I recently won a 'broken' vintage ('81) phase 90 off ebay. I got the pedal for $50, which seems like a good deal to me since a lot of the vintage (pre 82) MXRs go for upwards of $100. This one is particularly nice since it came at the time when MRX added new features (led), but before they changed the circuit.

Anyway, as I suspected the only apparent problem with the pedal is a couple of de-soldered wires. I'm sure i can trace the pedal and figure out where they go, but I wanna be sure.

Can someone point me towards a wiring diagram of a vintage phase 90? I am particularly interested in connections coming off of the board, but connections between all internal components would be helpful.

Thanks in advance for any help anyone can provide.

OK, here's some more details, there are two cables with floating ends. I've decided what I think they are. Could someone verify this for me? Or at least tell me i sound correct?

The first unconnected wire is orange and is attached to the 'tip' portion of the power jack. There are two other wires connected to this junction. One goes to the battery snap and the other goes to the LED. So I am assuming that this unconnected wire is power to the circuit.

The circuit board itself only has 5 wires connected to it, and i think it should have 6. They would be power, ground, effect in, effect out, and 2 wires for the pot. I have accounted for the 2 pot wires, ground, and effect out.

The second unconnected wire is yellow and is connected to the circuit board. I think that this must be, by process of elimination, the effect in to the circuit. To further reinforce this fact, there is only one wire connected to the 'tip' connection of the input jack. I think i can see a place where this wire might have broken off of the input jack, but i am not sure.

Do these assumptions sound reasonable?

33 views, and no help.

I haven asked many questions in this forum, but every time I have I got at least some help. I am disappointed.

Oh well. I tested out my assumptions and it turns out I was right. So I'm only 2 solders away from having a working phase 90. Though, i don't think it's AS vintage as I once thought, but it will do for now.

I have another question, but I'll ask in a new thread.

You live in Rochester.  Certainly there must be *someone* from MXR in your neighbourhood who can help you out!

But seriously, the power jack will generally have 3 wires coming from it.  One will be ground, one will be a power wire bringing the supply voltage to the board.  The third will be the red wire from the battery snap.    With nothing plugged into the jack, the battery snap is connected to the power lead to the board.  Plug in a wallwart and the spring connection to the power lead is moved aside a little so that it is no longer in contact with the battery lead.  This enables the battery with no external power and disables it when external power is applied.

You should be able to figure where to connect the power wire up to by tracing the V+ pins on the chips back to some reasonable "starting point".  V+ will be pin 8 on any dual op-amps like 4558's or 1458's, and pin 7 on any singles like 741 or TL071 chips.    Find out where all the power pins link to and see if there is any diode or small value current-limiiting resistor in series with them.  The power wire will connect to the open end of that small resistor.  That advice will be valid for most pedals using op-amps.

Thanks, I checked the data sheet and the traces, and it does look like power.

Only one connection left.