Hissing from a Distortion Generator ***Solved***

[X100Ver10]

Yes, this is the same DG that I replaced all the switches and pots. I also replaced all 4 electrolytic capacitors.

I have a loud hissing from this DG. The hissing gets louder and softer as I manipulate any of the switches or pots. The only time the hissing goes away is when a footswitch is plugged into the Bypass in the back and I activate the Bypass (red bypass light turns on) by pressing the footswitch. The hissing will come right back if I turn off Bypass with the footswitch.

I have another DG and it doesn't hiss, so I know it isn't any other equipment.

If it helps, I believe the DG with the hiss was probably in a humid environment, due to the rusty/pitted switches and pots I replaced. Also, I have had this a while now. I don't remember if it hissed when I first got it.

Can anyone point me in a direction to resolve the hissing? Don't tell me to send it to David or to check for snakes!

Thanks.

[Fdesalvo]

Hey, there.

So, it’s a hissing/white noise sound vs a buzz correct? When did it begin making this noise? Pre of post cap surgery? Do any of the distortion modes make it worse or better?

Speaking in generalities, failing resistors can cause white noise and hissing- especially any carbon comp/carbon film resistors around the early stages near the input. I kind of doubt this is your issue, since many of the vintage amps I’ve restored got along fine and with acceptable noise (given their age) on the old carbon comp resistors they came with.

[X100Ver10]

Hey, there.

So, it’s a hissing/white noise sound vs a buzz correct? When did it begin making this noise? Pre of post cap surgery? Do any of the distortion modes make it worse or better?

Speaking in generalities, failing resistors can cause white noise and hissing- especially any carbon comp/carbon film resistors around the early stages near the input. I kind of doubt this is your issue, since many of the vintage amps I’ve restored got along fine and with acceptable noise (given their age) on the old carbon comp resistors they came with.

Hi Fdesalvo,
Thanks for replying. Yes, a hissing/white noise. Not a buzzing or hum. I can't remember if it hissed before the electrolytic caps replacement. Its been too long. I'm sure it did though. There are only 4 caps. 2 were for the power. I'm sure I checked if the new ones were good before installing. I usually do.

It seems like all the switches and sliders either make it better or worse depending on where I put them. Higher seems worse. Lower better. I figure that is only because the volume is increasing and decreasing though.

Do you recommend taking nearby Input resistors out of circuit and testing them?

[Fdesalvo]

You can check the resistors in circuit as long as there’s no current flowing through them. Do you have a schematic or a band decoder? Forgot to ask - does it hiss with no plug in the input jack?

[X100Ver10]

You can check the resistors in circuit as long as there’s no current flowing through them. Do you have a schematic or a band decoder? Forgot to ask - does it hiss with no plug in the input jack?

No schematic but I have used the band decoder at Digi-Key. I became very familiar with it and band codes when I was figuring out what was wrong with a Rockman Acoustic Guitar Pedal.

Yes, it still has the hiss with no plug in the input jack.

[Fdesalvo]

I hear ya! I have a little engineering app on my phone that has some Ohm's law features and a resistor band decoder, as well. Can't live without it.

Is she still still hissing? Sorry I haven't been so helpful. I'm new to these units (rookie).

[X100Ver10]

Fixed it!

It turns out the TL074CN IC at location U1 was the culprit.

It helps to buy another, working DG, to compare voltages.

[Fdesalvo]

Nice work! Can ya give us a shot of the little bugger?

[X100Ver10]

Nice work! Can ya give us a shot of the little bugger?

Sure. Here you go.

The culprit:

IMG_20180311_112926.jpg

The new one installed:

IMG_20180311_113016_1.jpg

[Fdesalvo]

Excellent troubleshooting. Looks like it’s near the input, so it would have a huge impact on noise. Any changes in tone?