Strange power supply behaviour

[Fancy Lime]

Hi everyone,

I noticed something quite bizarre: I built a fuzz with reverse biased NPNs. Schematic here:

http://www.diystompboxes.com/smfforum/index.php?topic=118360.0

It works fine with a battery and with all of my wall warts, except one. The one that does not work works with all my other builds and pedals. Only this specific pedal with this specific power supply does not. The status LED works but the circuit makes no sound at all, not even noise. The power supply delivers 300mA and the circuit draws a measly 75µA, so that can't be it.

Any idea what the funk is up with that, anyone? Not like this is a practical problem, the pedal works fine with a Visual Sound 1-Spot, which I usually use anyway. But I would really like to understand what sort of voodoo is going on there for future reference.

Thanks,
Andy

[rezzonics]

Minimum charge required?

[thermionix]

What voltages are you getting with the suspect wall wart compared to the other power supplies?

[Fancy Lime]

Slightly over 9V, just as for the others. The circuit runs fine on 6V from a different supply as well.

[thermionix]

That's while it's powering the circuit?

[Fancy Lime]

No, that's with the power supply not connected to anything. Good point though, I should try what the voltage is when connected to the device. However, since the thing only eats 75µA, I would not expect much difference.

[anotherjim]

Minimum charge required?

I would say "minimum load" and that's a good first call given the behaviour. But it seems the output voltage can be measured ok without anything plugged in so not likely.

Is there a possibility that the DC jack centre pin is not making contact in this pedal? So when other pedals are used with it, it gets supply negative via the audio jack screens from the others. Not all 2.1mm plugs/sockets are a perfect match.

BTW, 75uA suggests an extremely efficient activity LED in that pedal - or is that only the circuit?

[Fancy Lime]

Hi Jim,

75µA is just the audio part without the LED. The center pin contact was something I thought might be the problem, but there are 9V going to the board, says my multimeter. So thats not it. At this point I'm almost convinced it's just cursed. Always suspected that the whole electricity thing is witchcraft anyway...

[rezzonics]

Hi Jim,

75µA is just the audio part without the LED. The center pin contact was something I thought might be the problem, but there are 9V going to the board, says my multimeter. So thats not it. At this point I'm almost convinced it's just cursed. Always suspected that the whole electricity thing is witchcraft anyway...

Does it work when you plug your multimeter while reading 9V?

[R.G.]

The preceding comments are good, and need to be checked. In fact, they're more plausible than some things that come to my mind. But just for completeness, here are a few less-probable things.

But before I get started, do try eliminating that power supply minimum load thing. Hang a 2.2K resistor across the power to your pedal. Some power supplies get funny (="go into oscillation") at either zero load or loads near zero. It's worth eliminating.

Now back to the weird stuff.
A pedal that otherwise produces sound and then stops under some conditions may be oscillating at frequencies so high you can't even see them on "utility grade" oscilloscopes. You could possibly, maybe, have hit one of these unlucky conditions with that one power supply. If so, the extra loaad resistor would change things, if not cure it.

Does that one power supply have issues with any other pedal, too? It seems more like we're debugging a power supply than debugging a pedal.

Have you tried swapping in different KSP2222s? The reverse connection does work, but it, too is "funny". The reverse breakdown voltage on the base-emitter junction for normal silicon transistors is often 5 to 7V according to the data sheets. To get the high forward gain and high frequency response, the base-collector junction is doped differently from the base-emitter, and the difference in zener voltages is one consequence of this; the difference in gain is another. It is possible that you have one transistor that is on the edge of zenering the base-emitter and that's causing an interaction with the power supply. The "works fine at 6V" comment makes me suspect this. Another transistor might have a slightly higher base-emitter breakdown and not show the problem, expecially if it's a different type or from a different batch of transistors of the same type.

[anotherjim]

Always suspected that the whole electricity thing is witchcraft anyway...

We know that witchcraft is just technology that's too advanced for us. I think Arthur C Clarke said something like that.
Actually it was "Any sufficiently advanced technology is indistinguishable from magic".

Anyway, I'd not give up yet. No specific ideas, just hunting for evidence.

RG's zener theory is a good one.

That fuzz circuit. Try bypassing D3, the 1N4001 going in. Maybe it will stop working with other supply then too.

Compare transistor voltages when working and not.

Put your DMM +probe on the boards +9v connection and go around with the -probe all the places that should be 0v and you should read 9v at all of them. Check your audio jacks screen contacts also.

Does the battery negative go to the in jack ring contact as usual?

Does the battery positive go to the middle contact of the DC jack?

Is there anything different about the supply that don't work with it? AC cord ground/earth pins - some have connected and some don't. Anything on the label like "for office IT equipment only".

[Fancy Lime]

Hey everyone,

thanks for the ideas and input. I don't have answers to all your questions but will do some controlled testing tonight and try to answer the points one by one. Heres what I can tell you real quick at this point:

The "bad" power supply is the only one I own that is not switching. Just a plain old (quite old in fact) iron core with wires and a rectifier. Don't see how that would make it not work but then again, I don't know that much about the potential quirks of power supplies.

I don't suspect the built is to blame because it did not work on the breadboard and does not work now completely boxed.

To test R. G.s zener theory, I'll try different transistors. I'll also try to use them "correctly". That will sound horrible because it has way to much gain but should work with the circuit and show, if the reverse biasing is the problem in conjunction with this supply.

Cheers,
Andy

[Fancy Lime]

Soooo,

back from the testing lab. Turns out its not magic, I'm just stupid! Well, I guess that explains a lot of things that at first seem like magic. Or as Jim cited Arthur C. Clarke: I am apparently not sufficiently advanced for what I'm doing.

Let me explain: It turns out the contact between the ever so slightly too large plug and the ever so slightly too small pin was shaky. It works when the box is turned on its back, as it was when I measured stuff. It looses most of the contact most of the time when the thing stands on its feet, as it was when trying to play it. The little bit of rest contact every few seconds is apparently enough to charge the power supply ripple smoothing capacitor C10 enough to light the LED but not enough to make a sound. When I play while pushing the connector in with one hand, it works most of the time. So the fact that it always seemed ok when measuring and never when playing seems due to positioning and possibly a slightly crooked pin in the DC jack. The fact that this was the case on the breadboard as well (where I only checked it once as compared to many times in the box) was probably just a coincidence. It boils down to a jack and a connector being ever so slightly off in different directions. Lesson learned: There is no way around proper testing with all parameters controlled and only changing on parameter at a time. Damn it, I should know this, its kind of my job.

So thanks again to all. I promise I'll try to avoid asking embarrassing questions about rookie mistakes in the future. Still, I learned a lot about what else could go wrong, so that's something, I guess...

Andy

[thermionix]

 

[R.G.]

We've all either been there before or will be there. The big thing about mistakes and finding subtle flaws is to note that Mother Nature absolutely insists that positively every little thing be right before She'll let things work right. Sometimes the every-little-thing can be things that are unapparent or mysteriously small.

[Fancy Lime]

Hi R.G.,

thanks for the kind words. Means a lot especially coming from a veteran such as yourself. Those people complaining that the anonymity of internet forums turns people into screaming hate-goblins must be frequenting different forums. They really should start building fuzz boxes. It's good for the soul, I think...

Cheers,
Andy

[anotherjim]

I don't think you were being stupid, just a little too quick to believe it was beyond you. Stick around, we'll cure you of that.

[Tony Forestiere]

[bluebunny]

Stick around . . .

Ha ha!  Like he has a choice...