MFOS Noise Toaster DIY Troubleshooting

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Re: MFOS Noise Toaster DIY Troubleshooting

Post by guest » Wed Feb 05, 2020 8:22 pm

the JFET side of R45 should be the same as the gate voltage you were measuring before. so maybe the resistor isnt connecting to the JFET, or you werent measuring the right pin on the JFET? do you have any pictures of your board?
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Re: MFOS Noise Toaster DIY Troubleshooting

Post by feedbackl00p » Wed Feb 05, 2020 8:33 pm

guest wrote:
Wed Feb 05, 2020 8:22 pm
the JFET side of R45 should be the same as the gate voltage you were measuring before. so maybe the resistor isnt connecting to the JFET, or you werent measuring the right pin on the JFET? do you have any pictures of your board?
It is the same voltage as the gate, i think i omitted the negative symbol.

Both the drain and source pins show zero voltage.

I dony have a photo but I can make some later. You can assume its the same as the layout i posted a couple of posts up, ive checked all the physcial connections many times.

What do you make of the filter sweeping correctly once on startup, but then fails to loop?

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Re: MFOS Noise Toaster DIY Troubleshooting

Post by guest » Wed Feb 05, 2020 8:40 pm

oh, i see. the gate is at -1V to -2V? that makes a lot more sense.

at startup the voltages havent settled out, so its possible that the JFET is fine and it goes through a decent state at startup before settling into a bad state. or its possible that the JFET is busted inside, and risidual charge internally makes it work fine to to begin with, and then that charge moves off and it goes bad.

try removing R44 entirely and see what happens. does it sound any different?
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Re: MFOS Noise Toaster DIY Troubleshooting

Post by feedbackl00p » Wed Feb 05, 2020 8:49 pm

guest wrote:
Wed Feb 05, 2020 8:40 pm
oh, i see. the gate is at -1V to -2V? that makes a lot more sense.

at startup the voltages havent settled out, so its possible that the JFET is fine and it goes through a decent state at startup before settling into a bad state. or its possible that the JFET is busted inside, and risidual charge internally makes it work fine to to begin with, and then that charge moves off and it goes bad.

try removing R44 entirely and see what happens. does it sound any different?
Interesting about the power.

Removing r44 and shorting that connection to BN makes cutoff stop working entirely.

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Re: MFOS Noise Toaster DIY Troubleshooting

Post by guest » Wed Feb 05, 2020 8:56 pm

2$ is a bit expensive, but JFETs are a little hard to come by these days, so not suprising. i find jameco to be the best place to get outdated things. depending upon where youre located, there might be a store with a decent substitute.

so, cutoff was full open before, and now its full closed? what if you dont short that resistor, and just leave it open?
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Re: MFOS Noise Toaster DIY Troubleshooting

Post by feedbackl00p » Wed Feb 05, 2020 9:11 pm

guest wrote:
Wed Feb 05, 2020 8:56 pm
2$ is a bit expensive, but JFETs are a little hard to come by these days, so not suprising. i find jameco to be the best place to get outdated things. depending upon where youre located, there might be a store with a decent substitute.

so, cutoff was full open before, and now its full closed? what if you dont short that resistor, and just leave it open?
Cutoff wasnt fully open before, the cutoff knob responds properly. Its just the modulation of it that isnt working.

With R44 removed and shorted it sounds like cutoff is fully open. With R44 not shorted the cutoff kinda works, but theres no resonance.

The cheapest ive found from a reputable dealer is about $45 AUD delivered for 10pcs, which is twice as much as the cost of the rest of the project. And unfortunately the places that sell them dont sell everything else i need so theres no way to mitigate the expense.

Standard problems for Australian hobbyists.


edit: just found an Oz store that sells them for $2 a piece, but ships for the price of local postage. Ive taken a punt on a few and will report back.

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Re: MFOS Noise Toaster DIY Troubleshooting

Post by guest » Wed Feb 05, 2020 9:50 pm

it sounds like its a matter of tweaking components to get that JFET into is right operating range. also, the resonance is coupled to the cutoff, so as the cutoff closes, the resonance decreases (or at least i think i have that the right way).

so, do you have a small amount of cut off control with R44 removed and not shorted? if so, try shorting across R39 with some alligator clips and see if you get more range.
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Re: MFOS Noise Toaster DIY Troubleshooting

Post by feedbackl00p » Thu Feb 06, 2020 6:13 pm

Another question about biasing.

If im trying to find the right range for the Jfet, what components should I play with? Can i add a trimmer or the like instead of a resistor to make it simpler?

