Vintage Foxx Tone Machine

[vmazz38]

Ive got a vintage Foxx Tone Machine on my bench. It had a wire out of place which made it functional. Its got fuzz sound and passes signal, but it is fairly low output and there is not much bass. The controls all function as they should. It made me suspect the old electrolytics where going out of tolerance. Anyone with experience with this variation of the fuzz? It seems input and or output caps could be shaving off to much signal. particularly lower frequency.

I think this is close to original schematic
http://fuzzcentral.ssguitar.com/foxx.php

[thermionix]

Never messed with a FTM, but any electro from the 70s(?) is way past its life expectancy.  Were those made in Japan?  They look like it from the pictures I've seen.  My experience is mostly with tube amps, and the old Univox amps had some really low quality caps.  Elna, off the top of my head.  Maybe another brand too.

[yeeshkul]

How does the filter in the negative voltage feedback work (from Q2 to Q1)? Fuzz Face or amps from the era use just 100k.

[thermionix]

Is it a band pass filter?

[Mark Hammer]

My sense is that the filter in the feedback path is what allows the FTM to have such a robust octave.  It provides differential gain for the fundamental.

[Danich_ivanov]

I haven't spent much time with foxx in particular, but i did with octave fuzz concept. Besides biasing and filtering i would assume that it also stabilizes the emitter output in relation to collector output, by leaving that cap throughout the whole thing, hence making a perfect phase splitter is not that easy. Also, i'm not entirely sure why, but emitter can oscillate a tiny bit, and i bet it helps with that as well, i would think by providing negative feedback.

Can't help much though, it can be a number of things, caps or a bad solder joint probably.

[vmazz38]

Thanks for the input. I have reflowed all solder joints.   So on to the caps.

[vmazz38]

New caps in. Works like new! Pretty thick over the top fuzz! Octave is great! Sound like Fu Manchu...Thanks!
Also when I took out the first 10 uf cap I tested on my eico ...tested very weak about .1uf!

[Mark Hammer]

I've found that the stock design can yield a better non-octave fuzz if the octave switch is wired to select between connecting the second diode (producing the octave), and bridging the remaining diode.  Stock, the signal from the phase splitter always has crossover distortion, since it MUST pass through the remaining diode, even when the octave is off.  Some might like that, but I found it to be annoying, and liked the normal clipping produced by the diode pair to ground.

That said, I have also found that using Schottky diodes (e.g., Bat 41/46, 1N5817, etc.) for the phase splitter and octave production tends to get a more reliable octave, and also better octaving further down the fretboard than one is accustomed to (right down to the 5th fret and even below).  If you DO decide to try Schottky diodes in that way, there is no need to wire up the octave switch in the manner described above, since the crossover distortion is negligible and barely discernible with such a low forward voltage.

[pinkjimiphoton]

i'd replace the electros. a ftm should be huge phat and chewy, like a cross between a big muff and a fuzzface. if its really thin, check the octave switch too, as often switches break down over time and it may be either stuck in or bleeding thru the octave fuzz part of the circuit.

that said, i have a foxx fuzz wah on my bench that only works if plugged in backwards... lol... so ....