Dean markley overlord not switching

[TarheelTechinTraining]

I just acquired this pedal and its not switching when the button is pushed. The led comes on and off when the button is pushed. The switching is ran by two FETs but I'm not sure which one to check first. Any help is appreciated, Greg

[Enzo]

Before throwing parts at it, go directly to the gate legs of the JFETs, and see if the voltage there is toggling on and off as you push the switch.

Is the LED hidden under the smudge by R10? It seems to be a separate point that the JFET control, so if for example R6 was open, the LED would work, but not the channel switching.

[Dave H]

The led comes on and off when the button is pushed.

To clarify does the LED come on and then go off when the button is pushed and released or does it latch in the opposite state? ie. If it's off and the button is pushed and released it comes on and if it's on and the button is pushed and released it goes off. If it doesn't latch the left section of IC2 could be stuck at either positive or negative saturation and you'll have to replace the IC. If it latches check the FETs and R6.

[TarheelTechinTraining]

Yes the LED latches. Push button LED lights, push again and LED goes off. Enzo I get -.063v at Q1 and -.43v at Q2..

[Dave H]

I get -.063v at Q1 and -.43v at Q2..

That's not negative enough to turn the FETs off. What is the output of IC2 doing at the top of R6? Does it go more negative than the gates? One of the FETs could be shorted. I'd remove Q1 and then check Q2 gate voltage as the button is pressed. Q1 could have been destroyed by something connected to the output jack.

[Enzo]

You gave me one voltage at each gate. My question was is it toggling? So when you stomp the switch off and on, does that gate voltage stay at that, or does it change? And if so, from what to what?

What Dave said, what is happening on each end of R6?

[TarheelTechinTraining]

OK I'm getting
LED on
-.06v @ Q1
-.42v @ Q2
-.42v @ R6
LED off
0v @ Q1
+.92v @ Q2
0v @ R6

[Dave H]

It looks like you could be measuring at the D3 end of R6. Check the voltage at the other end or at pin 1 of IC2. That should switch between about +/-12V. If it doesn't check the power supplies +V2 and -V2 (pins 8 and 4 of IC2) they should be about +/-15V. The tube heaters are series connected across the transformer so I'm guessing it's a 12V AC winding giving about +/-15V DC after rectification.

[TarheelTechinTraining]

At IC2 I've got
LED on
Pin1 -14.02v
Pin4 -15.4v
Pin8 15.65v
LED off
Pin1 15.4
Pin4 -15.45
Pin8 16.12v
So I'm assuming Q1 is bad, right?
Thanks so much for your help!!

[TarheelTechinTraining]

I'm not sure if it matters. The schematic shows Q2 as a 2n4383 but on my pedal its a bsk30a. Does that work or is it the problem?

[Dave H]

So I'm assuming Q1 is bad, right?

It looks like it. I'd remove it and check R6 and D3 to be sure. I can't find a data sheet for either of those transistors. The schematic is not clear. I think the BSK30A could be a 2SK30A. Check the print on the actual device. It may just read K30A

[Enzo]

2SK30, yes, K30 used to be everywhere.

Are you sure you are looking at the right JFET? One is a K30, and the other is a 2N4383. They both connect their gate to the same place - point A - via a diode. And it looks like either type would work either place anyway.

So I am curious how you decided Q1 was the bad one? The IC does seem to be toggling. Either one could be shorted, and not letting the other work. By disconnecting the gate diode to either JFET, you could then see if the other JFET works.

Or just replace them both.

[TarheelTechinTraining]

Thanks for your help with this. The transistors on the pedal both say K30A even though the schematic says different. I figure Q1 is bad because its more difficult to reach than Q2, Murphy's law. I'll just replace both since I have the parts. Thanks again you guys are the best!!

[g1]

That 2N4383 is probably a typo anyway. Near as I can tell, a 2N4383 is an NPN bi-polar, not a FET.

[TarheelTechinTraining]

Thanks g1. Also I found a couple of discrepancies. C3 on the schematic says4.7uf, on pedal its 47uf. C4 on the schematic says 2.2uf, on the pedal its 220nf(224). Do ya think I should change them to match the schematic? Thanks again, Greg