fuzzface problems

[rocket8810]

so i've got a si fuzzface and i've got a few problems that i've been debugging for a few days and i can't figure out for the life of m. e what's going on. it's basically a stock fuzzface with an output cap blend and part of an affect i've designed. everything was breadboarded before the board was made, except for the output cap blend. everything else for the circuit worked including the fuzzface part, and on the breadboard i got no oscillations from the fuzz and it worked as it should.

problems:
1 - way below unity output. upon tracing with an audio probe i have strong signal up to the output caps, but lost it at lug 2 of the cap blend bot. the pot is wired similar to a tonebender so that i have 1 cap going to lug 1, the other cap going to the lug 3, and lug 2 goes to lug 3 of the volume pot. both caps are connected to the 8.2k, 330r junction. the pot is a 100k pot.

2 - it oscillates like a mother if you're not playing. when i traced the signal i found the oscillation occurs at the collector of Q1 and the leg of the 33k resistor coming from +9V that's going to the collector of Q1, not on the leg attached to +9V. adjusted the bias and it's spot on, tried lower gain transistors same issue.

the low output is making me thing that the output cap blend is either not wired right, or the pot is acting like a series resistor and cutting the signal. i'm completely stumped with the oscillation issue, and need to have some sort of tone control on the output. the idea is to have a variable highpass filter that will go from the stock whoolly tone up to 350Hz at the output. i figured the cap values needed to do it with a cap blend, but something is obviously a miss.

any ideas? btw, thanks in advanced as always.

[Cozybuilder]

It would be helpful if we had a schematic to look at.

Your description makes me think you may have out of phase output signals at each end of the blend pot.

[rocket8810]

hey cozy, here's the schematic for the fuzz section



edit. just realize R3 isn't labeled. right now it's 330r, but was originally planning on upping it to 470r or 1k for more output to match the clean side when it was on the breadboard, but with the current oscillation i didn't change it.

[antonis]

Try to place a low value cap (47-100 pF, say) between Collector & Base of Q1..
(if Q1 is a BJT..)

If some oscillation remains, try also the same for Q2..

[Cozybuilder]

This schematic looks like it should work, although I expect the signal out will be quite thin. I would suggest using a cap capable of passing the full signal at one end, and a small cap at the other- maybe 1 nF and 100 nF for example. Your blend would be more of a tone control then. At this point I suggest checking the wiring, component values, and cold solder joints.

I agree with Antonis to try snubber caps on the transistors to tame the oscillation.

[rocket8810]

antonis yea they are, i didn't realize i used the FET schematic symbol when i quickly put the schematic together. i'm planning on using BC108 transistors gains are ~250hfe, and tried lower gain BC183 transistors with ~160hfe. i'll give the small caps a shot. what makes no sense to me is where the oscillation occurs when i audio probed it.

Cozybuilder, do you think the the 900pF cap is too small and that's causing the issue of low output? literally when audio probing it, the signal is strong throughout the fuzz, and going into lugs 1 & 3 of the cap blend pot, but a lug 2 it's like nothing. i've gone over the board a few times now, didn't find any cold joints, and the wiring should be right, but might as well check again. even had a buddy double check the layout and the schematic to see if i made an error, and he didn't spot anything that was off.

it's maddening that on the breadboard it worked right, and on the board it doesn't.

[Cozybuilder]

The 900pF will give a thin reedy out, maybe use 4.7nF instead and 100nF at the other end of the pot. Maybe try a much smaller value of pot too- say in the 10K to 50K range. Or look at the SWTC for more ideas:
http://www.muzique.com/lab/swtc.htm

Edit-
Is the blend pot good? A bad wiper would give low out vol.

[Gus]

Are you using a battery or external power supply?

Have you tried a 100uf cap from 9VDC to ground.

What does your layout look like?

Short wires long wires?

Layout matters with FF like circuits.

You layout most likely changed from breadboard to board.

[rocket8810]

hey gus. using a DC jack, same issue happened with a battery, there's a 100uF filter cap +9V to ground, the wires aren't really long or short, kinda medium i guess. really used just as much as i needed to make it to the pots with not a lot extra. the layout has both parts of the circuit on it, fuzzface on the bottom the clean on the top. i've heard about the fuzzface being temperamental, but until now i never had any issues with it before with any of the fuzzface layouts i've made. since you said that the layout matters what matters with fuzzface what's important to watch out for?

cozy, i was thinking about it possibly being a bad pot while i was working on my car this afternoon, was thinking about swapping it out and see if that fixes it. if i shrink the pot, wouldn't that decrease the sensitivity in the sweep? yea the 900pF will thin it out a lot, but when i went to calculate the cutoff for the high pass filter including the output cap and the volume pot to get 350Hz cutoff that was the value of the cap. i originally was thinking about having a 100nF on one end and 1nF on the other, but that didn't give the desired cutoff.

[Quackzed]

if you connect the blend pot middle lug to its bottom lug it'll be more like a standard input cap blend setup, always going straight through the 900pf, and 'blending' in the big cap in parallel with the 900pf  via the 100k variable resistance pot , that way there shouldnt be any big vol drop except for low freqs which is i think what your after... all the way up and the 2 caps are in parrallel 'added' so just under 11n and all the way to the other side just 900pf, with 1n/lows essentailly dialed out due to the series 100k...

[antonis]

i was thinking about it possibly being a bad pot while i was working on my car this afternoon

May I also presume you think about bad spark plugs while you're working on your PCB..?? 

[rocket8810]

May I also presume you think about bad spark plugs while you're working on your PCB..?? 

haha. nah, but oddly enough i was working on my replacing my headers, spark plugs, and wires on my chevelle when the idea of a bad pot came to mind. i do think about my things i need to work on car while i'm working on circuits. maybe i should be concerned about some crossed wires.