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Hi!

I have this Coron Distortion III which I got in an auction lot with a few other pedals.

Since I haven't seen a schematic for this online, and I wanted to practice doing a trace, I thought I'd give it a go.

I've added my trace files in case you can spot any mistakes.

Furthermore, I haven't been able to identify Q5, but my guess is that it could be a j-fet. It's marked 304 F1L and comes in a TO-92 case. The middle pin goes to ground and the other two measures 4,5v.

Diodes 1, 4 and 5 looks like the same type of diode, but haven't got any markings on them from what I can see. CX2 sits under the board and is a small ceramic cap, but it's impossible to read what value it is.

To sum up, it looks like a fairly common distortion pedal with a single 4558-opamp and clipping diodes to ground. However, surprisingly those clipping diodes are 5,6v Zeners.

It doesn't sound great, but not bad either. The biggest issue however is that the bypass is terrible. I can quite clearly still hear the pedal in the background while it's off. Would it be possible to improve the bypass on this one?

D2 and D3 are very unlikely to be zener diodes! They wouldn't ever conduct. Check again what they are. The "bleed through" of the effect isn't uncommon with cheap FET-bypassed circuits. This is why many people demand "true bypass" with a real switch!

mictester wrote:D2 and D3 are very unlikely to be zener diodes! They wouldn't ever conduct. Check again what they are. The "bleed through" of the effect isn't uncommon with cheap FET-bypassed circuits. This is why many people demand "true bypass" with a real switch!

I thought so as well, but the only things I could make out, as I recall now, was that it looked like "5,6" was written on them and that they looked just like the 5,6 zener diode that's connected to the LED. But I figured that it couldn't be right, but then I read this article: http://www.muzique.com/lab/zenmos.htm, and thought it might be possible anyway?

I'll desolder them and measure their voltages when I get the chance.

Here's a picture from another angle that shows both the clippers and the LED Zener.

mictester wrote:D2 and D3 are very unlikely to be zener diodes! They wouldn't ever conduct. Check again what they are. The "bleed through" of the effect isn't uncommon with cheap FET-bypassed circuits. This is why many people demand "true bypass" with a real switch!

An electronic magazine in the '80s here in Italy, and I supposed half Europe, showed a Distortion+ like circuit with zeners instead of GE diodes.
I remember I tried several lower voltage zeners to achieve more distortion back then...

What's the poster should check is they should go to Vbias instead...

Hi.
I had the "chance" to see that pedal and mod it to be true bypass.
I took the opportunity to trace it again and here's what I have (there were a few mistakes in the previous schematic, I didn't check most of the values, I copied them from the previous schem) :



The weirdest part for me is the tone control, it looks like a tilt / active big muff stack... but reversed.
Has anyone already seen something like that ?
I haven't been able to check the pedal a normal volume yet, but I'll report on how it sounds. I might install a tone bypass switch as well to turn it into a "normal" 250/D+.

I could turn the volume up a bit this afternoon.
It's quite similar to a big muff (nyc) tone stack to my hears. Not as big of a mid scoop, but there still is one and you can feel it. Kinda "metal"-sounding. More gain than a usual 250 (but that's expected with the differences in the circuit. tone bypass sounds huge, as always (shortcut from after the 10uf to before the 10k output resistor, so no output cap smaller than 10uf...).

By the way, I wrote "4.5V" but I should have written "Vb", since it's something like 4.9V (like in the other coron-made 250 inspired pedals i've seen)

re: the suspected "zener diodes" for clipping.

IF you are correct they are 5.6 volt zeners, they would not clip using the avalanche response at 5.6 volts, even with doubled supply voltage (+18volts).

But they WILL clip at around plus and minus 0.6 volts, even with a 9 volt supply. Why?

Zener diodes will conduct in the opposite direction to the polarity you normally apply when you use it to clamp a voltage, acting like any regular silicon diode will. Since they are in anti-parallel, the voltage can't ever reach the zener threshold. Would they sound different than plain vanilla 1N4001? Maybe. Some people will use a mosfet's intrinsic substrate diode for clipping and think the difference in sound is worth the additional cost.

FWIW

mr coffee wrote: ↑10 Aug 2022, 05:54 re: the suspected "zener diodes" for clipping.

IF you are correct they are 5.6 volt zeners, they would not clip using the avalanche response at 5.6 volts, even with doubled supply voltage (+18volts).

But they WILL clip at around plus and minus 0.6 volts, even with a 9 volt supply. Why?

Zener diodes will conduct in the opposite direction to the polarity you normally apply when you use it to clamp a voltage, acting like any regular silicon diode will. Since they are in anti-parallel, the voltage can't ever reach the zener threshold. Would they sound different than plain vanilla 1N4001? Maybe. Some people will use a mosfet's intrinsic substrate diode for clipping and think the difference in sound is worth the additional cost.

FWIW

I still have the pedal, I'll desolder them and check.

Matlevo12 wrote: ↑09 Dec 2022, 19:03 I still have the pedal, I'll desolder them and check.

They are zeners, 5.6V zeners, with a forward voltage drop around 735mV.