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mlgshadow20 13th January 2013 05:38 AM

PLEASE HELP! Distortion Pedal problem!
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I built a great sounding pedal not too long ago using the attached Distortion Plus schematic. I built another one for a friend, but this one has issues. When I play through it, my tone is extremely dry and very "trebley" when the gain is half or less.

One of most important issues is that when I turn the gain DOWN it creates a lot of hiss. When I turn the gain to max, it sounds nearly as good as the functional pedal, but still a bit "dry." The weirdest thing is that when I crank the volume past half with the gain about half or less, I get an extremely weird feedback, similar to the sound of tuning a radio, but I do not actually pick up a station like I have experienced before. :confused:

I am using a 9V battery to power this, BUT I did connect a 9V power adapter to this and that is when I noticed that it didn't work correctly. I don't recall if it had the issues before I connected the power adapter. The LED came on, so I know that my polarity was correct with my jack.

I started troubleshooting by comparing some measurements between the two pedals and nothing seems out of place.

I also tried replacing almost every capacitor on the board. I replaced the IC as well, but nothing had any effect. I know that I am using the correct values as they work in the other pedal.

If anyone has any suggestions or if you've "been there," PLEASE let me know. I've got another one built for someone else, but I would really like to fix this one so I won't have to make another one. I can send pictures by private message or whatever else is necessary.

epicyclic 13th January 2013 07:03 AM

Hi mlg

it sounds like the circuit is oscillating , check the value of all the components in the feedback loop ,C5 etc ( maybe you have put an incorrect resistor value in ) and check the rail decoupling .....another thought are you using a different IC to the original .

mlgshadow20 13th January 2013 03:36 PM

I know that my IC is correct because it is identical to the other one that I used. A resistor value could very possibly be my problem but unfortunately for me, I didn't use color-coded ones, so I may just have to measure them. I omitted C5 from the circuit as well because other similar schematics I have viewed do not have a C5 and the schematic itself states that it is optional. I'll start by checking my resistors and get back. Thanks for the help!

Tesla88 13th January 2013 09:35 PM

Also check if grounding is correct and also check if the enclosure is grounded . I'll also check input and ouput connections , did you use shielded cable for Input ? Try add C5 , it made a roll-off for HF rejection.

mlgshadow20 13th January 2013 10:34 PM

All of the wire I used for both pedals is cloth-covered wire. I'm not sure if I have room on my board for C5 and the closest thing I have to a 10pF is 2pF Thanks!

Dr-Joned 13th January 2013 10:35 PM

What do you mean by "you didn't use color-coded one"? Are they precision with writing on them?

turk 182 13th January 2013 11:00 PM

somewhere there's a bad ground
was this built on a pcb or was this built point to point
i would try resoldering most if not all connections.
after that find a magnifying glass and look over everything again checking for any fine solder bridges especially between the ic pins or across component pins

Tesla88 14th January 2013 10:08 AM

for C5 you can go up to 100pF , is less trebly but you can debug if problem is related to too much gain at high frequencies.

Anyway , the problem is a self oscillating opamp due to high gain and parassite cap or a bad grounding , also an incorrect layout or bad wiring cause this.

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