MXR DYNA COMP True Bypass

[mikabodcrane]

I have seen an older version of the MXR Dyna Comp [non-script logo] on Ebay which does not have the LED as the current production units have.  Does anyone know if these older units were true bypass?  If so I am wondering if anyone has experience with adding an LED to these units with the use of a 3PDT fulltone switch to maintain the true bypass, or if this is even possible?  Thanks

[Mark Hammer]

Just about anything that was produced before 1980 tended to NOT be true bypass.  Sometimes, ironically, they would even have a DPDT stompswitch and use only one set of contacts for signal switching.

By using only one set of contacts, and leaving the circuit permanently connected to the input jack (switching involved selecting what went to the output jack only), the effect would sometimes load down the signal considerably, even when in bypass mode.  When such effects are converted to true bypass, the effect sound is invariably a different volume than the bypass sound because the circuit was designed IN ORDER to provide level balance within those constraints/circumstances.

While EVERY effect from the past that has used SPDT bypass can be converted to DPDT true bypass, that does not mean it will also maintain level balance when converted.  Achieving this balance may require some small modifications to the circuit.

Using a 3rd set of contacts to switch a status LED on and off is certainly a feasible modification to an older pedal, and should not affect the sound at all.  Be forewarned, though, that LED's *do* require current, and that battery life can be noticeably shortened if that current drain is high enough.  If you use a wall wart to power it, then current drain is inconsequential, but if you regularly use a 9v battery, you should explore use of superbright LEDs which require only a fraction of the current that "regular" ones do.  They cost a little more, but you end up saving the cost in battery life.

[BDuguay]

Super bright LED's use less current? I would've thought the opposite. Do you have any specific mfg's/values that I might look for Mark?
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[Mark Hammer]

The thing to look for is brightness rating.  I try to aim for those with >2000mcd (millicandles) ratings.  The standard red LED most of us are accustomed to is generally in the <500mcd range and frequently <300mcd.  With a 9v battery supply and one of the standard red LEDs, it is typical to use current limiting resistors of 3k3 and less to provide acceptable brightness and visibility.  With superbrights, I can easily hike that up to 15k-18k and still have something to see easily.  The point is not to squeeze the full 2000mcd out of one, but to run a POTENTIALLY very, very bright one at much less than its full brightness.  It is under those conditions that it results in less current consumption.

Note that a lot of superbrights tend to be completely clear plastic domes.  To diffuse the light evenly and improve visibility, you'll need to buff the plastic dome with steel wool or something.

[BDuguay]

Well that makes perfect sense to me now. I've seen the 2000mcd range LED's at my local Sayal Electronics store but it never occured to me to use them at less than their rated performance.
Not only are you a gracious forum contributor, your pretty smart too! :wink:
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