LED not lighting up - SOLVED

[Addy Bart]

I'm stumped by a bizarre problem with an LED...

I've built David Rolo's Twin Peaks tremolo. It sounds great but the LED for the bypass foot switch does not come on. Everything seems to be working ok with the pedal itself... Sounds great and looking inside, the home-rolled vactrols and tap tempo LED flash in time. The bypass LED however does not light up. I swapped the CLR down from 6k8 to 3k9 and measured the voltage: I'm getting 9v on the wire before the LED and 7.5v on lug 1 on the 3PDT (so the LED seems to be doing it's thing) but the LED just doesn't light up. I've tried three different LEDs, reflowed the solder at all relevant points, and the voltages would surely read 0v on the 3PDT if I had the LED connected the wrong way around, or if there's a bad solder joint somewhere.

Urgh. Any ideas guys? Never had a problem like this before...

[jimilee]

Got any pictures?

[Addy Bart]

Ah, it's working! I had a last ditch solder reflow. Phew.

[Addy Bart]

Spoke too soon.
The LED is reducing the voltage from 9.6v to 7.5v on lug 1 on the 3pdt. It doesn't light up though. Could it be a bad 3pdt? It's not a bad connection if I'm getting voltage from DC jack to lug 1, is it?

Here's a video of the problem:

https://youtu.be/JGAyb53vpFA

[Addy Bart]

And the LED itself seems ok. Can I conclude that the 3pdt is to blame?

https://youtu.be/0tlCPDXwD2I

[m-Kresol]

looks to me like the voltage/current is passing the LED fine. try to measure the voltage of the middle pin of the 3pdt in the LED row to see if any voltage is there, it should be on GND. If you have any voltage then your GND connection is faulty, if you have 0V, I would suspect a faulty switch. try and measure continuity of the outer and middle pin of the 3pdt in both positions to make sure.

[Addy Bart]

Ok... So I have 7.5v on lug 2 of the 3pdt (the ground). And when I switch the 3pdt on it goes to 0v. Hmmm, does that mean a dodgy switch? I've tried reflowing the solder on all the ground connections on the 3pdt.

[m-Kresol]

hmmm... isn't GND ALWAYS connected to lug 2? I think it's time for a photo of the 3pdt switch, it's not visible in the video

[jimilee]

Is the D.C. Input grounded to the stomp? Definite picture of the stomp wiring.


Sent from my iPhone using Tapatalk

[Addy Bart]

I did the Madbean method for the wiring...

[m-Kresol]

the wiring looks good, but as the GND wire is fixed to the middle lug, you should ALWAYS have 0V on that pin. that's most likely the culprit. Check your gnd connections. Where is the other end of that GND wire hooked up to? reflow that joint and use the continuity setting of your multimeter to make sure you have a good connection.
good luck

[Addy Bart]

Right! That's good to know, thanks Felix. I suspect it's a bad switch as the other end of that ground wire just connects to the output jack sleeve and from there to the input jack sleeve.

The only other possibility I can suggest (and it's a slim one) is I'm using one of David's prototype PCBs, which has an extra ground pad for the tempo indicator LED. I noticed on the build document for the project that this pad is missing from the finished PCB. The tempo LED is instead grounded directly to the momentary switch. I doubt it makes much difference though. The pedal seems to work fine, so I guess it's a crappy 3pdt.

[m-Kresol]

it actually can't be the 3pdt from what you told us. If you have 7.5V on the middle lug of the switch, than it clearly isn't grounded properly. Make sure it is. maybe you forgot a wire somewhere (ground to jack or something). take the continuity setting and make sure that all the ground points have continuity to the DC jack (enclosure, pcb, stomps, jacks...).

just to be sure if the switch operates correctly, you should measure continuity there too. it should have continuity between the middle and outer pin only in one of the switching states (the second outer lug is hardwired, so always connected).

hope you get it fixed. David's projects are too good to let something like this stand in the way.

[Addy Bart]

I have some spare time tomorrow so I'll check the continuity on the switch. I'm fairly sure all the grounds are connected where they should be, but I see your point about it being an issue with a bad ground. I'll try reflowing the connections and see what happens. Keep you posted.

And thanks very much for the help too... this seems like an amazing tremolo so it would be great to get it sorted out. 

[Addy Bart]

Thanks for the help Felix... I redid some of the ground wires and everything is hunky dory now