Other Equipment

Vintage Marshall Supa Fuzz pedal issue. I could need some tech help.

1

I have a ca. 1970 Marshall Supa Fuzz pedal. As I know it's worth some money and I never use it I was preparing it for a sale. Since I once acquired it, it always had that rustling noise when engaged. I was thinking of dirty pots and cleaned them now but it's still there. Cleaned the jacks and the switch (as much possible) too... but nothing. The bypass signal when disengaged is fine. All original parts. No harm to be seen. I run it with a fresh 9V battery.

Does anyone have an idea? Could the switch been broken? Does this sound a killed transistor? (that would be bad)

Thank you.

Sascha

2

It's one of those (not mine pictured).

3

Such a cool pedal, an old work mate gave me a one years ago, I sold it for £400 quid. I've no idea what is wrong with yours though.

4

That rustling, or rush-of-wind-over-the-telephone sound (if I'm interpreting you correctly) is really indicative of dirty pots. How did you clean them? Could also be old electrolytic caps. Easy to replace, but I'm not sure how it might affect the value. If you're not comfortable with a soldering iron, I'd be happy to replace them for you

5

I used quality contact cleaner and then a shot of WD40. It didn't change anything at all surprisingly. The fluttering noise increases when I turn up the 'Filter' control. So it might be more than just dirt in the pot. The volume pot (that's exactly as old) appears to be okay. Changing a component heavily affects the sales price unfortunately and I'd like to be sure what the actual cause is. I'm okay with a soldering iron with voltages and current this low. Thanks a lot for the offer! I might be a bit far-off, though.

What's the chance it might be the switch or can this be ruled out?

6

damn that's a cool looking pedal... no ideas tho...

7

In my opinion it's very unlikely to be the switch. You can rule that out.

This is what we're looking at:

More info here

Your observation that it changes with the "Filter" knob (on this schematic it's labelled "Attack", but it's the same) really points us in that direction. That "fluttering" type of sound is indicative of changes in DC making it into the signal. Typically (if all parts are OK) turning that pot would not change the DC voltage at all, because that pot only changes the voltage seen by the 10uf cap, and caps don't pass DC. However, old electrolytic caps (which that one is) dry out over time and begin to leak current. If that cap is leaking current, then turning that pot will adjust the DC bias of the transistors, and you will get the stereotypical "whooshing" sound. It's likely that the leakage is not steady, so you'll get that sound even without turning the pot.

I hope that made sense

IMO the best way to test this would be to measure the voltage of the emitter of Q3, and see if it changes when you adjust the knob (or if it changes without adjusting the pot). If the cap is fine that voltage won't change, conversely if the voltage changes then the cap is your culprit. The good news is this is an easy fix, but as stated, may affect the value.

Alternatively you may try measuring DC resistance across that cap. It should be very high, like infinity high. If it has a measurable resistance then it's probably leaking. It may be intermittent though, which means this test is difficult and not necessarily definitive. Your best bet is to measure the transistor voltage. Note if the transistors are Germanium all of your voltages should be negative (and the electrolytic caps should have + pointed to ground).

Do you have any photos of the guts? If you can, post some close-up images of the electro caps. It will be hard to tell from photos, but maybe something obvious will stick out.

8

Thanks so much, Otter. I had a look at the inside and took two pics. The caps look good to me from the outside. Amateur question: Measuring voltage in series? So I have to desolder one leg of the component, right? How I wish I can avoid that.

To view a large version of the pic open it in a new tab and delete anything after the first .jpg in the URL.

Now that I look at the photo... The blue cap 2nd from right looks greenish but that's just a shadow.

10

What kind of price will you be asking for this?

11

Sascha, no need to desolder anything for now! Set your meter for VDC (20V range), and measure the voltage across the "Filter" pot, the two outside legs.

  • One should be grounded, place the black probe on that one (or any other convenient ground location; sometimes I like to just shove the black probe in one of the enclosure screw holes).

  • Then place the red probe on the other outside leg, what does it read?

  • Turn the pot all the way down, what does it read?

  • Turn the pot all the way up, what does it read?

If the cap is good, every voltage you measure here should be rock solid, probably around -0.3 to -0.7 VDC. Don't worry about the specific voltage, it's more important that it doesn't change when the pot is turned.

12

Sascha, no need to desolder anything for now! Set your meter for VDC (20V range), and measure the voltage across the "Filter" pot, the two outside legs.

  • One should be grounded, place the black probe on that one (or any other convenient ground location; sometimes I like to just shove the black probe in one of the enclosure screw holes).

  • Then place the red probe on the other outside leg, what does it read?

  • Turn the pot all the way down, what does it read?

  • Turn the pot all the way up, what does it read?

If the cap is good, every voltage you measure here should be rock solid, probably around -0.3 to -0.7 VDC. Don't worry about the specific voltage, it's more important that it doesn't change when the pot is turned.

– Otter

Thanks, Otter! I'll give this a try tomorrow.

13

What kind of price will you be asking for this?

– Setzer

As much as I'd like to do but this time I need the money and so I can't let this one go for a GDP mate's price. Last one I saw that actually sold had the switch and knobs replaced and went for €560 which is short under $700 and not overly much for one of these. A slightly earlier version recently sold for much more.

14

Understood. I was curious. I've got a old colorsound tonebender that I play around with.


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