Problems with audio buffer

[RB26DETT]

Hello! This is my first post in this forum, although I have been reading this forum for a long time I'm Néstor, from Spain (sorry for my bad English haha), and I'm trying to build a homemade Tremolo. I have designed a Tremolo based on a 555 and a LDR/LED. I have tested it on the breadboard and everything worked. I Then I tried to build it on a PCB and the oscillator part works (I divided the PCB into parts, one has the oscillator and the LDR and in the other is the audio path) but the audio way doesn't seems to work correctly. It is based around the MXR Distortion Plus, but I have removed the clipping diodes, because I only want a buffer/booster of the signal. I have choosen it because I have built an MXR Distortion + so I could compare the results between them. Here is the schematic:


I tested it with the Oscilloscope. The input is a 3V sine wave at 5KHz. Everything until the input of the Op Amp LM741 (I get the same input plus and offset in the input of the Opamp) But everything goes wrong at the output: I get an amplified half wave. I can change the gain of the output wave turning the pot. It's like I had some kind of diode in the feedback of the op amp or something like this, but I only have resistors and capacitors. Here is the log of the scope:



As you can see, there is only half wave I'm worried because this is a high school project and I have to solve this problem so any help would be very greatful.

Thank you!

[Gurner]

Most likely to be an input biasing problem....your schematic hasn't posted up properly?

[slacker]

Here's the schematic he was trying to post, weirdly now I've looked at it it displays in the original post.

http://www.freeinfosociety.com/electronics/schematics/audio/pictures/mxrdistp.gif

Like Gurner said it looks like the LM741 is not biased correctly. I would check that the three 1M resistors and the 1uF capacitor are connected correctly.

Oh yeah there's nothing wrong with your English

[Gurner]

Here's the schematic he was trying to post, weirdly now I've looked at it it displays in the original post.

http://www.freeinfosociety.com/electronics/schematics/audio/pictures/mxrdistp.gif

I've clicked on your link, but it says forbidden!

[slacker]

Sorry, looks like you have to copy and paste it into the address bar to get it to work, that's presumably why it doesn't show up in the original post. The freeinfosociety are obviously happy to steal other peoples work, but don't like it when you steal their bandwidth by hotlinking 

[Gurner]

Bingo....that did it (copy/paste)

RB26DETT - if your scope screenshot wasn't on an AC setting we'd be able to see the DC level your signal was witting on ...which would have helped here, but in the meantime as slacker says check the voltage at the junction of the two 1M resistors & the the opamp's +ve pin....it should be approximately half your supply (just under in fact).

[RB26DETT]

Yeeeeeeeeeees! Thankyou! Thankyou so much! One of the the copper tracks was broken. I couldn't see it because it was behind a wire. By now the audio is working  Tomorrow I will try the whole tremolo. Thankyou! I was hopeless about this circuit, I was thinking of building one new.

[runmikeyrun]

FYI I keep a light up magnifying glass at my bench just for stuff like this- it literally takes 2-3 minutes to carefully go over a circuit board, and often saves me much more time than that when I find a broken or shorted trace.  Anytime I have issues I look it over; also when I build on vero I go over the whole board before powering up the first time.