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Discussion Starter · #1 ·
Hey all,
I’m hoping there’s some experience out there with the first two versions of this classic pedal. Im wondering how much audible difference there really is. I know they used different chips and the boards defiantly look a lot different. There aren’t too many videos available but it seems to me so far that maybe the original is a bit warmer/darker, maybe a bit juicier. Anyone with actual experience of the two pedals, I would love to hear your thoughts. Are either of these worth the money with all the technology available these days? I really don’t need a lot of modern functionality— I need a delay, for the most part, for a subtle set it and forget it kind of thing; it’s all about the tone for me on this particular search.
Thanks!!!
 

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The original M118 implemented something that I guess MXR co-founder Keith Barr became a little infatuated with, and that is switched-resistor filtering. As you probably know, all BBD-based delays use a high-frequency clock to step the signal through the delay chip. Typically, analog delays would pick a filter rolloff frequency to keep noise out of the worst-case final result, and fix it at that. Barr/MXR used the clock - divided down - to turn solid-state switches on and off, in a way that mimicked a tunable filter. The result was a "tracking filter" that allowed for more bandwidth when the delay time was shorter and reined in bandwidth when the delay time was long (i.e., when the clock was pulsing at a lower frequency and moving the signal through the BBD more slowly).

The delay chips used in earlier versions of this clever delay pedal are long out of production, hence not available in quantities more than one or two here and there at sky-high prices.

Miniaturization allows for companies to build in variable filtering of the delay path at very modest cost, and in a package that does not demand big enclosures and the various costs associated with size. Can they "nail" the tonal specifics of an original M118? Who knows. It is always worth noting that the wide tolerances of capacitors - especially as they age (and caps on an original M118 are 40 years old...at least) - can make for unit-to-unit tonal differences in ostensibly identical pedals, that people mistake for consistent features of ALL pedals of that make/model/issue. So it is quite possible that a particular M118 unit is "darker, warmer, juicier". But are they ALL like that? You'd really need to try many examples of that model/issue to know for certain. And the odds of that happening are slim. To be fair, it's not like the difference between germanium fuzz units of the same model, but there can be audible differences.
 

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Discussion Starter · #3 ·
Thanks so much! That’s really helpful. Is it sort of silly to chase that vintage delay tone given the scarcity of parts if/when something fails? Or do these chips last forever? I have heard the filter cap on the power supply should be changed. I’d hate to drop a bunch of dough and then have a broken pedal that can’t really be fixed or sold.... thanks again!
 

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Thanks so much! That’s really helpful. Is it sort of silly to chase that vintage delay tone given the scarcity of parts if/when something fails? Or do these chips last forever? I have heard the filter cap on the power supply should be changed. I’d hate to drop a bunch of dough and then have a broken pedal that can’t really be fixed or sold.... thanks again!
Electrolytic caps (those cylindrical ones with a + and - terminal) have a tendency to dry out over several decades, and be off-spec. Chips, much less so, if at all. Bucket brigade chips need to be biased properly in order to work, and it can happen that jostles and kicks over several decades nudge a trimpot inside the pedal such that the biasing is a little off and needs to be reset. But often that's the brunt of what can go wrong with an analog delay....at least under normal circumstances.

If a pedal is >35-40 years old, and sounds a little "off", replacing the electros - especially the larger-value ones - can often restore the pedal to original form. The caveats to keep in mind are that: a) you don't want to damage the printed circuit board by applying too much heat in removing the old caps, and b) electro caps have gotten smaller in size over the last few decades, so the spacing of the leads might not easily match the spacing of the pads on the circuit board.

Having nursed several people through a repair of their beloved green box over the years, one of the things that can also go wrong with them is that one or more of the CMOS chips in the pedal are fried and need replacing. Both the chips used to divide down the clock for switching, and the chips that DO the switching of the tracking filter, are CMOS, which are both static sensitive, and intolerant of supply voltages higher than +15Vdc. Happily, they are cheap, still in production, and plentiful, and replacing them has restored many a pedal.
 

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Always wanted to try one of these, even just to compare to something like a Carbon Copy. I've read that Neil Young uses one, and IIRC Keef as well (Some Girls). Sounds great in those instances.
 

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Discussion Starter · #8 ·
Electrolytic caps (those cylindrical ones with a + and - terminal) have a tendency to dry out over several decades, and be off-spec. Chips, much less so, if at all. Bucket brigade chips need to be biased properly in order to work, and it can happen that jostles and kicks over several decades nudge a trimpot inside the pedal such that the biasing is a little off and needs to be reset. But often that's the brunt of what can go wrong with an analog delay....at least under normal circumstances.

