3-legged dog help:

[waky]

paul:

yes, i have a hfc4049UBE (thats the unbuffered right?). ive readed many threads form double d's or red llamas saying that that chip can replace the cd, just a different manufacturer

btw what exactly does "buffered /unbuffered" mean in this case? in stompboxes i get is as matching impedances, idk if thats the case with the chip.

gez:

al the inverters at the right side of the chip are input-grounded, the left side are the three invertes being used up.


i noticed that the drive pot started to oscillate at a pot resistance of 10k, so i replaced the 1m resistor with a 10k pot, it stopped the noise but it doesnt really change the drive at such a low value.


ive been analyzing a little the circuit with my little electronics understanding,   so correct me if im wrong but basically the drive pot controls the amount of the guitar signal going into the first inverter, at 0 ohm resistance all the  signal would go trough the pot skipping the first gain stage, as soon as i start rising the drive resistance some of the signal starts going into the inverter, so if i turn the pot higher the amplitude of the signal into the gain stage raises so it clips harder.

so... could the oscillation be a loading problem?
could it have to do with the j201?
could a different jfet stop the oscillation?

=/ im almost giving up on this pedal ...

[FlyingZ]

Almost every volume control ever made will have lug 3 connected to ground, it's the whole principle of it working as a voltage divider to attenuate the signal.

+1

For the rest of us it's lug 1

[gez]

btw what exactly does "buffered /unbuffered" mean in this case?

Buffered chips have an extra inverter in-front and after each individual inverter.  So, you end up with 3 inverters in a row.  It means that each 'unit' has higher gain, which makes switching faster (inverters were intended, primarily, as switches).  They're also less susceptible to loading as the inner inverter (and preceding inverter) are protected - 'buffered' - from any heavy load by the final inverter.  Because of the extra phase shifts (3 inverters for the price of one), it's very easy to get oscillation if you set the gain of of circuit utilising a buffered chip too high.  They'll also clip harder, due to the increased gain. 

ive been analyzing a little the circuit with my little electronics understanding,   so correct me if im wrong but basically the drive pot controls the amount of the guitar signal going into the first inverter, at 0 ohm resistance all the  signal would go trough the pot skipping the first gain stage, as soon as i start rising the drive resistance some of the signal starts going into the inverter, so if i turn the pot higher the amplitude of the signal into the gain stage raises so it clips harder.

On the face of it, you'd think so.  It actually works as a compressor, though, increasingly loading the signal from the previous stage as its amplitude increases (there's a whole thread on this somewhere).  In order to counteract this, you'd need to stick a resistor between the 10n and the input of the 1st inverter.  That would restore the neg earth effect, providing that said resistor is at least the same size as the max resistance of the pot that sets gain.  Min resistance on the pot would give you no output.  However, as this setup would only give you unity gain with max pot resistance, that's not going to help you to create much distortion.

Perhaps that is not such a bad idea, though.  What PUs does your guitar have?  It sounds like they might be driving the circuit too hard; in which case, reducing gain throughout the circuit may not be such a bad thing.  Probaby not as much as unity in the first stage, but a small value input resistor for the first stage might help.

so... could the oscillation be a loading problem?
could it have to do with the j201?
could a different jfet stop the oscillation?

Sounds like the overall gain throughout the circuit is too high and needs taming.  The oscillation might be cured by keeping your wires short/using screened cable. 

[waky]

so... could the oscillation be a loading problem?
could it have to do with the j201?
could a different jfet stop the oscillation?

Sounds like the overall gain throughout the circuit is too high and needs taming.  The oscillation might be cured by keeping your wires short/using screened cable. 

that might be it..? since i was at debugging phase i still have that long unshielded wire. should i shield the input only? or the outputs/drive pot too?

[gez]

that might be it..? since i was at debugging phase i still have that long unshielded wire. should i shield the input only? or the outputs/drive pot too?

See what you can get away with, in terms of doing the least amount of work (try one thing and, if still no joy, keep going).  The wires to the drive pot and the input wires are the most important.  Keep as short as possible and shield input if necessary.  Output of this circuit isn't low impedance so shielding its wire might be necessary too.

[waky]

ok (i know its been some time) i shielded all wires out of the pcb and i still get the nasty oscillation, so could i tame the thing by reducing the feedback resistor of the first inverter to get a lower gain?

[gez]

Try reducing the feedback resistor in the last stage (the 1M).  The other stages have resistors to ground to create DC offsets.  Varying resistance in 2nd stage would change the DC bias point.  May or may not make a difference, but I'd try the final stage before tweaking anything else.

Bear in mind that taking the feedback resistor down to 470K gives unity gain, so you wouldn't really want to take it that low.

[waky]

yeah but i think i get way too much gain for some reason.. i mean its TOO loud for a stompbox when i raise the volume O.o, ill try and get back to ya

[gez]

yeah but i think i get way too much gain for some reason.. i mean its TOO loud for a stompbox when i raise the volume O.o

That's because the output hasn't been divided down and CMOS inverters deliver a large peak-to-peak output.  Stick a resistor before, and in series with, the pot to make it useable.