Faulty SM9 - replacement bypass FET advice

[acromarty]

Hi all,
I have an Ibanez SM9 that has stopped passing the signal when the effect is off. It works fine when on.
I'm guessing that since the on/off switching seems to be ok, it's the bypass FET that has gone.
Can anyone suggest suitable replacement FETs, and where I can get some? Oh and exactly which one is it, I can't find a circuit diagram.
Thanks
Andy

[R O Tiree]

The JFETs used quite often in Ibanez circuits of this vintage were 2SK118, an N-channel JFET with a quoted V_GSoff figure of -0.5V to -5V. I'd guess that something like a 2N5457 would do the trick (-0.5V to -6V) or similar.

Various sources I've trawled on the 'net assert that the SM-9 is identical to the (later) MS-10.  The only schem for that out there is a scribbled, hand-drawn affair (Google is your friend).  If this is true, then find D9's anode and you should find that it is connected to the Gate (centre pin) of a 2SK118 JFET, which is the one you'll need to change out if, indeed, it has given up the ghost.  This seems unlikely, though... Use your DMM to measure the voltage at the Gate as you switch the pedal over and back again.  You should find that it goes from pretty much 0V (effect on) to +9V (effect off).  Use an audio probe on Drain and Source to see if signal is passing when the effect is off.  If it is, then the problem is downstream.  If it isn't, make sure it's reaching the Drain of that JFET.  If so, then the JFET is bust.  If it's not even getting that far in the circuit, then the problem is earlier in the circuit.

Hope this all helps.

[acromarty]

Thanks very much :-)
I'm ok with how to try and track down the problem, once I have some sort of circuit diagram or at least an idea of which component I need to look at.