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Re: MFOS Noise Toaster DIY Troubleshooting

Post by guest » Thu Feb 06, 2020 6:49 pm

there are 2 things that need adjusting: offset and gain. R44/45 do most of the work here. personally, i would remove R39/40/44/45. then take a potentiometer (50k or 100k) and connect the two ends from ground to the BN. connect a 47k resistor from the wiper to the gate of the JFET. the pot will allow you to adjust the offset, and the 47k sets the gain (make it a bigger value if you need more gain). next, take another 47k from the gate and go to the wiper of the mod depth pot.
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Re: MFOS Noise Toaster DIY Troubleshooting

Post by feedbackl00p » Thu Feb 06, 2020 7:48 pm

guest wrote:
Thu Feb 06, 2020 6:49 pm
there are 2 things that need adjusting: offset and gain. R44/45 do most of the work here. personally, i would remove R39/40/44/45. then take a potentiometer (50k or 100k) and connect the two ends from ground to the BN. connect a 47k resistor from the wiper to the gate of the JFET. the pot will allow you to adjust the offset, and the 47k sets the gain (make it a bigger value if you need more gain). next, take another 47k from the gate and go to the wiper of the mod depth pot.
Fantastic help, thank you again. Ive got a project for the weekend it looks like!

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Re: MFOS Noise Toaster DIY Troubleshooting

Post by feedbackl00p » Fri Feb 07, 2020 4:26 am

guest wrote:
Thu Feb 06, 2020 6:49 pm
there are 2 things that need adjusting: offset and gain. R44/45 do most of the work here. personally, i would remove R39/40/44/45. then take a potentiometer (50k or 100k) and connect the two ends from ground to the BN. connect a 47k resistor from the wiper to the gate of the JFET. the pot will allow you to adjust the offset, and the 47k sets the gain (make it a bigger value if you need more gain). next, take another 47k from the gate and go to the wiper of the mod depth pot.
You bloody genius, it worked straight away. :hail:

It seems to sound best when the 100k pot reads about 75k. How do I integrate this knowledge into the existing circuit with R39, 40, 44,45?

At least it solves the mystery and I know the FETS are out of spec.

I very much appreciate the time youve taken to help. Ive learned a lot during this process. Assuming the JFETs i get next week are the real deal i should have a fully functioning unit.

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Re: MFOS Noise Toaster DIY Troubleshooting

Post by guest » Fri Feb 07, 2020 11:35 am

glad its working. so, if you take out the pot (or just disconnect the wiper), and read the resistance from the wiper to the 2 ends, you should get 2 resistances that add up to the total of the pot. for example, 75k and 25k for a 100k pot. you can then just replace the pot with those fixed resistors.

the JFET might not technically be out of spec, its just that the spec for this part is over a wide range, and the circuit provides now way of adjusting for this. you can actually measure the JFET to see if its in spec or not. remove it from the circuit, connect the gate and source to ground, and the drain to a 1k resistor. the other side of that resistor goes to 9V. then measure the voltage across that 1k resistor. this will tell you the current into the drain, and is known as the "zero gate voltage drain current". and this is the way they sort JFETs. if it is between 1V and 5V, then its within "spec" for the part (but obviously not for the circuit).
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Re: MFOS Noise Toaster DIY Troubleshooting

Post by feedbackl00p » Sun Feb 16, 2020 8:58 pm

So the Centronics 2n5457s have arrived, and all problems were fixed by replacing the apparently fake or out of spec Fairchild ones I got from the net.

Ive now got the noise toaster modular fully working and its ridiculous fun for analog patching on the couch.

Ill be posting the project once ive cleaned it up a bit.

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Re: MFOS Noise Toaster DIY Troubleshooting

Post by feedbackl00p » Sun Feb 16, 2020 9:01 pm

guest wrote:
Fri Feb 07, 2020 11:35 am
glad its working. so, if you take out the pot (or just disconnect the wiper), and read the resistance from the wiper to the 2 ends, you should get 2 resistances that add up to the total of the pot. for example, 75k and 25k for a 100k pot. you can then just replace the pot with those fixed resistors.

the JFET might not technically be out of spec, its just that the spec for this part is over a wide range, and the circuit provides now way of adjusting for this. you can actually measure the JFET to see if its in spec or not. remove it from the circuit, connect the gate and source to ground, and the drain to a 1k resistor. the other side of that resistor goes to 9V. then measure the voltage across that 1k resistor. this will tell you the current into the drain, and is known as the "zero gate voltage drain current". and this is the way they sort JFETs. if it is between 1V and 5V, then its within "spec" for the part (but obviously not for the circuit).
Thanks so much again for your help. Authentic 2n5457s have cleared the issue, but your help with biasing JFETs is an incredibly useful tool i can see myself going back to many times.

Its interesting that in Rays book, he mentions that 2n5457s are end of life, and being replaced by the NTE457. Fast forward a couple of years, and no major stockist seems to have the NTE457, and the market is flooded with out of spec versions of the 2n5457, and Centronics started making them again with the old designation.

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