Bypass LED won't work - Help troubleshooting

[seten]

I just finished putting together a Quaverato tremolo kit and my bypass LED won't work. Everything else seems to work fine, theres just no power to the bypass LED (even the other LED's work fine). I've checked all the solder connections for shorts, tried a new LED that I tested with another circuit, and now I'm stuck -- that's about the extent of my troubleshooting capabilities. Completely at a loss, only thing I can think to do is resolder everything one at a time -- which I'd rather not do. I traced around the PCB a bit and looking at the schematic it seems the bypass circuit is just Led-->Resistor-->Ground 2 (which is just a part of the PCB board?) and then the other end of the LED is going to the big ol IC microcontroller -- and thats where I'm lost. Not sure how its getting power or connected to the bypass switch. Schematic is at the end of the PDF below

https://www.zeppelindesignlabs.com/wp-content/uploads/docs/Quaverato-Assembly-Instructions.pdf

[Kipper4]

Simple though it may seem are you sure you got the correct orientation of the led?
Always look for the simple things first.

Does the pcbs +rail make a connection to the leds anode.
Unplug the power and check for continuity on the trace.

Does the 1k CLR (current limiting resistor) have a good connection at both ends ?
(Led cathode and ground respectively)

[seten]

Simple though it may seem are you sure you got the correct orientation of the led?
Always look for the simple things first.

Does the pcbs +rail make a connection to the leds anode.
Unplug the power and check for continuity on the trace.

Does the 1k CLR (current limiting resistor) have a good connection at both ends ?
(Led cathode and ground respectively)

checked all of that, no luck.

[ElectricDruid]

If you've put the -ve probe on ground and checked with +ve probe that you've got continuity to the resistor, and that you've got 1K resistance at the other end (high side) of the resistor and that the LED is correctly orientated, the only remaining thing to do is check that the processor chip is actually outputting the 5V it should be. With the pedal powered up, test for 5V on pin 11 of the processor. Obviously you need to switch the bypass on and off a time or two to make sure it works.

If Pin 11 is ok, it *has* to be the LED, the resistor, or the tracks or solder between them. There just isn't anything else.

HTH,
Tom

[R.G.]

Does the LED work when it's hooked up to a 9V battery through a 3.3K resistor in seires?

Sometimes LEDs get damaged. Not often, but sometimes. The 9V battery plus resistor also tells you for absolutely certain which way is anode and cathode for the LED.

Then your multimeter can tell you if the place on the circuit board where the questionable LED goes is (a) getting any voltage at all and (b) is voltage in the correct polarity.

[seten]

If you've put the -ve probe on ground and checked with +ve probe that you've got continuity to the resistor, and that you've got 1K resistance at the other end (high side) of the resistor and that the LED is correctly orientated, the only remaining thing to do is check that the processor chip is actually outputting the 5V it should be. With the pedal powered up, test for 5V on pin 11 of the processor. Obviously you need to switch the bypass on and off a time or two to make sure it works.

If Pin 11 is ok, it *has* to be the LED, the resistor, or the tracks or solder between them. There just isn't anything else.

HTH,
Tom

Interestingly enough, the LED in question's solder pads didnt have continuity with either the resistor or the IC chip. Howecver, that wasnt the issue; I used hookup wire to connect a new, tested LED to the resistor and touched it to the pin on the ic chip and while it worked on the other pins that had power (and pulsed in time with the tap tempo led when touching its pin) there was no light at all when touching Pin 6 (I know you said 11 and so i checked that too, but when i looked at the schematic it seemed like the bypass led was pin 6 and the one underneath it was 11. check me if im wrong.) But I'm getting a steady FOUR volts () out of that pin regardless. and my multimeter is working because it gives 5v on all the other pins which DO light up the my test LED and a varying voltage when touching the pulsating tap tempo's pin, pin 5.

And yes, I tried everything again after switching the bypass switch. and again and again and again - Ive learned a lot so far but I'm giving up for the day as right now I am hopelessly lost

[seten]

So, now I have Four problems of varying concern:

1 why wont pin 6 light up my test LED pulse when it shows theres 4v there?
2 Why wont the bypass switch turn on and off the voltage to pin 6? It turns the pedal on and off just fine.
3 why were there no connections to the LED socket on the circuit board?
4 why does pin 6 measure at 4v instead of 5v?

