tube screamer woes

[msclford]

I repackaged my ts-5 and since I took it apart yesterday, it distorts even with the drive knob at 0. What might cause this?

[casey73]

short?

Maybe somehow you shorted the connection on the drive control so it has max resistance in the feedback loop.

[Mark Hammer]

Alternatively, depending on the way the pot is wired, if the little rivet securing the middle lug to the wiper is loose now then the pot can behave like it is always at max. I often find I have to get in there and "pinch" the rivets of some pots to make them behave again. Just be gentle.

[msclford]

Alternatively, depending on the way the pot is wired, if the little rivet securing the middle lug to the wiper is loose now then the pot can behave like it is always at max. I often find I have to get in there and "pinch" the rivets of some pots to make them behave again. Just be gentle.

Well, it's not always at max, it increases throughout the range. I also figured out that the bias voltage is at 9v, and it should be at 4.5v.

[Enzo]

I think you have two threads going on the same repair. I mentioned elsewhere that you likely have an open resistor in the voltage divider that provides that bias V.

[msclford]

I think you have two threads going on the same repair. I mentioned elsewhere that you likely have an open resistor in the voltage divider that provides that bias V.

I checked the resistance of both of the resistors in the voltage divider, and they both showed 10k ohms. They should be working, shouldn't they?

[msclford]

If you look at the schematic I've attached, you can see what I have done to the circuit. It's hard to see in the clipping section, but I show the switch between sets of diodes. The circled components are removed. Also, should I remove R27 and R28 since those transistors are removed (In other words, does it still need that bias voltage)? Is there anything that I should not have done? I put in a tpdt switch for true bypass.

Also, see this thread for a little more background.
HC Forum thread

[Enzo]

Where is that voltage divider, I don't spot it? Is there 4.5v at its center? Measyre at each end of each resistor, and there should be 9v at the top, 4.5at the junction, and zero at the bottom.


Where is the path for emitter current through TR3? All I see is caps.

If you removed the switching JFETs, then R27 and C15 seem irrelevant. If that is a jumper in place of Q2, then R28 is no longer needed.

[msclford]

I was wrong about the voltage. It does measure 4.5v, I just measured it wrong. That eliminates one problem, but I still think that it has a lot of distortion going on, it sounds like a metal distortion pedal instead of a tube screamer! It doesn't seem like it had this much gain before. I did swap C10 for a slightly different value, but that wouldn't cause it to boost the gain, would it?

About the emitter current, there is a 10k resistor to ground off of that leg of the transistor. The person who drew up that schematic just missed it.