Deflector hisses when Density knob isn't maxed

[falco_femoralis]

Hi all, just put together a Deflector. I get a hiss as the Density knob is turned counter clock wise. The hiss is also affected by the mix knob, with it getting louder as the mix is turned clockwise. I've checked my component values and it all seems okay. I've searched here and google and this seems to be a common issue, but I can't find a definitive answer.

Any smoking gun type thing I should look into?

Thanks,
Brian

 

[falco_femoralis]

I spent some time pouring over the schematic and data sheets as well as more google searches and from what I can tell the CD4049UB chip provides the 3.3v to run the FV-1, which rather than using its internal programming, uses one loaded onto IC3, and the 4049 is also set up as an adjustable frequency generator for the FV-1's X1 input, rather than the crystal the data sheet suggests.

From searching the forums, the consensus is that the TI 4049 chips are noisy and I should try an NOS brand. I wonder if a 4007 can be substituted? I don't know much about it other than the pins are slightly different.

The hiss noise I'm getting happens with the Density knob in any position except full; increasing the mix knob makes it more obvious. So the voltage output part of the chip seems to be functioning without issue, it's only when changing the frequency of the clock that the noise comes into play. I've double checked my resistor and cap values in this region.

I just wish I understood more about how the clock circuit works.

 

[Robert]

I wouldn't worry about the 4049 being noisy, that doesn't apply here since it isn't in the signal path.

3.3V to the entire circuit is supplied by the L78L33 voltage regulator, not the CD4049.

It's normal for there to be noise at the low extreme of the Density pot, but it shouldn't be noisy throughout the majority of the rotation.

 

[jwin615]

L78L33 provides the 3.3V
The 4049 is out of the audio path so, though new production 3039s can be noisy in some circuits, I doubt that's the issue here as it's just clock.

Just to be clear. Is the hiss louder at 100% wet?
Also, gut shots. And make sure your in and out wires are ran AWAY from the pcb. The "hiss" could just be reverberated clock noise bleed near Nyquist.

 

[falco_femoralis]

Re: the power supply - oh yeah, I see it now. Pin 1 of the 4049 is fed by the L78L33

The Density knob introduces hiss when turned counter clock wise. As the wet knob is turned clockwise, the point at which the hiss introduced by the Density knob changes. For example, when the wet knob is at full clockwise, the only position the Density knob can be at and not produce hiss is full clockwise. With the wet down to half, the Density knob will be mostly silent down to about 12 o'clock and then start introducing noise.

The hiss is a white noise type of sound. It is not affected by input signal from the guitar and is constant regardless of whether I am playing or not.

Some of my pots are off board mounted but the Density, Wet, and tone controls are mounted to the board

I suppose I can start by removing R16 and seeing what that does to the signal?

 

[jwin615]

I would move all our off board wiring as far away from the pcb as possible(outside of the enclosure) and see if it goes away.
I wouldn't mess with r16

 

[falco_femoralis]

I took the pedal back apart and removed the 4049 chip. I installed a socket so I can try different things and I put the chip back in. I was probing around with my finger, trying to recreate something I'd read in another thread. I discovered when I touch pins 2-5 of the 4049, the exact same hiss noise becomes louder. Presumably due my finger grounding perhaps pin 3 against pin 4. I'm not sure what to make of it yet. Nothing else I touched made the hiss noise.

I don't understand why there is no noise when the control is on full. What I know is that there is no resistance across the pot or R16 when that happens. I may try to source a crystal and install it in such a way to bypass the 4049 and its RC circuit to either eliminate or identify it as a cause of the noise.

Studying the data sheet of the 4049, it says that unused inputs should be connected to either VCC or ground, whichever is more convenient. The schematic for the Deflector doesn't reflect this, though I don't have the pedal in front of me to confirm. The data sheet also lists the maximum soldering temp to be 265 C, and I don't typically socket my IC's, so it's possible I may have cooked the chip installing it. I believe I have an extra one around and will try it out when I get home tonight.

 

[falco_femoralis]

I tried some stuff. I ordered a new chip and installed that. No change.

I studied the diagrams and noticed pins 7, 9, 11, and 14 were not connected to VCC on the pcb trace. They were connected to each other but not to VCC, so I made that correction. The hiss seemed to be a bit less, but it was still there. I then reflowed every solder joint and replaced the Density pot. I checked the value of C14. It's all good.

I play thru a Deluxe Reverb and I have the bright cap on a switch, and when it's on it amplifies high pitched hissing noises. I usually play with it on for what it does for my cleans, but with the bright cap bypassed the hiss is reduced. I'm having the same scenario with my Mach 1 build atm.

So now I have decent functionality with the Density knob maxed and then down to about halfway before it gets too hissy. The volume of the hiss is affected by the mix control and, it seems, also my guitar playing input. Overall I believe it's at an acceptable point now, but it does bug me that it's not 100%.

I'm trying to understand what the 4049 chip is doing to set the frequency going into pin X1 of the FV-1 chip. I understand that when the Density control is maxed, it bypasses the 100k ohm resistor between the two amplifiers arranged in its circuit. I just don't understand how they create an oscillation. I don't have a background in electronics theory but I do enjoy learning, especially during the troubleshooting process.