Magnavibe issues: lots of noise and hiss. Please help

[guile]

I built a Magnavibe the other day but it's much too noisy to be usable.  
Only thing I changed from the layout is swapping all the 1uf caps for 2.2 uf to make the rate slow down. This worked fairly well but don't know if this causes the noise and hiss. Can't swap them again for 1uf as this will ruin the board for sure.

Any wisdom would be very much appreciated!

[nocentelli]

Oscillator cap swap shouldn't affect the noise, and the magnavibe is usually pretty quiet (I think it rolls off some treble and some people find it muddy without a buffer so hiss is unusual). Is it unity volume? Double check all resistor values in the audio section,  maybe swap the transistor.

[guile]

Thanks for your reply.

It is unity volume. What transistor could I swap it for?

[nocentelli]

If all the resistors and caps are correct values, I'd initially just try another mpsa18 out of the bag, that's what I've always used without much hiss: There's a slight chance the particular one you've used is at fault. Any similar npn si would probably work, however - 2N5088/BC549/BC108/etc

[guile]

Thank you, I will try that.

[anotherjim]

You do have it in a box sealing out all light don't you?

[guile]

You do have it in a box sealing out all light don't you?

Yes, I do. With or without light doesn't change the hiss.
I tried different transistors and that helps, but still noisy. The board is fried now so will have to start again.
Thanks so far!

[guile]

I built the entire board again and the same rhytmic saw-like noise.
The problem seems to be in the on/off led. Leaving it out helps a lot. Still need the on/off led. Is there any way around this?

[nocentelli]

Post the layout you've used. Are the 2u2 polarised? I can't think how a status/ on-off LED would make ticking noise worse. Can you characterise the noise a bit more? Your first post implies hiss/noise which suggests an issue with the audio transistor gain; hum would suggest a ground issue, but rhythmic saw noise suggests it's related to the LFO - does the rate pot affect the rate of the noise? Is it ticking?

[guile]

Layout: http://pedalparts.co.uk/docs/MagicVibes.pdf

Are the 2.2n polarised? yes

I can't think how a status/ on-off LED would make ticking noise worse. Can you characterise the noise a bit more? No ticking at all. It's pulsating noise that vanishes the moment I cut the on/off led

Your first post implies hiss/noise which suggests an issue with the audio transistor gain; hum would suggest a ground issue, but "rhymic saw noise" suggests it's related to the LFO - does the rate pot affect the rate of the noise? Yes, but only when on/off led is attached

Is it ticking? No

[guile]

I changed the on/off LED resistor value to 20k and used an extra bright led. This solved the pulsating noise issue. Still some noise but useable now.