What could be wrong with my LED?

[SixStringSlinger]

I have a loop switcher with an LED for each loop to indicate when it's engaged. All the LED's work fine except for one. If I click it on and off nothing happens (the LED remains off), unless I leave it alone for a while and then click it on, in which case it flashes weakly for an instant before fading away.

The loop switcher is connected to a power supply, and as I said, all the other LED's are fine.

Any idea(s) what could be wrong before I start poling around?

 

[edvard]

I have no idea what kind of unit you have, but my first thought is that the switching circuit for some reason has a capacitor in parallel with the LED (maybe to absorb current spikes when switching? Dunno...), and that particular loop has either a bad solder connection, defective LED, or bad switch. Here's how that might work:
- The capacitor gets charged up when you "leave it alone for a little while", but a bad connection doesn't allow the charge to drain or the LED to light.
- You finally step on the button, and a temporary connection is made somewhere that allows the capacitor potential to drain through the LED, lighting it for a brief time.
- If the LED is bad, same deal, but it's acting more like a low-ohms resistor than a light source.

That's all I can offer without knowing specifics. If you are the kind to open it up and take a look, check for bad solder connections or obviously burnt spots. If it's a newer piece of gear, the main "brains" of the thing are most likely built with surface mount components, so any off-board wiring points are probably the only place where you may be able to do something about it. If the LED is an off-board deal and not a surface-mounted bright spot, maybe unsolder the LED and check it is operating normally with a multimeter that has a diode testing function. If possible unsolder the switch and check it for proper operation.

If you don't trust yourself with a soldering iron, take it to your local guitar shop repair guy, and if he charges too much, just buy another one. Too much stuff is made disposable these days...

 

[SixStringSlinger]

I have no idea what kind of unit you have, but my first thought is that the switching circuit for some reason has a capacitor in parallel with the LED (maybe to absorb current spikes when switching? Dunno...), and that particular loop has either a bad solder connection, defective LED, or bad switch. Here's how that might work:
- The capacitor gets charged up when you "leave it alone for a little while", but a bad connection doesn't allow the charge to drain or the LED to light.
- You finally step on the button, and a temporary connection is made somewhere that allows the capacitor potential to drain through the LED, lighting it for a brief time.
- If the LED is bad, same deal, but it's acting more like a low-ohms resistor than a light source.

That's all I can offer without knowing specifics. If you are the kind to open it up and take a look, check for bad solder connections or obviously burnt spots. If it's a newer piece of gear, the main "brains" of the thing are most likely built with surface mount components, so any off-board wiring points are probably the only place where you may be able to do something about it. If the LED is an off-board deal and not a surface-mounted bright spot, maybe unsolder the LED and check it is operating normally with a multimeter that has a diode testing function. If possible unsolder the switch and check it for proper operation.

If you don't trust yourself with a soldering iron, take it to your local guitar shop repair guy, and if he charges too much, just buy another one. Too much stuff is made disposable these days...

It's a Loop-Master unit I bought used on Reverb. I'm ok with an iron and I don't mind breaking into the switcher, but it's held together with rivets I'd have to drill out (and replace with screws later) so I'd rather have a game plan before I break into it.

 

[edvard]

Ugh, rivets (shiver). Good luck, and I really mean that. The last time I had to drill out a rivet was on my 2003 VW Passat (RIP) to replace the door speaker. I thought the rivets were aluminum. I thought wrong.

 

[tubejockey]

It is possible that you just have a switch with dirty contacts. It may be worth dribbling a little DeOxit in around the switch actuator (assuming it is a mechanical stomp switch).

 

[wildschwein]

Oh yes rivets generally equal disposable or at least some re-engineering. If you drill out the rivets you will have to find a way to replace them with something. Ruling out a dirty switch it’s more than likely you have a cracked solder joint (most likely scenario), a kinked cable, or a failing LED and/or resistor. You will have to get it open to confirm.

 

[SixStringSlinger]

Geez, the manufacturer's website makes it sound simple; drill out the rivets and replace with screws. But then lately I'm skeptical about them. They make a good product, but there have been issues.

 

[SixStringSlinger]

I got everything hooked up, and the switcher works fine, including the loop that employs the apparently bad LED. So the only issue seems to be that it won't light up (leading me to believe that the switch is not the issue).

 

[Peegoo]

@SixStringSlinger

The thing to do is pop the unit open and test the voltage across the LED. It should be around 1.5v to 3v or thereabouts.

LEDs do have a lifespan, and older-style red ones seem die the fastest based on how they're constructed.

If you have voltage on the LED, get a replacement the same size and color, rated for that voltage, and solder it in there.

If there's no voltage across the LED, trace it through the board to the power supply and you'll find the bad component (resistor, cap, etc.). It will be something associated with that single LED (probably a 220-Ohm resistor that failed open, or a bad solder joint or cracked trace, or maybe a failed switch that grounds it or supplies it). I mention this because all the LEDs are probably powered by a single common supply rail on the board.

 

[teleplayr]

Touch-up the solder joints first and see if that helps.

 

[CirrusBand]

I got everything hooked up, and the switcher works fine, including the loop that employs the apparently bad LED. So the only issue seems to be that it won't light up (leading me to believe that the switch is not the issue).

In all likelihood it is the switch.

Those loopmaster boxes are very simple.

Each footswitch is probably a 3pdt switch, which is the most common type of switch to wire up a basic true bypass switcher. a 3pdt switch is 3 switches in one box, and you can wire them up in a way that lets you either have true bypass, or shunt the signal over to whatever you have plugged into the loop, AND at the same time the switch connects the LED to ground, allowing current to flow and the LED to light up.

Most probably, one of the 3 little switches in that footswitch is malfunctioning mechanically. The fact the LED sometimes lights up for a moment when you push the switch makes that more likely - while you're pressing down on the switch, the physical force might be enough for the malfunctioning connector to briefly make contact and complete the circuit.

The 3dpt switch will need to be replaced, which is a job for someone who can solder.

 

[Charlie Bernstein]

xxxx