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

[stephanovitch]

Check the voltage between the 1M resistor and the diode on Q3 gate.

You must have 12V for a position of the switch blue/solo swap and 0V on the other position.

[rockhorst]

The side of the 1M not connected to the diode shows -12V or +12V depending on the switch position. The side connected to the diode shows voltages of roughly -0.2V or +1V. I've noticed that my measurements on Q3/diode aren't always consistent. I'm expecting swings between minus to plus something, but sometimes it doesn't go below zero. Might be my DMM probe though. I've replaced the cap and the diode once more and will give it a listen in the morning.

[stephanovitch]

Ok for the voltage 12V and -12V.
I think you must replace the 1M resistor too.
Remove it to mesure is resistance.

Stef

[rockhorst]

Stef, first of: thank you very much for trying to help. It is much appreciated.

The 1M resistor seemed to be fine, but I've replaced it anyway. So, by now I've replaced the J175 transistor (P channel), the 100nF tantalum (with a film cap), the diode and the resistor. Those seem to be the essential parts of this part of the switching. Still, it doesn't work

What I find weird is that I got it working once, just with a few seconds of latency where the solo channel 'fades in'. That was when I replaced the diode for a 1N4148. I then tried a 1N4448 again and it didn't work anymore. Put back the 1N4148 hoping it would work again but no it doesn't. Somehow, I feel there's a hint in this. Basically what is happening is that the jFET switch is normally closed (jFET on) and it can't open up (jFET off). The source and drain sit at 0V, so how far would the gate need to be above that?

[slacker]

The side connected to the diode shows voltages of roughly -0.2V or +1V.

I think something is wrong here, the junction of the 1M resistor and the anode of the diode should be the same as the other end of the resistor either + or - 12 Volts, you might read about half that because of the impedance of your meter but not that low. With 12 Volts on the resistor you should get a bit lower than 12 Volts on the gate of Q3, with -12 Volts you should get around 0 Volts.
I'd maybe try removing the diode and see if you then get the right voltages on both end of the resistor, that might help narrow down where the problem is.

Does the "blues" LED work?

[rockhorst]

All the LEDs and other switched functions work as expected.

[rockhorst]

I've disconnected the diode. It's been desoldered about 5 times now, so the pads are starting to lift

Directly after switching, at the 1MOhm it reads +/- 1.5V and slowly climbs up/down. It takes several minutes to stabilize, which I guess is due to the capacitor.
I noticed that even without the diode connection, there still is a 0.6V potential difference between ground and the gate of the jFET. That seems weird to me?

Edit: I did a measurement at the gate with the diode connected (with some alligator clips) again: 0.4V to 1.4V, so that's different from before.

[rockhorst]

And then...for no particular reason whatsoever...it's just working. Put in another diode (BAW62, 1N4448 workalike) and it does its thing. Weirdness, but I'm not complaining