Trouble adding LED to old pedal

[mr_sooty]

I've added an LED to a friends pedal. It's a Dimarzio 'very metal' pedal, which sounds better than it.... sounds.... er.... is a better pedal than the name suggests.

Unfortunatetly I don't have a schemtaic for this one, it's a pretty oddball pedal from 1987 - 1988 ish.

Anyway, I used some other pedal diagrams for tips, and I connected the hot end of the LED to the positive point on the adaptor input, and the other end to a 3PDT switch so that it connects with ground (with a 750ohm resistor connected to the hot end of the LED - didn't have any 1k ones, figured this would be enough to protect the LED).

First attempt, the LED swtiched successfully, however, it seemed to be shorting out the signal. Pedal bypassed properly, light off, pedal on, light on, no sound.

So I kept the negative end attached to the ground switch, and unattatched the positive end of the LED. With the pedal on, I poked around the circuit board with the LED's positive end until I found a point where the LED stayed on as well as the signal, and didn't seem to affect the sound of the pedal. Strangely (it seemed to to me), that point connected to one of the transistor legs.

I gave it back to my friend, but he says the volume of the pedal is affected, and it no longer goes any louder than unity gain. Oops, didn't notice that.

I noticed in the Blues Breaker wiring diagram I used for reference, that there is a diode immediately after the LED in the circuit. However, this doesn't seem to be the case in the 808 wiring diagram I have (I'm not so good with schematics). However I'm wondering, if I went back to the original plan, with the LED connected to the positive point on the adaptor input, and then to ground, and had a diode somewhere to restrict the direction of signal flow, could this work? Which way around would it go? I'm thinking it would be orientated so that signal went to the LED but not back from it?

Of course, it also occurs to me that an LED is a diode anyway, so maybe I'm on the wrong track here?

[JohnH]

Hi sooty
The right way to do it is so that the LED gets switched onto the battery or power input, and does not connecr to the main board at all in any other way. The 3rd pole on the 3pdt does this, so you have two poles to do the true bypass switching, and the 3rd pole is + to resistor to LED to switch to ground (those components in any order)

This should work with any old box.

John

[mr_sooty]

This is exactly what I did, but I guess the problem is that the power input then goes to the circuit (?) on this pedal. I know it's pretty difficult for you guys to see where I've gone wrong without a diagram, but yeah, I put in a 3PDT switch, transferred the wires from the old switch onto the first two poles (this worked fine), and connected the LED so that the positive leg went to the hot of the power input, and ground was either switched to nothing when off, or to one of the grounds when on. The LED worked fine, as was predicted, but the pedal had no signal when on. So somehow my LED seems to be shorting the circuit.

[flateric]

At times like this I go back to the Beavis Audio site and recheck my wiring:
Have a quick look here.......... beavisaudio.com/techpages/StompboxWiring/

[mr_sooty]

Is the resistor purley for protecting the LED from blowing? As I say, mine is only 750 ohm.

[flateric]

yes thats right, depends on the led of course but normally you use anything from 1.2 to 3k, higher value means not so bright but on some of these boxes the LED draws more battery juice than the actual circuit so if u go with a bigger resistor for the LED you help get a bit more life from your battery.