. This means that R28 on a SMT board is not the same R28 on a non-SMT board. That being said, these mods apply only to the non-SMT component models.
Please note before continuing that these mods will NOT turn your reissue in to a script logo version. They will however, help improve the sound of the circuit.
The first, and most common mod, is to remove the feedback resistor, R28 (24K). This will make it sound somewhat like the classic script version and pretty much eliminate the distorted mid-boost. If this is not enough, then remove C11 and C12, this mod will remove the pre and de-emphasis (noise filtering).
On the reissue board photo (link below), C11 is .01uf & C12 is 680pf. R7 (150k), which controls output level, is directly above C12 in the photo. If you remove all three (R28, C11, and C12), lowering R7 to 130k gives unity gain in most setups. Re-adjusting the trimpot afterwards will help too. Mark the trimpot first with an index line so you know what the original setting was.
Reissue Board
Switchable
If you want to add a switch to go between script and block logo with the R28 mod, follow these steps (diagram included).
DPDT
- Remove R28 from it’s current location.
- Connect the resistor across two of the outside lugs of a DPDT on-on switch.
- Connect a wire to each of the middle lugs.
- Connect each wire from the middle lugs to the holes were the 24k resistor was connected to the PCB.
SPST
- Lift one leg of R28 from it’s current location.
- Connect the resistor to the outside lug of a SPST switch.
- Connect a wire from the middle lug to the hole were the 24k resistor was connected to the PCB.
Output Buffer
Does your Phase 90 suck tone like a stock wah? Here’s a simple and easy solution to the problem, an output buffer. The buffer will sit between the DPDT switch’s output lug and the output jack’s tip lug. The signal is then passed through the buffer and the lost high end and definition is restored, both bypassed and when the phaser is in use.
First, choose your buffer. The smaller the better, as space is limited inside the Phase 90 enclosure with the PCB in place. My choice was a simple IC-based buffer that I had built using a very small and compact layout. More information on this buffer can be found on the General Guitar Gadgets web site.
Next, wire the buffer in place. The IN and OUT refer to the buffer’s input and output. The ciruit is to the bottom right of the picture, right above the battery. It’s a tight fit but it works. With longer wires and a little foam insulation it could probably be placed in a few other spots as well. The +9V wire may need to be moved to the solder point to the left of where it is indicated in the picture.
And finally, cut the trace connecting the DPDT switch’s output lug to the output jack’s tip lug. It is circled in the photo. Use an exacto knife or sharp razor blade. The finish on these PCB’s is thick so it may take a little extra work. These are multi-layer PCB’s so be careful not to go too deep or another trace may be accidentally cut.
This is a nice alternative if you don’t want to cut a section of the PCB out and put in a 3PDT, which has been done by some, or are limited to space on your pedalboard leaving out the true bypass box option. It’s quick, simple, and effective.