Dunlop TS-1 Tremolo question

[stevenrb718]

I took in a buddy's TS-1 Tremolo that's giving me a hell of a time. At this point I'm more annoyed by it, and just bugged to figure out the issue. The intensity knob doesn't change the effect. Already adjusted TP-1 with no outcome either. The LED of the LDR *is* affected by both TP-1 and the intensity knob, but the effect is not. The effect otherwise works fine. I feel that it may be an issue with whatever controls the blend of the non-effect signal. I already tested several components, along with swapping two ICs with no avail. Also, I'm not familiar with the pedal, but the bypass signal has a decent volume drop, as does the effect.

[Helmholtz]

Schematic?

[stevenrb718]

Not sure how "official" these are...

[Mick Bailey]

When you say the LED of the LDR is affected, is it modulating correctly?

[stevenrb718]

When you say the LED of the LDR is affected, is it modulating correctly?

Seems like it. I don't have an oscilloscope so I put an LED to the LED-side of the LDR, and also an audio probe on the sensor side to hear the pulsing, and both alter with the controls.

[Mick Bailey]

Here's the audio signal path.

U1a and U1b are voltage controlled amplifiers with the control voltage on pins 3 & 8. This is the section where the audio signal is being modulated. The voltage range is 0v=full output, 1v=maximum attenuation. You should see the control voltage varying with the speed and depth controls. If not, something in the oscillator section isn't operating correctly.

[stevenrb718]

The Speed and Shape controls function as they should, it's the intensity knob that doesn't affect the sound. It's all full intensity the entire time. I assume this is supposed to act as a blend knob.

[Mick Bailey]

There's no blend function, as the circuit doesn't mix between the dry and effected signals. Take a look at the schematic; U1a and U1b are acting as electronic volume controls. Imagine an electronically controlled hand on your amp's volume control turning it up and down. That's basically the effect. If the LED on the pedal modulates, then the oscillator is working. The intensity control alters how much Q2 is turned on, which in turn alters how much VR1 (as well as the pedal LED6) is modulated. In a normally functioning pedal it looks as though the intensity control alters the brightness of the pedal LED - does yours do this?

[stevenrb718]

There's no blend function, as the circuit doesn't mix between the dry and effected signals. Take a look at the schematic; U1a and U1b are acting as electronic volume controls. Imagine an electronically controlled hand on your amps volume control turning it up and down. That's basically the effect. If the LED on the pedal modulates, then the oscillator is working. The intensity control alters how much Q2 is turned on, which in turn alters how much VR1 (as well as the pedal LED6) is modulated. In a normally functioning pedal it looks as though the intensity control alters the brightness of the pedal LED - does yours do this?

The LDR is sealed, but as I mentioned before, when connecting an external LED to it, the LED brightness does change with the intensity control, as does the audio probing on the sensor side. Also, obviously, adjusting the trim pot changes the minimum intensity as it should. U1 was also already one of the ICs I replaced.

[Mick Bailey]

I'm talking about the pedal LED - the one on the top that flashes in time with the rate and turns of when the pedal is bypassed.

What voltage do you have on the wiper of RP1 with the intensity control at zero, and at maximum?

[stevenrb718]

I'm talking about the pedal LED - the one on the top that flashes in time with the rate and turns of when the pedal is bypassed.

What voltage do you have on the wiper of RP1 with the intensity control at zero, and at maximum?

Oh I gotcha. No, that LED only changes with the speed and shape knobs, but doesn't change brightness with intensity.

I'm getting 1.2V up to 3.2V with the TP1 at zero, and 2V up to 3.2V with TP1 at max.

[Mick Bailey]

It's RP1 you need to measure - the intensity pot. The voltage on the wiper controls how much Q2 is turned on.

[stevenrb718]

Those were from the intensity pot. Oh but you want the wiper...so 1.2V through 0.7V with TP1 at zero, and 2V through 0.7V with TP1 at max.

[Mick Bailey]

D7 clamps the voltage applied to the intensity pot to 5.1v and I was expecting a greater voltage range from the wiper. For the purpose of testing set TP1 to its mid position and leave it there.

I would lift one end of R141 to remove the base voltage from Q2. The pedal should now stop modulating altogether. If it keeps working at maximum, either Q2 is likely to be shorted or there's a problem elsewhere. I'm unfamiliar with this pedal, so watched a few videos and the pedal indicator LED behaviour doesn't fit to how it would operate from the schematic. As its in series with VR1 it should change the maximum brightness with the intensity control, but in the videos it doesn't. Maybe the schematic has errors?

[stevenrb718]

I pulled R141 and nothing changes. I tested Q2 earlier and I'm pretty sure it's ok. Plus as I mentioned before, I applied an LED externally to VR1 and it changed intensity with adjustments to the pot and trimpot. Therefore all of the power stage into the photocell should be fine. Now if it's not getting enough voltage is another thing entirely. I haven't come across anything in the schematic yet where it's different or has errors, but the part numbers don't jive.