Red Witch - Scarlett Overdrive

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Red Witch - Scarlett Overdrive

Postby reddesert » 16 Apr 2017, 23:14

Red Witch has a line of "Seven Sisters" - very small, rechargeable pedals with a lithium battery inside, moderately priced. I happened to get a Scarlett Overdrive and decided to trace it to figure out what the circuit was, and because I thought the technology was interesting. Of course, deciding to start by tracing a tiny SMD pedal may not have been the most brilliant decision.

Rechargeable pedals don't seem to have taken the effects world by storm, but maybe this is an avenue that will see more development in the future. I'm pretty impressed that all of this fits inside an enclosure smaller than a 1590A.

Here are two photos of the circuit board. The battery has been removed for these pics - it's a Maxell 3.7V Li battery that plugs into the white 3-pin connector.
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Re: Red Witch - Scarlett Overdrive

Postby reddesert » 16 Apr 2017, 23:27

And here is the schematic that I traced. Because tiny SMD is a pain and traces run under components, etc, I wound up verifying essentially every connection by checking continuity with a multimeter. I tested the small capacitor values in-circuit with one of those 328A component testers - this yielded plausible values except for the cap in the feedback loop.

Perhaps unsurprisingly, the overdrive circuit turns out to be essentially a tube screamer with a couple of minor changes. The internal B50K trimpot is the tone control - the pedal manual tells you to adjust it for more treble/more gain or less/less to taste. I checked multiple times and one end of the trimpot appears to be unconnected, unlike a TS.

Anyway, I expected the power/charging to be the more innovative part of the pedal. It uses a 3.7V Li-ion battery and a MAX1044 charge pump to produce a supply voltage of 5.8V - double 3.7 less two diode drops. I measured Vb to be 2.2V (not sure why it's not half of 5.8V).

The switch is a DPDT, presumably for size, and it achieves true bypass + LED switching with something that looks like a descendent of the Millenium bypass, using two transistors.

The battery circuit has an IC that I assume is designed for battery charging labeled "LCLG 09," something that must be a regulator, and a bicolor LED.
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