MXR dynacomp 3pdt led mod?

[Mrwizard]

Newbie here, trying to mod a 1975-1978 mxr dynacomp to add an on/off led indicator. So far, I understand I need a 3pdt switch. I have that and all the other parts required. The question is, which poles do I wire on the switch to what wire? If anyone has any schematics or helpful tips it would be greatly appreciated.

[Mrwizard]

I found a good resource at stew mac. Thanks anyway to everyone. I got what I needed. Here's the link if Anyones interested.

Free information : Fulltone 3PDT Stomp Switch at Stewart-MacDonald

[Mark Hammer]

A word or two of advice...

LEDs require current. LEDs also vary considerably in how much current they require. Some of the lower efficiency/brightness ones can require as much current as the very effect they are being used with, while others can achieve the same brightness with but a mere fraction of that current.

As many have found out, installing truebypass with an indicator LED can sometimes resulting in unpleasant loud popping when switching. The reason is that the LED needs enough current to turn on that it produces a spike on the supply lines shared with the audio portion of the circuit. One of the ways to avoid this and get more life out of a battery is to seek out and use a "superbright" LED. I'm not sure if there is any formal criterion, but I generally consider anything rated at 2000-3000 millicandles or higher to be "superbright". The classic little red things found in Boss pedals and such are likely on the order of 500mcd or less. With a superbright LED, the current-limiting resistor usually described as anywhere from 1k-3.3k can be as high as 12k-15k and still be as bright as a Boss.

Finally, the amount of current you need to use to achieve visibility is a function of the context in which the LED will be viewed. I recommend use of a black mounting bezel to provide maximum contrast for the LED. This will allow it to show up as lit with very little current draw.

[Mrwizard]

Thanks for the reply Mark. I am currently working on the monteallum vintage dynacomp mod and planned on using the blue led that came in their kit.

Monte Allums Mods - Individual Pedal Mods

I'm still a little fuzzy on where to tap into the power for the led. It looks like a straight connection to the battery + from the designated pole on the 3pdt. Any specifics on this or would you need to see a pic of the pcb?

[Mark Hammer]

I'm assuming there will likely be a red wire going to the board, carrying 9v. Anywhere along the trace that wire connects to will be fine. Depending on where the trace meanders, you may be able to simply solder one end of the current-limiting resistor to it, and the other end to the LED, without any intervening wire. Use either insulation stripped from other wire, or else thin gauge heat shrink, to insulate the two leads of the resistor. Use another piece of shrink/insulation to cover the lead of the LED that is to be soldered to the resistor, andreserve a third piece of heat shrink, of a slightly larger gauge, to slide over the solder joint, such that there is not risk of shorting anywhere between the point where the resistor is attached to the board and the body of the LED.

Trust me, when folks are doing mods or builds for the first time, one of the most common phrases heard is "I don't know what happened. It worked before I boxed it up." And in a great many of those cases, the problem is the result of unanticipated shorts when things that were not cramped suddenly get cramped during chassis closure. It is useful to assure they are prevented from making electrical contact with things.

As for the switch, there are a zillion ways to connect the LED/resistor network to the switch. The contact break the switch provides during bypass could be between the LED and ground, between 9v and the resistor, or between the resistor and the LED. Doesn't make a pinch of difference electronically. From an ease of installation perspective, though, I find it easiest to solder the resistor to the board and the LED, and the LED to the switch, then run a wire from the switch to the most convenient and least obtrusive ground point. Depending on where you wish to install the switch, YMMV.