high pitched oscillation!?

[blanik]

i just debuged my Tube Reamer 2 (thanks to MartyMart ) but as soon as i turn either the volume or drive pot, it starts to oscillate like crazy (like a starved FF)... it stopped for a couple of seconds when i was playing around with the alligator clips but ame back immediately?

what makes a 4558 get into oscillation?

R.

[Rob Strand]

It's the same old problem with any high gain effect!  The output signal somehow get coupled back to the input signal and that forms an oscillator.

Given the volume or drive level affects the problem my guess it's the input and output leads being too close together, especially around the switch.

There's many solutions, depending on the degree of the problem, most of which you should be able to find in the archives.

For that effect you should be able to just keep the input and output leads well spaced.

Next check your foot switch.

Next try shielded cable on the input and output leads.

For more options search archives.

The drive pot wiring should be kept away from the input wiring too, but I don't think this is the main problem in your case.

Good luck!
----
Edit.  Keep the input wiring away from the volume pot as well.

[blanik]

hi, yeah, i searched the archives before and found many treads about and most of them were mentionning shielded cables etc... but i was testing the circuit with aligator clips, straight in-out with the wires far apart... when i was playing with the power wire it did stop it for a second...   i did many high gain circuits before (much higher than a TS variant) and never got this problem before...

one of the posts was interesting, it was sugessting reducing a small cap across 2 legs of the opamp... there's a 100pF and a 150pF on the Reamer 2... i might try one of those

and i don't get it why input and output wires should be really far apart, i don't think there's much  electromagnetic field around stompbox circuit... 

R.

[gez]

and i don't get it why input and output wires should be really far apart, i don't think there's much  electromagnetic field around stompbox circuit...

Capacitance is the problem. 

[A.S.P.]

i did many high gain circuits before (much higher than a TS variant) and never got this problem before...

it was the very first problem I ever had 

got your powersupply decoupled?

[blanik]

got your powersupply decoupled?
[/quote]

hun? 

wazzthatmean? 

[A.S.P.]

caps across the rails: an electrolytic and a small one at the PS, and a small one near the chip.

[blanik]

caps across the rails: an electrolytic and a small one at the PS, and a small one near the chip.

yeah, i did... i followed RM Rivers vero layout:
i guess C8 C7 an C4 are what you're talking about...?

[A.S.P.]

I`m not a big friend of strip ore vero,
but in this case I`d solder C7 directly to pins 4 & 8 of the chip.

And I`d omit C1, which is obviously soldered in pallel to the link
between pin 4 and "volume1"  (shorted).

But anyway: I don`t know the schematic, so what worth the thoughts ?

[blanik]

I`m not a big friend of strip ore vero,
but in this case I`d solder C7 directly to pins 4 & 8 of the chip.

And I`d omit C1, which is obviously soldered in pallel to the link
between pin 4 and "volume1"  (shorted).

But anyway: I don`t know the schematic, so what worth the thoughts ?

yeah, Marty Mart pointed to me that the vero had an error, it needed a trace cut right above R3, i did it, that's when the layout started to work...

i think C7 is already on IC 4 & 8... jumper between C2 and R4...

here's the schem:

[A.S.P.]

HF layoutwise, C7 is far apart from those pins: not directly
(A lot can happen between the IC pin and the "Ground" connection).
But it`s good, that R.M. Rivers put in C7&8 there at all
since they`re missing in the schematic. 

[gez]

Blanik, do you have the right value resistors wired up for the last op-amp: 12k and 51k?  If there's too much gain in this stage then that might be causing the oscillation.

[gez]

PS  Check the joints of your diodes (if they're not connected properly you're not limiting gain in the first stage).