Rebote 2.5 Delay Debugging help required!

[the59sound]

Hi, hopefully someone can help...
I have built the Rebote 2.5 Delay using the Tonepad PCB. I am getting a very low output (original signal and repeats). The whole output signal also sounds like it has gone through a Low Pass Filter.
I have audio probed the circuit and the part where it seems to fail is at the 33k resistor bringing the delayed signal back to the direct signal as the delayed signal completely disappears. Strangely though, the resistor measures ok and in terms of continuity seems ok...
The output of pin 2 of the level pot sounds fine. Passing through the 1uF Capacitor it's still fine then disappears at the 33k resistor. Confusingly though, at the 15k resistor controlling repeats, there is a very strong 'original+delayed' signal.
The output of pin 7 (direct signal) of the TL072 is very good. The output of pin 2 is like the output signal described avbove. Could it be a faulty TL072? I have tried another resistor in place of the original 33k and no change in the results but don't have a spare Tl072 to try yet.

Battery - 8.6v

Circuit Voltage - 8.37

IC1 PT2399:
1:  5.01
2:  2.50
3:  0
4:  0
5:  2.73
6:  2.50
7:  2.00
8:  2.27
9:  2.50
10: 2.50
11: 2.50
12: 2.50
13: 2.50
14: 2.50
15: 2.50
16: 2.50

IC2 TL072:
1:  4.22
2:  4.19
3:  4.16
4:  0
5:  2.94
6:  4.13
7:  4.19
8:  8.32

IC3: 78L05
1:  8.28
2:  0
3:  5.01

Schematic: http://www.tonepad.com/getFileInfo.asp?id=98

I have checked all resistor values, used a magnifying glass to check for dodgy solders and am sure all is ok on that front although I am double checking these again just to be sure!

Hopefully someone can point out where I may have gone wrong.
Thanks,
Toby

[StereoKills]

I'm thinking it's the weakened battery as I had the same issue when not using a full 9v battery. Seems to work best on a wall wort.

[the59sound]

Thanks very much for your reply. I must admit, this was a brand new battery pumping out about 9.4v but I had the effect plugged in for so long trouble shooting that it ran down to that current value. The low output was the same when on the new battery. I'll buy a PSU and see if that improves things...

Could anybody who has built one (and has the time!) do an audio probe at that 33k resistor and let me know if there is any audio at all? I would be very appreciative!

Thanks,
Toby

[jmwreck]

I guess we both have same problem, but i've already used it for a month or so, and it sounded good, but the other night, as i play along, the delay is gone. Ive finished troubleshooting it, but i cant seem to find any right clues to help me out, ive tried a battery and a PSU, still it wont work.  I haven't touch anything or done anything wrong aside from tweaking the knobs, as what we usually do.. then it suddenly stop working, i'm hoping its the TL072, so that i can fix it in an instant, I also wonder if the thing (components) inside will get damaged even if I have already used it for a month, with no problem at all, except the other night. 

[the59sound]

On mine, I can 'bodge' an output by connecting the output to the 0.1uF capacitor next to the 15k resistor controlling repeats and I get a much clearer sound although it is still much quieter overall than it should be. It's a shame really as the delays I can hear when I probe the output of the PT2399 is stunning - very impressed with them!

Does anyone think it could be a faulty TL072?

Thanks

[JKowalski]

By the way, there should be no signal after the 33k resistor.


The way the op amp after it works is that it looks at the voltage on the (-) input, and then outputs current, which goes through the resistor between the output and (-) and cancels out the voltage present on the (-) pin. So it outputs whatever voltage it takes to bring the voltage on (-) to zero - and since you need the opposite to cancel it out, the output of the op-amp is flipped (hence "inverting" input). People usually refer to the pin used in this way as a "virtual ground", because it's not connected to the actual ground but it stays 0V nonetheless. This property is very useful in that you can attach many input resistors to the point to mix signals, and the signals cannot go back into the other inputs. The rebote uses it as a mixer - see, you have one resistor from the pedal input, and one from the PT2399.

Might as well go further in my explaination - If you think about it, you can see how a gain stage works with a op-amp. If the resistors going into the (-) input and the feedback resistor (the one from output to (-)) are equal, then the op-amp outputs the same voltage going into it. See, to get the current going to the (-) pin canceled out by the current coming from the output, while each resistance is resisting an equal amount, then you need equal voltages. Hence, it follows the input (but flipped).

If we want the op-amp to amplify, then we make the feedback resistor bigger then the input resistor. This works because it makes it harder for the signal on the output to cancel out the input, so the voltage on the output needs to get bigger in order to push the same current as the input resistor is letting pass through the bigger resistance (V=IR). It still has to keep the same shape as the input, just amplified.



Annnyways, try to replace the op-amp. If you have some spare, and a socket installed, whats the harm? I've had a number of tl072's go out in one of my rebote builds, but I think it was caused by a newly installed millenium bypass - a problem which to this day I don't understand.

Semiconductors are the most likely parts to fail, so always troubleshoot them first.

[the59sound]

Brilliant - thanks very much for that. It might take me two or three reads to properly understand it but it definately points towards the chip being the problem. Really appreciate your input.

Thanks.

[StereoKills]

Keep us posted how you fair with swapping the opamp, I just got word from the recipient of my rebote that it's having that problem again.

[the59sound]

Ordered new TL072s (and also 4558) to test. SHould arrive tomorrow. Will let you know how I get on... Fingers crossed.

[the59sound]

Neither op amp solved the problem. Might build another one as the board I have is looking a bit messy following component removal/replacement!

[StereoKills]

Hmm...I thought using a fully charged battery had solved the problem and gave the pedal I built as a birthday present to my father. Apparently overall volume level still doesn't meet unity even with a power supply. I hope I can fix this without taking it away from him for too long.