Phase 90 mods

[cdhopkin]

Hi guys,

I've just bought a Dunlop Reissue Phase 90 from "The Bay" (I had an original block logo Phase 100 and really kick myself for selling it a few years ago) - now I know about the R28 mod but is it possible to add a switch to the Phase 90 to select 2/4 phase stages (i.e. changing the Phase 90 to a Phase 45)? 

Cheers

Chris

[Mark Hammer]

Yes.  The trick will be in identifying where the output of the 2nd stage is.  I'm unfamiliar with the Dunlop unit's layout so I couldn't advise in that regard.  I'll let somebody else handle that.

I will draw your attention to the fact that a 2-stage phaser, à la P45, has no recirculation path and probably shouldn't.  So, once you identify the output of stage 2, whatever switch you install should also disable the regen path when you switch from 4 to 2 stages.  easy to do since it is just a matter of cutting a connection.  A DPDT toggle will easily accomplish both tasks.

[Sweetalk]

I have some pictures from a MXR Phase 90 Reissue that can be helpfull... are from the component side and I put the tracks over it for easy-looking and modding. I used this image that I made to do the Vibrato mod, also you can build one with this layout!. The pictures are big, sorry!!

Components + Tracks



Components only


Tracks only


And this is the pedal...



If my memory doesn't betray me, the IC from the left it's the input buffer and LFO. The 150K resistor (Brown-Green-Yellow) labeled as "Vibrato" it's the one to lift if you want the vibrato mod, lift the left side of the resistor, going to pin 1 of the Left IC.

The Center and Right IC are the phase shifting stages. The 24K resistor going from Center IC pin 1 to Right IC pin 6 it the "Feedback" resistor that Mark Hammer pointed to lift if you want to make 2 stages phaser.

So, the Center IC must be the last stages, pin 1 the output for the last stage. You have to find the output of the second stage and you are very close to make it. A DPDT toggle will do the trick as Mark Hammer said.

I hope it helps!!!

[R.G.]

Also be aware that there is another difference between the P90 and P45. The P45 uses a more complicated JFET circuit for the variable phasing resistors. They each have a couple of additional resistors and a cap that give local feedback and linearize the JFET so it doesn't distort on signals until much later than the P90 version.

[mac]

Also the mixers resistors are different, and the input stage.
Given the number of differences, I´d go for a P45 clone, IMHO.

mac

[cdhopkin]

Yeah...I think you are right there...I'm going to give it a good test and I think I'll do the R28 mod if I have to and then see what else i can do.

Thanks for all the suggestions )

[R.G.]

@ Sweetalk:
I tried to email you, but it bounced. Can you send me a private message or an email address where I can reach you?

[Sweetalk]

@ Sweetalk:
I tried to email you, but it bounced. Can you send me a private message or an email address where I can reach you?

Send me a PM!!! I been having problems with the mail lately 

cdhopkin I think you can still have a 2 stages - 4 stages selector, not 100% phase 45 but still very usefull mod. With one circuit of a DPDT lift the 24K Feedback resistor, and with the other circuit switch between the output of the second stage and the output of the fourth stage. I think that will do it very well...

[cdhopkin]

Sweettalk sounds like a good idea...time to get dust down my schematic reading skills   Is the 500k pot log or lin because I could always replace the speed pot with a 500k push pull to do that...Would look a bit more elegant than bodging a switch on.

[Sweetalk]

Sweettalk sounds like a good idea...time to get dust down my schematic reading skills   Is the 500k pot log or lin because I could always replace the speed pot with a 500k push pull to do that...Would look a bit more elegant than bodging a switch on.

The pot it's Reverse Log... yo can replace it with a Lin or Log also... the control will not be as "sensitive" as is now but you can use it very well... I build a couple Phase 90 with Lin pots and it's OK the speed control.

A Push/Pull will be VERY nice and no appereance change, good idea... If you don't like it just put the original pot back and nothing happened!!!... BTW, your Phase 90 it's the same that in the pictures I post?

[Solidhex]

A push pull would be nice but depending on which type you get you might not be able to fit the pcb back on top of it. Its pretty tight in those things...

--Brad