Modified MXR Phase 90

 

Read this first!

It seems that Dunlop has changed the standard Phase 90 again, so all models (except the handwired 1974 vintage model) now use the circuit board from the EVH model. Its construction style means that I am at the moment not able to modify them to true bypass or Stage 2. Further down the page, you'll find information on how to identify which pedal you have, and if it can be modified.

 

Standard

The Phase 90 is one of the true classics - it has been around since the mid '70s, and is still sold in heaps. Although the exterior has stayed more or less the same, there has been a few different versions throughout the years. The first series was the "script logo" and is easily recognized from the logo in script writing. Around 1978 MXR switched to the look they still have ("block logo"), and around the same time a feedback path was introduced in the phase stages, making the phaser more pronounced. Later, the circuit was also redesigned. Instead of using six single op-amps, MXR streamlined the circuit to use three double op-amps. This means that there are block logo pedals that have the script logo circuit inside, both with and without the feedback path. There are plenty of information (and pictures) on the web, iif you want to check yours or one you're thinking of buying.

After that, not much happened - during the first half of the '80s, pedals with LEDs and adapter jacks (3.5mm plug) started appearing, but the actual circuit didn't change too much until the company went bust in the mid '80s. Dunlop later bought the rights to the entire line of MXR pedals, and started production right where the previous owners had stopped.

Around 1990 Dunlop redesigned the entire MXR line, and started mounting the jacks, switch and pot(s) right on the circuit board. This was of course done to speed up production and save money (no hand-soldering was needed). At the same time, a buffer stage was added to the input of the circuit, in an attempt to minimize the signal loading mentioned below. The pcb-mounted jacks and switch makes modifications to true bypass a bit harder, but it is still doable. However, as of right now (2008), Dunlop seems to have changed the design again, using the EVH circuit board for all models (except the handwired 1974 model). These pedals are currently not possible to mod.

All Phase 90 - regardless of version/age - has the simpler form of bypass system, switching only the output signal. This means that the pedal has a tendency to steal signal in bypass mode, especially if it is placed first in the chain. Placing an active pedal in front of it helps, but the the pedal's ideal placement in the chain is decided by other factors than simply where the effect sounds best. When modified to true bypass, the pedal becomes 100% independent - when bypassed, it is completely out of the chain, as if it weren't there in the first place.

 

Modification stage 1:

I start out by modifying the pedal to True Bypass. On the older pedals (circuit board packed in rubber foam and free-standing switch with wires to/from it) it is a relatively easy mod, and if the pedal didn't have an indicator LED and adapter jack, that can be arranged too. On the newer pedals (circuit board like the picture on the right, either green or red with a Boss-type barrel plug) it is quite a bit harder. I need to cut out part of the circuit board to fit the new switch, something that is naturally reflected in the cost.

On modern (Dunlop reissue) pedals I also remove the feedback resistor, to make it sound closer to the "script logo" pedals. The modification itself is really simple - I just remove a resistor (R28) - and I don't even use the soldering iron. It is quite easy to lift the copper traces when removing or replacing components, so in this case (where I simply need to remove a component), a pair of sharp pliers is the best tool. Anyway, he difference in sound is quite prominent - after the mod, the pedal is much softer and less pronounced.

Please note that the second half of the stage 1 mod (the "script mod" part) may not appear to all older pedals. Some of the earliest Dunlop pedals, as well as some of the original block logo MXR pedals, might not have the feedback resistor in question. However, since that part of the modification is as simple as it is, this will not make any difference in the modification cost - you will still only pay for the true bypass mod, regardless of if R28 needs to be removed or not.

 

Modification stage 2:

I start out by doing the stage 1 mods, and then replace a couple of components to change the phase shift a bit. This gives the pedal a little of the "bubbly" character of the Uni-Vibe, and reduces the pitch alterations the stage 1 pedal has. The stage 2 Phase 90 loses a little of that glimmering phaser sound, and gains a more uniform, pulsating sweep. As the detuning effect is reduced (most notably on the lower strings), the stage 2 version works beautifully in front of an overdrive for Hendrix/Robin Trower type sounds. This mod works on all Phase 90, regardless of age or version.

 

Which pedals can be modified?

The newer pedals (the EVH model, Custom Shop Script with or without LED plus the regular Phase 90 with the Rev B circuit) can not be modified. These pedals are easily recognized by the "Rev B" legend at the output jack (image). They also have far less solder points, as the components are surface mounted on the inside. If you plan on buying a brand new M-101 to mod, you need to open it up first to see if it has the "right" circuit board.

Do it yourself?

Why not. The "script" part of the mod is very easy, and you don't even have to solder to do it. Here is an excellent instruction on how to mod a modern reissue Phase 90 (LED and Boss type adapter jack). All you need to do is snip one end of the resistor off - you don't even have to remove it. Do not desolder the resistor - you'll risk damaging the copper inside the holes, which doubles as a pass-through between the circuit board front and back. Just clip one end off, and you should be fine.

 

Prices:

Original MXR and older Dunlop
New (reissue) Dunlop/MXR
Stage 1: 250 SEK / 23 EUR
Stage 1: 400 SEK / 37 EUR
Stage 2 (incl TB): 300 SEK / 27 EUR
Stage 2: 450 SEK / 41 EUR
Add LED: +50 SEK / 4.50 EUR
Add LED and adapter jack: +100 SEK / 9 EUR
 

 

Prices do not include shipping.

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