If a pedal is >35-40 years old, and sounds a little "off", replacing the electros - especially the larger-value ones - can often restore the pedal to original form. The caveats to keep in mind are that: a) you don't want to damage the printed circuit board by applying too much heat in removing the old caps, and b) electro caps have gotten smaller in size over the last few decades, so the spacing of the leads might not easily match the spacing of the pads on the circuit board.

Having nursed several people through a repair of their beloved green box over the years, one of the things that can also go wrong with them is that one or more of the CMOS chips in the pedal are fried and need replacing. Both the chips used to divide down the clock for switching, and the chips that DO the switching of the tracking filter, are CMOS, which are both static sensitive, and intolerant of supply voltages higher than +15Vdc. Happily, they are cheap, still in production, and plentiful, and replacing them has restored many a pedal.
Amazing info, and reassuring. Thanks so much!! I guess there’s really no cause for concern about parts then. How can one tell if the biasing is off?
 

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Amazing info, and reassuring. Thanks so much!! I guess there’s really no cause for concern about parts then. How can one tell if the biasing is off?
The audio signal needs to ride into a BBD atop a DC voltage. So, for instance, in the DM-2 schematic below, one can see that the input to the BBD (IC3) comes via C15. C15 blocks out any unwanted DC voltage. The Bias trimmer takes the supply voltage (the solid black triangle), divides it down, and feeds that resulting voltage through R18, to sum with whatever the audio/AC signal is.

When the bias voltage is either too high or too low, no audio will pass through the delay chip. As one gets closer to the right bias voltage - either a little too low or a little too high - one will hear a distorted and lower-volume delay signal. As one gets closer to the optimum bias, the delay sound will get clearer and louder. Ideally, one would adjust the bias using a known audio signal of predetermined amplitude and frequency, and compare that signal to the output of the BBD on an oscilloscope until the "best" version of the signal is achieved. For the rest of us, that have to do such things by ear, the best course of action is to temporarily defeat the clean/dry signal so that one ONLY hears the delay. In this particular instance, the first stage, comprised of half of IC, sends a clean copy to the other half of IC1, that serves as the dry/wet mixer stage, via resistor R32. One would temporarily unsolder/lift one end of R32, such that only the wet signal is audible at the output. This would make it much easier to hear the delay signal begin to get cleaner or distorted. Once the bias is properly set, the resistor is soldered back into place. Note that the delay chip can not be damaged by monkeying around with the bias voltage, so twiddle away until you find delay bliss.
 

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Discussion Starter · #10 ·
The audio signal needs to ride into a BBD atop a DC voltage. So, for instance, in the DM-2 schematic below, one can see that the input to the BBD (IC3) comes via C15. C15 blocks out any unwanted DC voltage. The Bias trimmer takes the supply voltage (the solid black triangle), divides it down, and feeds that resulting voltage through R18, to sum with whatever the audio/AC signal is.

When the bias voltage is either too high or too low, no audio will pass through the delay chip. As one gets closer to the right bias voltage - either a little too low or a little too high - one will hear a distorted and lower-volume delay signal. As one gets closer to the optimum bias, the delay sound will get clearer and louder. Ideally, one would adjust the bias using a known audio signal of predetermined amplitude and frequency, and compare that signal to the output of the BBD on an oscilloscope until the "best" version of the signal is achieved. For the rest of us, that have to do such things by ear, the best course of action is to temporarily defeat the clean/dry signal so that one ONLY hears the delay. In this particular instance, the first stage, comprised of half of IC, sends a clean copy to the other half of IC1, that serves as the dry/wet mixer stage, via resistor R32. One would temporarily unsolder/lift one end of R32, such that only the wet signal is audible at the output. This would make it much easier to hear the delay signal begin to get cleaner or distorted. Once the bias is properly set, the resistor is soldered back into place. Note that the delay chip can not be damaged by monkeying around with the bias voltage, so twiddle away until you find delay bliss.
Wow amazing info and detail! Thank you so much for sharing and taking the time!! Are you for hire to bias my almost new old mxr delay?!?? Toronto area perhaps?
 

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Wow amazing info and detail! Thank you so much for sharing and taking the time!! Are you for hire to bias my almost new old mxr delay?!?? Toronto area perhaps?
No, on both counts. But I'm happy to guide you through it. I like helping folks become more independent and self-sufficient. They generally go on to help others, so the debt gets repaid/
 
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