[ElectricDruid]

I know you said 11 and so i checked that too, but when i looked at the schematic it seemed like the bypass led was pin 6 and the one underneath it was 11. check me if im wrong.

Ah yes, my mistake. The numbering is above the line, not underneath it, you're right.

But I'm getting a steady FOUR volts () out of that pin regardless. and my multimeter is working because it gives 5v on all the other pins which DO light up the my test LED and a varying voltage when touching the pulsating tap tempo's pin, pin 5.

That sounds like something is dragging the voltage down. But I'd still expect to see something happening, even if the pin wasn't getting to the full output voltage. Very odd. Could the pin have got fried somehow? I've managed to burn out a single pin's output driver circuit once or twice, but it's not easy to do - they're pretty robust.

I'm giving up for the day as right now I am hopelessly lost

Good idea. Taking break and coming back to things with "a fresh set of eyes" often helps a lot.

[seten]

, I think i just may have found the issue, but im not sure how to solve it. pin 6 is continuous with ground but only when the pedal is powered up. So are pins 13 and 14 which I saw are the relay pins. Also, I finally tried running a guitar through it for the first time since i started troubleshooting and now the pedal doesnt work anymore. just clean signal. Does this mean theres a short somewhere? Where could it be to mess up pins 6 13 and 14?

[ElectricDruid]

If it's "only when the pedal is powered up", it can't be a short, since that wouldn't be affected by whether the power was on or not.

Instead, what you're seeing is that the microprocessor is outputting a low level on those pins - so it's the same as ground.

Is the processor running correctly? If you measure resistance across the Tp1/Tp2 and Tp3/Tp4 points on the board, do you see the LDR vary with the LFO?
Are the jumpers JP1-JP4 in place? (Best if not for testing the optos).

[seten]

Not sure how to check if the processor is running correctly, but yes, the resistance varies in time with the LFO led and the jumpers are in place.

[EBK]

, I think i just may have found the issue, but im not sure how to solve it. pin 6 is continuous with ground but only when the pedal is powered up. So are pins 13 and 14 which I saw are the relay pins.

You're not using a multimeter to measure continuity (or resistance even) on a powered up circuit, are you?

[seten]

, I think i just may have found the issue, but im not sure how to solve it. pin 6 is continuous with ground but only when the pedal is powered up. So are pins 13 and 14 which I saw are the relay pins.

You're not using a multimeter to measure continuity (or resistance even) on a powered up circuit, are you?

I am indeed. I'm guessing thats a no-no. Is that for the safety of the circuit or the safety of my own human body?

[EBK]

, I think i just may have found the issue, but im not sure how to solve it. pin 6 is continuous with ground but only when the pedal is powered up. So are pins 13 and 14 which I saw are the relay pins.

You're not using a multimeter to measure continuity (or resistance even) on a powered up circuit, are you?

I am indeed. I'm guessing thats a no-no. Is that for the safety of the circuit or the safety of my own human body?

Not a safety issue, unless safety of your multimeter counts.  Best case scenario, you get meaningless results.  Worst case scenario, you burn out your meter.

[seten]

Ahh okay phew. Why does it give meaningless results?

[EBK]

Ahh okay phew. Why does it give meaningless results?

Because your meter is attempting to send a small, known current through the circuit and measure the resulting voltage drop across the leads.  If you have other voltage/current sources in your circuit, then they may change the results.  It is kind of like pressing your thumb on a scale while you are trying to weigh something else.

[ElectricDruid]

Ok, if the optos are responding and the circuit seems to work properly except for the Bypass LED then the microprocessor chip must be running ok.
So if the Bypass LED doesn't work, it can only be:

1) The pin burnt out for whatever reason
2) The resistor
3) The LED
4) The ground connection

My first step would be to desolder the resistor and test to see if there was a changing voltage on the pin. We've established that there's no changing voltage on the pin with the other parts in place, but if there's a short they could be the problem.
There's no point messing with the rest of the circuit, since it seems to work fine and it isn't related to the Bypass LED anyway.

Incidentally, in your first post you mentioned that you couldn't see any connection between the bypass switch and the LED - that's entirely correct. The "connection" is done in the firmware on the processor. When it detects a press on the switch, it turns on the LED. There's no "electrical" connection between the two things outside the firmware.

HTH,
Tom