Thursday, 29 June 2017

MXR EVH Flanger

Another eBay pickup. This is a slightly modified re-issue of the MXR M117 with an "EVH" button that adds a fixed preset for a Van Halen sound. Dead, as usual.

This pedal uses an 18V supply or two 9V batteries, like some of the original M117s. The insides however are the standard modern MXR stye - almost entirely surface mount, a red PCB and board-mounted jacks and pots. It's a very tight fit to the enclosure, but it looks fairly robust as everything lines up very well. Turning it on lights up the LED but there is no signal.

Before soldering the new DC jack

The fix for this one was fairly simple. The DC jack was broken, so that was replaced. Still no power at any ICs. The traces from the DC jack & battery harness run to a 15V linear regulator, which feeds a 9V linear regulator downstream. Replacing the 15V regulator (78L15) brought it back to life. The LED is powered from the unregulated 18V supply, everything else needs working regulators. Drop-out voltage on a 78L15 is usually only ~1.5 - 2.0V so if you use batteries you probably can't drain them below ~8.3V each before the pedal will stop working.

It's possibly that this was killed by an incorrect power supply, but there is a reverse polarity diode which is still intact and the 15V regulator should handle up to 30V

The insides are pretty packed but this is more-or-less a standard Flanger design. Here is an overview of the ICs used:

V3204. A Bucket-Brigade Delay chip, almost definitely made by Coolaudio, but they don't list it on their site. As the original MXR Flangers used a Reticon SAD1024 (dual 512 stages) this is probably a 1024 stage BBD. Coolaudio make a V3207 chip with 1024 stages though, so I can't be certain.
SA572D compandor. Compressing and expanding before and after the BBD for better SNR, Electricdruid has an article on how this works.
MC33178, MC33179, TL072: opamps.
MC14504b level shifter. I was surprised to see this, I think it's translating low voltage clock signals to the higher voltage ranges that the BBD wants. This could be done with discrete FETs but maybe this solution was cheaper.
HEF4013 dual D flip-flop. I would have guessed that this used for bypass, but looking at older 117 schematic this is used with an opamp LFO for generating the complementary BBD clock signals.
HEF4053 triple SPDT analog switch. This switches out some of the pots for fixed-value resistors when the EVH button is pressed. This is a neat solution, I think more pedals could use this for "channel switching" by having two sets of pots.

It sounds great, it's a classic design and the controls are broad enough to go from subtle to over the top which I always enjoy. I can't hear any clock noise or bleed-through. The EVH button may be a bit of a gimmick as it's not easy to activate by foot, if you bend down to push the button you could just turn the knobs.

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