Debugging EBS Octabass problems (noise when pedal should be inactive).

[kennedylanduk2]

Hi all,

I picked up a faulty EBS Octabass pedal. It works, but when you are not playing, there are constant blips/bops. Essentially, if you even touch a string, you get some signal being generated.

I've done a fair amount of reading, and recall somewhere that it could be a pull down resistor (which seems logical to me). Is there an easy way to find which resistor it is from a schematic (the schematic is available online)?

Thanks in advance for any help/suggestions.

Tony.

[antonis]

Hi & Welcome..

I can't see any pull-down resistor but, if it's there it should be placed somewhere on CE10/C8 junction..
(right after input jack tip.. - like R11 on output)



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[anotherjim]

Does the "leakage" noise reduce when you turn down the Octave and Normal pots?

[kennedylanduk2]

Does the "leakage" noise reduce when you turn down the Octave and Normal pots?

Thanks for the replies.

I only get the noise when the octave pot is turned up.

If I even touch a string to give a minimal signal, I get a fast tick/tock type noise, and this slows down fairly quickly. It's a definite tick/tock type noise. Mighht that indicate a pull down resistor before one of the op-amp circuits?

All the best,

Tony.

PS. My electronics knowledge is pretty basic, soldering skills are good, and I can do some simple debugging. This is beyond me sadly, so thanks for any help.

[anotherjim]

An input pull-down resistor probably won't achieve much, but you can try a 1M (down to 220k in extremes) added across C8. The lower that resistor is, the harder the guitar pickups have to work while also making it harder for noises - but it will affect the clean sound, reducing treble - known to all as "tone sucking".

The input is probably too sensitive to small inputs and really needs some kind of note 0n/off detection to keep the signal muted into the octave divider until a signal is strong enough to indicate actual playing rather than noise. That function can't easily be added in from what I can see, so all you have are tweaks to what is there already.

A small thing you could do is a change to 100p for C9 to create a little more filtering out of input noise, but the problem may be in the area of U3 and U7. The inputs of U7 pins 2,3, 5 & 6 will probably be at a similar voltage during silence and any noise might disturb one or more of those inputs to cause the following U3 flip-flop IC to change state which might be heard as clicks.
Be sure U7 is the correct TL064 and not some other quad opamp.
But, the octave method works by inverting the polarity at IC U6a by switching JFET Q1 at the right moments. During silence, that shouldn't make much noise but maybe a little switching noise is breaking through. A small cap (47p) between the gate and source of Q1 might help to filter fast glitches from U3 out.

A dumb easy cure is to use the FX with a true-bypass switcher. The FX is kept in an active state and the switcher diverts the signal away for clean sound bypassing the FX completely - it can make whatever noises it likes then, you won't hear them.

[anotherjim]

Actually, I wonder if Q1 gate shouldn't have a blocking diode like Q2 and Q3 do?