Tube Screamer Flip Flop Circuit Troubleshooting

[JFace]

I'm having difficulty figuring out the problem in a faulty Tube Screamer. The pedal is very difficult to turn on the effect, and disengages on its own within minutes. When I do manage to get the effect on, the audio circuitry works fine. The bypass works fine as well. I am not completely convinced it is the mechanical switch. I have attempted to short the switch lead to ground and it does not toggle the circuit. The problem occurs with either a battery or 9VDC adapter.

In addition, I hear occasional ticks (once every few minutes) whether engaged or bypassed. Scratch that...that's just my furnace kicking on. Dang Canada and your cold weather...

I measured the voltages of the two flip flop transistors:

Effect bypassed:
e1 = 0.57
b1 = 0.21
c1 = 6.44

e2 = 0.57
b2 = 1.18
c2 = 0.59

Effect engaged:
e1 = 0.65
b1 = 0.67
c1 = 1.25

e2 = 0.65
b2 = 0.24
c2 = 6.28


This image comes from RG Keen's jfet switching article from geofex.com

The flip flop seems to be working, it is the triggering that I can't figure out. I replaced the capacitor (C5)  that is shunted to ground at the switch input, but this did not fix the issue. I bent all of the components away from each other so they are not shorting. The solder joints all look fine. I am at a loss at this point, and would like to hear any advice you might have.

Thanks in advance.

[PRR]

> e1 = 0.57

?? The emitters are hard-tied to ground. How can there be voltage there?

[JFace]

The geofex schematic is the simplified version. Emitters are connected to the anode of a diode, in series with a small resistor to ground.

[antonis]

The problem occurs with either a battery or 9VDC adapter.

Does desengagment occur after same duration (more or less..) with both kinds of power supply..??

[JFace]

I haven't tested this extensively but I would say yes. The turn off time seems arbitrary.

[R.G.]

Just staring at the schemo, leaky C14 or C41 would gradually let Q6 turn on.  Leaky Q6 might do the same, maybe. So might some heavy board contamination.

That's highly speculative, but possible. These bistable latches rely on feedback being big enough and equal (well, OK, ENOUGH) in each direction to stay latched. Yours is coming unlatched on the "on" side, with Q6 off. Anything that gets them to start in the other direction will make them flip, as witness the small signal in the actual footswitch trigger that flips them.

Does it hesitate when footswitching it off?

[JFace]

Footswitching off is much easier. Some difficulty, but about 25% success rate as opposed to about 5% turn on success rate.

[JFace]

I wanted to do a follow up on this troubleshooting... turns out RG was right again! The flip flop transistor was faulty. I replaced it and it toggles fine. It's the one with the silver marking:


During testing I found that the mechanical switch was inconsistent. I checked the continuity across the leads and the switch was fused closed, even when I took it apart. I replaced it with a good ol' fashioned small bear momentary switch, with the aid of some nylon washers to cover up the oversized hole, it's now good as (better than) new!


Thanks for all your help.

[PRR]

> flip flop transistor was faulty.
> the switch was fused closed

TWO faults!! That's cheating.

Glad you got it sorted.