fuzz pedal - crazy hum/radio interference, only when fuzz dimed??

[drummer4gc]

hey all,

i am rehousing a jordan bosstone fuzz effect into an old MXR enclosure. When I turn the "attack" knob fully clockwise, I get a loud hum and radio interference, just like when there is improper grounding in a pedal/guitar. however, as soon as i back off a little, it goes away and works fine. does this have something to do with the design? I'm trying to figure out why this would happen only when the knob is turned full up. I've grounded it in a few different ways, connected the input wire straight to the pot instead of to the board, tried a small capacitor from input to ground, no luck with anything. can someone help? i feel like with this specific of a problem, there has to be a singular solution that i am just not seeing.

thanks!

[LucifersTrip]

The easy solution is to make it so that the fuzz can't hit 100% (small resistor on the attack pot), but I always think that's sacrilege...and as always, a very small cap (50pf - 200pf) can sometimes tame oscillation...but again, sacrilege.

Honestly, there's just so many ways that can happen....it could be as simple as a wrong choice or a mis-biasing of transistor(s)....or even the power supply (did you try a battery?).  It's probably not a grounding problem since it doesn't occur continuously.

Did you check to make sure the voltages are close to a working one?

[drummer4gc]

Thanks for the suggestions!

I did try a battery as well, same issue. The effect was a friend's who says that it wasn't doing this before. I did notice that the pots in the original effect only measured about 96K ohms instead of the full 100k...fairly normal for pots to be off a little, but I wonder if this would make a difference. I'm going to try to bypass the pot later today, hardwired to full on, to see if that changes anything, but I'm not too hopeful.

Any other ideas out there are greatly appreciated!!

[rousejeremy]

John Lyons "1k/470p filter at the input should do it si or Ge."

[LucifersTrip]

it's very doubtful the pots would be the problem...

just realized I didn't give complete info. my above comment should read:

"a very small cap (50pf - 200pf) across Q1 BC can sometimes tame oscillation...but again, sacrilege."

[R.G.]

The problem is almost certainly RF oscillation if it starts above some gain setting.

I would attack it two places. First, it's related to the wiring from the volume pot wiper. This wire is getting capacitively coupled back into the circuit, so re-routing it or shielding it should help. So might installing a small cap across the volume pot or wiper to ground to lower the output level at RF, but not audio.

Even better, lower the susceptibility of the circuit to RF.

- put a 0.1uF *ceramic* cap from +9V to the emitter of the first transistor
- install a small resistor, 10-100 ohms, value not terribly important, in series with the base of the first transistor; ideally, pull up the base lead, solder the resistor into the hole, then solder the base lead to the other end of the resistor
- if these don't work, put a quite small cap from collector to base of the first transistor; 10p to 100p

These tricks will not change the audio sound if done properly, but should kill any RF oscillation.

... maybe. I once had a Bosstone clone that resisted all my efforts to make it RF free.

[amptramp]

If the "Attack" pot has the normal construction with a metal shell on the back, check to see if there is continuity between the shell and the chassis ground.  If there isn't (and there often isn't), establish a ground.  Note that this may be aluminum, so solder may not work.  If this construction is used on both pots, ground both shells.  You may have to use a trick like bending one of the tabs back and wrapping a thin-gauge ground wire around it then folding it back down.  The cover is capacitively coupled to the resistive track and may promote feedback.

[drummer4gc]

Thanks for the suggestions all. I tried a ton of different ideas, got frustrated, and sinned - 5k resistor in series with the attack pot input. No trouble at full setting, still sounds as nasty as I'll ever need it.

Appreciate all the help!

[LucifersTrip]

The problem is almost certainly RF oscillation if it starts above some gain setting.

I would attack it two places. First, it's related to the wiring from the volume pot wiper.
Even better, lower the susceptibility of the circuit to RF.

before doing that, wouldn't it be a good idea to simply make sure the voltages are close to "correct" for the circuit?
I cannot tell you the amount of times I got some type of oscillation, noise, hiss or hum when volume or fuzz was maxed, before I biased the transistors properly.

On a related note, I recently finished this MKII:
http://forum.musikding.de/cpg/albums/userpics/16602/MKIIOC75schem.png

It oscillated in the final 5% of the fuzz. Ironically, I fixed the problem and wound up with MORE fuzz.  I rebiased Q1 to give me a hotter, more saturated fuzz overall, but the oscillation still existed in the same final 5%. I then added the "optional" resistor (measured & only needed 88 ohms)...which still had more fuzz than at the oscillation point before I rebiased Q1.

Since the designers knew there may be some oscillation at max fuzz, they sometimes included that optional resistor. Instead of adding that, I maybe should have tried a couple of your suggestions first....

if these don't work, put a quite small cap from collector to base of the first transistor; 10p to 100p
These tricks will not change the audio sound if done properly, but should kill any RF oscillation.

I've done this many times, but there is always a change in audio sound (mild taming or slight loss of highs). Is it possible to add this cap without doing that? I think I need a 100pf variable cap....

[R.G.]

before doing that, wouldn't it be a good idea to simply make sure the voltages are close to "correct" for the circuit?
I cannot tell you the amount of times I got some type of oscillation, noise, hiss or hum when volume or fuzz was maxed, before I biased the transistors properly.

Maybe. The two problems interact. If you get full-bore RF or ultrasonic oscillation, it can change the apparent bias point. Meters intended for DC read funny values sometimes when fed DC plus RF.

It oscillated in the final 5% of the fuzz. Ironically, I fixed the problem and wound up with MORE fuzz.  I rebiased Q1 to give me a hotter, more saturated fuzz overall, but the oscillation still existed in the same final 5%. I then added the "optional" resistor (measured & only needed 88 ohms)...which still had more fuzz than at the oscillation point before I rebiased Q1.

The current in bipolars can affect their high frequency/RF response.

Since the designers knew there may be some oscillation at max fuzz, they sometimes included that optional resistor. Instead of adding that, I maybe should have tried a couple of your suggestions first....

Whatever works is OK.

I've done this many times, but there is always a change in audio sound (mild taming or slight loss of highs). Is it possible to add this cap without doing that? I think I need a 100pf variable cap....

It depends on the rest of the circuit, and on the voltage gain. A cap from collector to base acts like a capacitor of voltage-gain times the actual capacitor from base to ground. This is the Miller effect. If the gain is high, you may need very tiny capacitors. If that's the case, use a real cap from base to ground. This position is not multiplied by the voltage gain, so has a non-multiplied effect on the frequency response.  

Frequency rolloff is frequency rolloff. If the cap affected audio tone, it either (a) had some rolloff in the audio band in that circuit or (b) was changing the RF response to shift the circuit action so the audio band was affected. If that happens, use a smaller cap, change the cap position from B-C to B-gnd or B-E. You might well need a 2pF to 10pF cap in the B-C position for some high voltage gains.

Also, making the circuit less prone to RF means there is more gain available in the audio band without oscillation. Clean power supplies and ground, and elimination of stray feedback capacitances mean you can use higher gain for audio, if that's what you want.

Everything oscillates if the gain is high enough.   

[LucifersTrip]

Frequency rolloff is frequency rolloff. If the cap affected audio tone, it either (a) had some rolloff in the audio band in that circuit or (b) was changing the RF response to shift the circuit action so the audio band was affected. If that happens, use a smaller cap, change the cap position from B-C to B-gnd or B-E. You might well need a 2pF to 10pF cap in the B-C position for some high voltage gains.

thanx much for the info...I'll consider that the next time I run into any similar problems. I honestly don't think I've used anything lower than 20pf yet and haven't used any B-E caps.