18V through protection diode

[garfo]

I am considering buying a EVH MXR Phase 90. The owner has run 18 volts through the pedal and it stopped working. Before I buy the pedal, I wanted to know if the protection diode that, as far as I know, is there to protect against wrong polarity, would let the 18 volts go through and burn the other components making the fix harder!?
Or is the protection diode also voltage sensitive?

[reddesert]

I don't know how that pedal is wired, or what parts of it are senstive to 18v, but most diode protection schemes are for reverse polarity and will not do anything to protect the rest of the pedal against 18v at correct polarity. 

[WormBoy]

In the standard Phase90, there are two diodes attached to V+, one is purely reverse polarity protection, so that won't do anything to protect against over-voltage. The other is a Zener, which makes the reference voltage ... that means that part of the circuit will/should not get any more voltage than the Zener voltage (5.1V in the standard Phase90). What will see more than 9V are the op-amps, but they should be able to handle it. However, if it has stopped working, apparently something blew. Don't know if the EVH version has an electrolytic cap to filter the V+ ... that might be blown then (which would be an easy fix). Buying a non-working pedal is always a risk, but if you can get it dirt cheap ...

[WormBoy]

And, who knows what the owner has done to it. Might be more than just hitting it with 18v.

[garfo]

Thans Wormboy.
Precisely, I have no idea what he has done to it so I offered 35 quid and he was asking for 50. I decided not to go any further because I have no idea what I'm looking at.