hiss when note is plucked

[tonesearcher]

I have an old Korg AX1500g multi-effects processor that has started to hiss when I pluck any note . The hiss also follows the decay of the note but the unit still maintains a soft, steady hiss when the strings are not being plucked.

The hiss is confined to the Drive Amp section that has the compression and distortion selections. The hiss disappears when I bypass this section.

Anyone knows what may be causing the hiss?

Thanks!

[R.G.]

I have an old Korg AX1500g multi-effects processor that has started to hiss when I pluck any note . The hiss also follows the decay of the note but the unit still maintains a soft, steady hiss when the strings are not being plucked.

The hiss is confined to the Drive Amp section that has the compression and distortion selections. The hiss disappears when I bypass this section.

Anyone knows what may be causing the hiss?

Thanks!

Parametric oscillation. The circuit settles into not-quite-oscillating when there is no signal, but is driven into high frequency oscillation by signal. The hiss is an audio artifact of RF oscillation that turns up sometimes. This can be a power supply decoupling issue, and that would make me suspect decoupling caps. But it could be some other root cause, too.

Put a scope on the output of that section and look for RF oscillation when there's signal.

[Mark Hammer]

Does the unit have some sort of noise-gate function, and if so, how is it set/applied?

[dumbassbob]

Hiss

My 100 w.Marshall started doing that after a hour.I took it to my amp tech and he sprayed 25 cents of Detoxit-5 all over the pots and output jacks.Water,beer,sweat,crud were all over it Bing worked like new then charged me $45.00

[tonesearcher]

Parametric oscillation. The circuit settles into not-quite-oscillating when there is no signal, but is driven into high frequency oscillation by signal. The hiss is an audio artifact of RF oscillation that turns up sometimes. This can be a power supply decoupling issue, and that would make me suspect decoupling caps. But it could be some other root cause, too.

Put a scope on the output of that section and look for RF oscillation when there's signal.

I had hoped might be a cap related problem since the 100uF 16V caps and a couple of transistors are the only discrete components on the board. I'll need to make some serious time to try and scope the circuit. The hassle will be trying to follow those tiny PC board traces and SMD components. I really miss the days of discrete components and DIP ICs.

Thank you R.G.!

[tonesearcher]

Does the unit have some sort of noise-gate function, and if so, how is it set/applied?

The unit does have a noise gate circuit. The gate was set at a medium threshold (5 out of 10) and it used to work fine. The hiss that tracks the plucked note and the slight increase in continual hiss just appeared recently, so I suspected some type of component failure.

I turned the noise gate sensitivity to "off", which made the continual hiss a bit louder but it did stop the note-tracking hiss.

Thanks Mark!

[tonesearcher]

My 100 w.Marshall started doing that after a hour.I took it to my amp tech and he sprayed 25 cents of Detoxit-5 all over the pots and output jacks.Water,beer,sweat,crud were all over it Bing worked like new then charged me $45.00

At first, I thought it might be a grounding issue so I did the cable tests and the DeOxit treatment. Bummed, the hiss was still there. But at least I had my own can of DeOxit spray.