Frantone Vibutron Depth Control

[Zouto]

I got a vintage Vibutron rev1 from a friend of mine. It‘s the model with two bias resistors instead of a trimpot. Sound is great and inspiring, but turning the Depth control completely cw to the right, there‘s still too much dry signal in the mix instead of just wet modulated signal.Is there a good fix for this? Reversed wiring of Speed and Depth pots is standard on Vibutrons. Would you also recommend replacing the bias resistors with a trimpot and how to do it? Unfortunately there‘s no schematic available, I sent a mail to Fran Blanche, but did not get an answer up to now.
Thanks a lot for help and support
Zouto

[Leo_Gnardo]

Given there's not many Vibutrons out there, I suspect the only expert on them is Fran herself. Yes I'd try the trim pot approach, but without schematic it's up to you to dope out the circuit & see where you could best apply a trimmer. "Backwards" controls, probably because Fran avoided using reverse audio taper pots. I've seen the same on some 50's - 60's amps. Also, for starters I'd make sure the depth pot is still working right, that the wiper goes all the way to zero ohms or near-as-dammit at each end of its travel. If a defective pot was the problem it sure would save you fiddling around with trimpots & guesswork.

No answer? Fran's new videos used to pop up in my YouseTube feed semi-regularly. Well I just put Franlab into their search window, and indeed she does have plenty of recent videos, so she's not on the missing list, not by far.

[Zouto]

Leo, thanks, you‘re right, that‘s the micro -invasive way to go. I‘ll check the pot, hoping for the best and Fran‘s answer.
Zouto

[dmeek]

There is a hand-drawn schematic here: https://www.freestompboxes.org/viewt...190176#p190176

[J M Fahey]

and indeed she does have plenty of recent videos, so she's not on the missing list, not by far.

Even better,she´s prominently featured in the just released "History of Effects Pedals" movie, go figure.
So she might be VERY busy as a consequence.

[dmeek]

What Happened to Frantone?

https://www.youtube.com/watch?v=cc00CpPvhlI

[Leo_Gnardo]

Even better,she´s prominently featured in the just released "History of Effects Pedals" movie, go figure.
So she might be VERY busy as a consequence.

In fact, I posted here about the movie and Fran's participation in it months ago.

To see what she's up to I checked out her most recent YT videos - commentary about the anticipated release of information about UFO's. Not much about pedals but who knows, maybe the little green men will have some kool outta-space pedal mojo to share. A dab of element 115 (touted by UFO expert Bob Lazar) ought to spice up the tone some, dont'cha think? My point of view: they should have Geraldo Rivera present the expose'. After months of highly anticipated buildup... "there's nothing there!" Just like inside Al Capone's safe. Another big nothing burger, concocted to sell advertising time. I hope to be proven wrong, but we've been to this movie before. Anyway it's all light years off the subject of getting the Vibutron fixed but maybe deserves a thread of its own here.

All the best to movie star Fran!

[Zouto]

Well, just back with my UFO from outer-space void to fixing my Vibutron, the 100k B depth pot Is ok. So what to look next for? Pix of the rev2 pcb here https://www.flickr.com/photos/354223...th/6926188298/

[Helmholtz]

The Depth control wiring doesn't allow for pure wet signal. For a wider mix range you may try a higher value Depth pot. e.g. 220k.

[Leo_Gnardo]

So what to look next for?

Looking at the hand drawn schematic, there's what looks like a MOSFET that's used to shunt signal to ground according to the waveform presented at its gate, simple enough. Two things to consider: First, from the "max depth" end of the pot, there's a 0.15 uF cap and 2000 ohm resistor "in the way" between the pot & MOSFET, so it looks as if one must expect the attenuation of signal will not be 100%. You could try reducing the value of the 2000 ohm resistor. I'd try paralleling that resistor with another one - gator clip in resistors say in the 220 ohm up to 1K range & see if one of those gets you the extra depth you're looking for.

Second, and maybe it should be first, there's what appears to be a 100K "TRIM" between signal generator and MOSFET gate. Mark where it's set so you can return to that point if you need to. If there's insufficient generator signal to cause the MOSFET to go to a near short-circuit, you'll need to dial in a bit more with the trim pot. If you have a scope you could observe a test signal at the MOSFET end of the 2K resistor and set the trim pot so the signal is fully attenuated one or the other extreme of the generator signal at its gate. If this condition isn't being met then any other adjustments won't help much.

Hope this helps. I'm no pedal expert but I did stay in a Holiday Inn...

[Zouto]

Looking at the hand drawn schematic, there's what looks like a MOSFET that's used to shunt signal to ground according to the waveform presented at its gate, simple enough. Two things to consider: First, from the "max depth" end of the pot, there's a 0.15 uF cap and 2000 ohm resistor "in the way" between the pot & MOSFET, so it looks as if one must expect the attenuation of signal will not be 100%. You could try reducing the value of the 2000 ohm resistor. I'd try paralleling that resistor with another one - gator clip in resistors say in the 220 ohm up to 1K range & see if one of those gets you the extra depth you're looking for.

Second, and maybe it should be first, there's what appears to be a 100K "TRIM" between signal generator and MOSFET gate. Mark where it's set so you can return to that point if you need to. If there's insufficient generator signal to cause the MOSFET to go to a near short-circuit, you'll need to dial in a bit more with the trim pot. If you have a scope you could observe a test signal at the MOSFET end of the 2K resistor and set the trim pot so the signal is fully attenuated one or the other extreme of the generator signal at its gate. If this condition isn't being met then any other adjustments won't help much.

Hope this helps. I'm no pedal expert but I did stay in a Holiday Inn...

Thanks a lot for help and good suggestions. Unfortunately my Vibutron rev1 has no trimpot between MOSFET‘s gate and signal generator, just a series-parallel three-resistor combination to achieve a value of fixed 38k6. I think a trimpot would be better as in later Vibutron versions, but „butchering“ the original version is quiet a decision…..

[Leo_Gnardo]

Thanks a lot for help and good suggestions. Unfortunately my Vibutron rev1 has no trimpot between MOSFET‘s gate and signal generator, just a series-parallel three-resistor combination to achieve a value of fixed 38k6. I think a trimpot would be better as in later Vibutron versions, but „butchering“ the original version is quiet a decision…..

Would be good to 'scope where I mentioned, to see if the signal is being fully attenuated. If not, and you don't want to disrupt the original setup with that resistor combination, then we're stuck with suggestion #1 - clip a parallel resistor onto that 2K & see if that helps. If it does, I s'pose it's less of a sin to tack solder the winning resistor across the 2K that's already there. After that I'm outta suggestions.

[Zouto]

Would be good to 'scope where I mentioned, to see if the signal is being fully attenuated. If not, and you don't want to disrupt the original setup with that resistor combination, then we're stuck with suggestion #1 - clip a parallel resistor onto that 2K & see if that helps. If it does, I s'pose it's less of a sin to tack solder the winning resistor across the 2K that's already there. After that I'm outta suggestions.

Big thanks Leo, 2k2 parallel resistor plus 100k trimpot were the right cure for better Depth control efficiency and a bit more weirdness.

[Leo_Gnardo]

Big thanks Leo, 2k2 parallel resistor plus 100k trimpot were the right cure for better Depth control efficiency and a bit more weirdness.

YAY! Meanwhile Fran is on a tear - she's gotten 6x more hits on her YT site since starting on her UFO meanderings. You know what's important: eyes on the page brings in $$$. So, good for her. Whatever makes a buck...

[Zouto]

Well, it‘s the omnipresent jootoop-shmoo-lube routine…..