Distortion + Problem

[Radamus]

Hello. I made a distortion + for a friend as a present, but it doesn't seem to be working. I used a 4066 chip for the bypass, which I've done a lot before with no problems. The problem is this: it works fine in bypass, it makes no sound when turned on. I probed it and found that there is clear (some buzzing, which I can't necessarily blame on the circuit) sound on the input of the 741 (the plus side of the amp), but no sound coming out of the other side. I'm coming from the GGG schematic on this. The voltage is fine on all of the IC's (around 9.5 on the input voltage, and 0 on the ground end). What could cause the problem here?

The voltages, as I began earlier, are 9.5 on pin 7 of the 741 and pin 14 of the 4066. The voltage is 0 on pin 4 of the 741 and pin 7 of the 4066. The 741 has around 1.5-2 volts on some of the other pins. No audio comes out of pin 6. Plenty of audio on pin 3. No audio on any of the other pins. I changed the chip out once already and the same problem persists.

What could keep the 741 from amplifying? Can I replace it with anything else?

Thanks
Conrad

[GibsonGM]

You'd have to check the parts associated with the 741, if you have audio at pin 3.  I would be suspicious of the 1M resistor in the feedback loop, along with the pot, cap, and other resistor there.   Of course, this is assuming the chip is not fried.  Are you sure it is in 'frontwards' and not turned around?   

Also try measuring the bias voltage on pin 3, to be sure it is present.
If everything above is ok, you should have audio at pin 6 when a signal is input.  If not, something is really off with the chip and/or its external components! 

[frequencycentral]

I would be suspicious of the 1M resistor in the feedback loop

Yup, when I built my D+ I forgot to add that resistor - took me a week to realise why it didn't work! Ooops!

[Radamus]

I'm pretty sure I have that resistor there. I'll check again. So what you're saying is that my problem is somewhere in the gain section or in the chip itself? I don't think I fried them both. For the record, the clocking point on the 741 is on the top with pin 1 to its left? If this chip is any different then that's my problem. I'll check the gain section. Is that the feedback loop? Could I have messed up the gain pot? I don't know where to get reverse log, so I either used a regular log or a lin. Shouldn't actually change anything right? I'll check all those solder connections.

How loud should the sound be on pin 3? When does the signal become distorted?

Thanks

[Ice-9]

Also if you have a signal at pin 3  but not at the output then assuming the op amp chip is fine you may have a short on the the clipping diodes or a problem with the cap in the same area as the diodes.

[runmikeyrun]

an audio probe really helps debug stuff like this, it helps you isolate where the problem is at.

[Radamus]

Also if you have a signal at pin 3  but not at the output then assuming the op amp chip is fine you may have a short on the the clipping diodes or a problem with the cap in the same area as the diodes.

That makes sense. If it's shorted to ground, then there shouldn't be much sound, right? That's another thing to put on my check list.

Thanks

[Radamus]

Well I found the problem in the no sound part. I accidentally bypassed the second 1m resistor to ground from the hot section, so current had no reason to go into the bias, I think. Now it makes sound, but it is either seriously sensitive to nearby frequencies, or it's making them on it's own. It screeches, even in the box. when I touch the ground, it calms down. When I touch the box in certain places, it calms down. When I move it onto my wooden table, it calms down. When I put it on the carpet, it freaks out and screeches. When I move my cell phone close, it screeches.

Does this sound like a room problem or a circuit problem?

I'm going to give it to him tonight. He's in school for electrical engineering, so maybe he can fix it. He was just too lazy to put the damn thing together. Otherwise, I'll work on it again this weekend.

[Ronsonic]

Sounds like grounding / assembly issues on the noise.

A handy answer to that gain pot, to make it more manageable is to use a log taper and just wire it backwards. That way you get a smooth easy to control gain taper AND you mess with the guitar player's mind and force him to think and listen to where he sets it. That makes him get a better sound than if you let him set it by force of habit by having a normal pot.

[Radamus]

I figured it had something to do with the grounding, but I have no idea where to start looking. All of the grounds are connected to one place on the board, mostly with jumper wires. I can try replacing some of that stuff and see what happens.