Tremolo whistling - power supply related?

[mark2]

I built this GGG EA Tremolo and it has a ~660Hz whistle, tremolo'ing at the rate set by the speed pot.

What's very interesting (to me) is when I plug in my little DSO138 scope into the daisy chain, the whistling stops.  So I'm guessing the scope must be doing some sort of filtering on the power supply?

Are there steps, e.g. an RC filter, I can take to make the tremolo behave nicely with this power supply?

[R.G.]

You don't say which power supply, but yes you could possibly stop this. I would put a small capacitor across the base-emitter of the tremolo oscillator transistor to cut it's high frequency response so it can't oscillate at higher frequencies. Size the capacitor experimentally. Start at 10nF, go down in value if the thing won't oscillate at low frequencies any more, up in value until the whistling stops.
You should also put a big(ger) power supply filter cap on the circuit. 220uF and up would be good.

[mark2]

Thank you; I'll try both of those suggestions.

The power supply is just a cheap daisy chain 9V adapter. Nothing else plugged in when it was whistling.

[Rob Strand]

The cause is probably your wiring.  Input wires and output  wires need to be kept apart.    That includes around the footswitch.    Twisting input and output wires together will be the worst.
 

[mark2]

The input and output wires are fairly far apart, and it's not affected by moving them closer or farther. So I'm guessing this isn't the issue.

I'm trying the caps now.

Anyhow, regardless of what this turns out to be, any ideas what the oscilloscope might be doing that it can cut this noise when I hook it up to the same power supply?

Edit: a larger power supply filter cap made a huge difference. There's still an oscillating noise but it's much quieter. I'm going to experiment with a cap on Q3 now.

[Rob Strand]

I'm trying the caps now.

Anyhow, regardless of what this turns out to be, any ideas what the oscilloscope might be doing that it can cut this noise when I hook it up to the same power supply?

Edit: a larger power supply filter cap made a huge difference. There's still an oscillating noise but it's much quieter. I'm going to experiment with a cap on Q3 now.

It sounds like the power supply itself.

A good design practice would be to keep the added caps.   However,  beyond that I think you are solving the wrong problem.    If the power supply is producing 600Hz then the power supply has a problem.     If you use that power supply for something else it's likely to produce a 600Hz artifact as well.   If it's an old or second-hand switch-mode supply,  the power supply might have bad caps inside and that's making it oscillate.   Another possibility is the power supply is misbehaving because of the light load.   You might try adding a resistor across the power rails.   Start with say 100 ohm and go up or down to see if you can shift or removed the oscillation.

Your other pedals might pull more current and that's why you don't see a problem.