Low Tide LED issue

[Devin from Athens]

This one’s been fun chasing but could use some help at this point. Low tide seems to work fine, but indicator LED stays dimly lit in bypass. If pedal is in bypass and you plug in the 9v supply, the LED is first bright for a second and then settles into dimly lit.

First thing I did was check for bridges and bad joints. Next, I took the switch out and found the LED still behaves the same way, flashes brightly and then stays on dim.

I’ve replaced the 22u cap in that part of the power circuit, I’ve replaced the LED. The two resistors are reading correctly with a meter. I feel like whatever I’m missing is staring me in the face, do any of my friends here have any thoughts or suggestions?

 

[Robert]

There's not much involved in the LED circuit aside from the LED, R54, R55, C21, and the footswitch itself.

Are you using a 3PDT breakout board?

Pics would be helpful.

 

[Devin from Athens]

Hey, sorry the pics are running late, should be attached now. You’ll notice I burned the LED cathode pad, so that’s running direct to R54 under the board.

I usually don’t use breakout boards, but did this time given it being included with the low tide pcb.

Since the LED was behaving the same without the switch installed, and I was able to beep out the switch/breakout board correctly with a meter, I believe it’s not the culprit and have reinstalled it (no change). I also can’t find an issue with the resistors; they read their value and are connected where it says they should be.

 

[andrewsrea]

The circuit is finding its way to ground somewhere. I'd suggest testing resistance to ground, of the 'switch' lug of the 3DPT stomp switch starting with a high DVM range, like 2M. Anything under 100K resistance continuity of cathode to ground will probably get the LED to light.

 

[Devin from Athens]

When reading resistance from switch ground to the six pads that connect the two resistors and cap with meter set to read up to 6M ohms, I get an open circuit at all connections save for C21 cathode; reads .8ohm. Resistance from switch ground to LED anode is 1.78K ohms.

 

[Devin from Athens]

I’ve swapped all parts in the LED circuit at least once. I think this will just have to be an always on LED.

 

[andrewsrea]

I’ve swapped all parts in the LED circuit at least once. I think this will just have to be an always on LED.

Remember that the stomp switch sends the LED cathode to ground (lit) or open (not lit). I am fairly certain that PedalPCB does not utilize a switched (+) on any of their designs.

In studying your pictures above, I may have found the issue. You have the LED anode (+) soldered into the correct 'A' spot, but the LED's cathode (-) is not soldered to the 'K' pad, which is the trace which eventually leads to the switch Lug#1-Column #2. You have it soldered to something else, which I am guessing is a path to ground. As you currently have it wired, the switch is doing nothing for the LED.

If the solder pad for 'K' is in good shape and you have continuity between that and the switch, then undo the current cathode solder joint and send that to the 'K' solder pad (use a lead wire if you do not have enough LED lead left). If that pad was damaged, then you can let the cathode lead 'float,' attach a lead wire to it and run that to Lug#1-Colum #2 lug, replacing the lead which is there.

Curiously enough, I experienced a similar issue of the LED being partially lit on a build I completed today (Median Compressor). It ended up being the LED making contact with a resistor pad on the circuit board, when it was all packaged up in the pedal. The LED had too much lead and was getting smooshed against the PCB and some solder pads (even though I had insulated that area with electrical tape, some resistor leads popped through. It made me look again at your pics.

 

[Devin from Athens]

Remember that the stomp switch sends the LED cathode to ground (lit) or open (not lit). I am fairly certain that PedalPCB does not utilize a switched (+) on any of their designs.

In studying your pictures above, I may have found the issue. You have the LED anode (+) soldered into the correct 'A' spot, but the LED's cathode (-) is not soldered to the 'K' pad, which is the trace which eventually leads to the switch Lug#1-Column #2. You have it soldered to something else, which I am guessing is a path to ground. As you currently have it wired, the switch is doing nothing for the LED.

If the solder pad for 'K' is in good shape and you have continuity between that and the switch, then undo the current cathode solder joint and send that to the 'K' solder pad (use a lead wire if you do not have enough LED lead left). If that pad was damaged, then you can let the cathode lead 'float,' attach a lead wire to it and run that to Lug#1-Colum #2 lug, replacing the lead which is there.

Curiously enough, I experienced a similar issue of the LED being partially lit on a build I completed today (Median Compressor). It ended up being the LED making contact with a resistor pad on the circuit board, when it was all packaged up in the pedal. The LED had too much lead and was getting smooshed against the PCB and some solder pads (even though I had insulated that area with electrical tape, some resistor leads popped through. It made me look again at your pics.

Thanks for responding and for the feedback. The LED was soldered into the correct anode and cathode pads previously. I burned the cathode pad at a point when I was trying my third LED (ran out of good ideas and started throwing crap at the wall, unfortunately).

I've pasted a screen shot from the circuit schematic below that shows the LED cathode ties to R54. That is where I have the LED cathode run to, bypassing the damaged pad. The LED does the same 'staying on dim' for both configurations, which should electronically be the same.

 

[jwin615]

Maybe c21 has a high enough esr to pass DC?

 

[Devin from Athens]

I swapped that cap with another 22uf, neither tested funny with a meter or my cheapo parts tester.

All the parts in the LED circuit have been swapped at least once and from what I can tell with a multimeter, the circuit is intact. Beats the heck out of me where it’s messing up, but the LED is quite dim on bypass, and the effect is quite nice, so I may hang it up on this one.

 

[andrewsrea]

I appreciate you including the schematic, as it was not available on the PedalPCB website. That is a overtly complex circuit for a basic indicator LED.

I personally would skip all the circuit between the R4 and pin #1 / column #2 of the 3DPT stomp switch, solder a 4K7 resistor to that lug, solder a lead wire to the other end of that resistor and solder that to the cathode of the LED.

IMHO, it sound like some type of short in that chain.