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.

10 comments:

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  3. I have one of these, same problem with voltage regulator. I replaced it and now voltages are ok there but still no sound with the switch pressed. Do you have any idea about this problem? Thanks man

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  4. Mine works but it does not give me the jet like swoosh sound. It just acts more like a chorus. Any suggestions to why that is? Do I need to get a new one?

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  6. Hello Krivx :)
    Thank You Very Much for this blog page. I am an electronics tech and for fun, I buy broken pedals off of Reverb and eBay to fix and use for myself at home. At the end of June, I snarfed one of these EVH-117 pedals for $60 shipped and figured it shouldn't be too hard of a fix - oh boy was I in for it. The symptoms were just as you described with no flange-effected audio. Long t-shoot process short, I would say that someone sabotaged the pedal and covered their tracks well.
    The 78L15 was bad as well as the 78L09 (stressed) - both had been removed and replaced. I replaced the 78L15 with an on-hand 7815, powered up and watched U2 MC33178 smt meltdown within a few seconds. Initially my fault until I realized that U2 had been replaced backwards previously. I ordered 33178s and replaced it - no smoke anywhere but no effect signal still. Had to put aside for a while till I could dedicate more time - yesterday. Using you photo from this page, I was able to verify that C22, C36, C46, R77, and R71 were all missing from the board with no sighs of ever having been there (solder & flux cleanup). The dual-battery-snap cable and the dual-battery tray were also missing. What convinces me that this was a sabotage is when earlier today, I found a cut trace on the bottom side of the board that feeds U12. With that tiny cut repaired, the pedal went from a crappy-sounding junk vibrato to it's normally glorious flanging abilities! Using your picture and a copy of the 1980 M-117 schematic, I was able to piece together what the parts are that were missing (I had to guess on C36). If your pic of the PCB was high-res, I coulda seen them hehehe.
    Also of note, the re-issue MXR M-117R uses the exact same board as the EVH-117. there are three additional small trimpots on the EVH that are missing on the M-117R - RP3, RP6, and RP8. The other EVH-117 parts are: DS2, SW1, C2, R1, R2, R52, and Q6. The U1 HEF4053 Tri-switch IC is actually installed on the M-117R but since the click-switch isn't installed, it is never switched and in default mode. I don't own a M-117R yet so there may be a few more parts missing. I look at it this way, this pedal was designed as the EVH-117 and then parts were removed to make a standard M-117R. I managed to find a hi-res PCB component-side pic of the M-117R and can see most of the differences via that.
    Thanks again,
    Dan Gibbs :)

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  7. The article is much informative which i was searching for.Nice intro good explanation thanks for sharing.
    Enrgtech

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  8. Can you please explain what the 4 trim pots do? I am getting a faint swoosh sound even with the pedal off and also feedback when I switch it on? Will these help it?

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    1. Trim pots are just to adjust EVH button settings

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  9. Hello,

    Thanks for the blog. My unit it exhibiting speed issues, it goes full blown and the speed pot won't adjust the speed setting. Any ideas? Thanks.

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