Mesa V-twin repair: no blues/solo switching

[Rockhorst]

I'm trying to fix a Mesa V-twin. The pedal is in good shape and most everything is working. When tapping the right footswitch to switch between the blues and solo channels, the LED changes but not the gain. I had found a schematic which I've temporarily uploaded here:

Mesa V-twin

If you look at page 5 you see that the channel switching is handled by Q3 and simply uses a jFET to parallel a resistor to change the gain of the opamp. The base of Q3 is connected to a diode (see page 6). Measuring at the diode shows a voltage of -128 mV when one channel is selected and over +1V when the other is selected. The voltage switches rapidly from negative to positive but then slowly rises to a constant value. No change in sound is heard. Measuring the resistance in the negative feedback loop of the opamp gives a resistance of 91 kOhm, consistent with a 100k parallel with a 1M. So basically, it's stuck in blues mode.

From these observations it seemed to me that the switching circuit was mostly functional and that the most likely culprit was the Q3 transistor. However, replacing it made no difference at all. So, I'm at a bit of a loss right now. Any and all suggestions welcome! Thanks

[Rockhorst]

Weirdness: I replaced the 1N4448 diode connected to Q3 with a 1N4148 as a test. Now, when switching from blues to solo, you slowly hear the gain ramping up to full. It takes about 2-3 seconds and then it's stable. As I understand it, it should be (almost) instantaneously.

I decided the original 1N4448 must've been faulty and replaced it with a new 4448 one (so taking out the 4148), hoping this would solve both problems. Now the original problem is back again: no switching at all!

I thought the 4148 and 4448 would be near identical in function in this type of circuit. Anybody that can tell me what's going on here?