Motorboating FF noise after negative ground conversion.

[Davefx]

I had to take my perfectly healthy pos. ground FF and convert it to negative ground, so a friend could use it in his pedalboard with his power supply(all germanium,btw).  I changed the board wiring, removed and added the jumper (JD's layout), reverse pol. of the input cap, and with fuzz control maxed, it makes this "motorboat noise". If I turn the fuzz pot to half it stops. While it's making this noise, you can hear the fuzz at least sound like it's working underneath it!

I've checked all my wiring 1000 times.. So therefore it has to be something stupid I forgot... I can build flangers and chorus's that work the first time, but I can't seem to keep this thing from kicking  my butt!

Has anyone ever experienced this, and have sollution?  

Please help!!

[Paul Perry (Frostwave)]

Maybe it was 'almost' doing it before. Two things that can cause a motorboating sound, one is power supply impedance not low enough (try a large cap say 500 mfd across the battery) or else ultrasonic oscillations, try shielding the input wiring.

[gez]

A few people have had this problem.  Paul's right, stick a large value cap across the rails (though it probably doesn't have to be that big - 220u?).

[Davefx]

Even with a battery?

[gez]

Even with a battery?

Especially with a battery! When using the other rail as ground in a circuit the battery's internal resistance will now be in series with the emitter (plus any emitter resistor) for AC signals, forming a divider.  As a battery flattens its internal resistance will increase and more of the guitars input signal is developed across it affecting the circuits performance (sometimes adversely).  

For this reason it's always good practice to decouple the supply with a large value cap.

[R.G.]

This one keeps coming up.

While in theory, the negative ground conversion for FF and other PNP circuits ought to work every time, there are a significant number of times where it causes oscillation, motorboating, etc. Sometimes you can clean this up by putting a BFC* across the power leads, sometimes you also need a low-impedance 0.1uF ceramic, too, and sometimes you need divine intervention.

There are people who will swear that it works every time - and it probably has, for the things they've done. I've had situations where it won't work happen, so I am less dogmatic at saying that you can always get a flipped-power-supply conversion to work.

Try it if you want, but be aware that there are conditions and layouts that will not be trouble free if you flip power and ground.

* Big Freaking Capacitor

[Davefx]

Thank everyone!  I'll try the BFC suggestion.  If not, I'll have to change it back to pos gr. and my buddy will have to suffer with battery operation, which I personally prefer with a FF, anyway!!

[bhealy]

I'm having the same problem but I built it negitive from the get-go (from the schematic on generalguitargadgets.com). If I use a brand new battery it makes a shreaking noise if the fuzz is turned up and you turn down the guitars volume pot all the way. When you raise the guitars volume pot it slowly goes away. When pluged in (DC wall-wort) it  has a higher shreaking noise that only goes away with the guitar volume up full.
If you use a battery that is say a bit older (7.8 V) it gets that "motorboat" sound at all volumes.
When you say put a big cap on the power leads, where? Between the battery red lead and the DC connector? If you have a link to a diagram that would be great to see it.

Help!

[aron]

FAQ

[bhealy]

FAQ? I checked before I even posted this reply. There is not a discription of my problem, or a chart to show the solution.
But thanks anyway...

[gez]

Large value electrolytic cap  between the terminals (not literally - connections made to your circuit board) of your battery.  + pin of cap to + terminal, - pin of cap to ground (-ve terminal).

[bhealy]

I tried the cap mod and it seemed to work untill I pluged in the wall-wort. The wall-wart worked well too, but it fried out something that made the battery not work anymore.

[zgrav]

motorbotin' in my experience working on home stereo euipment has translated into bad capacitors or sticking relays.  in you are somehow discharging the battery when the ac adapter is plugged in, then the battery is still "too connected" to the system.  one of the terminals needs to be completely out of the loop.

as for the motorboting here, it's obviously not a relay, but I might try changing out the electro caps just to see if it made a difference.  there can be so much accepted variance between the rated and actual values anyway, that it is worth a try.