Fulltone OCD v2.0 LED issue

drd · Post by **drd** » 23 Jun 2023, 10:49

Dear All,

I got a Fulltone OCD v2.0 for repair, the issue is, that the LED is all the time on, no matter where the footswitch or the bypass mode switch are. Also the LED seems to be quite dim. I have opened the box, and saw, that C14 is missing, however I have seen other pix on the internet with that cap missing too. Also I was foolish enough to remove the LED, only to find that it is ok, but I have destroyed a trace and a pad in the process, so I'll have to rework that too... Anyhow, I don't really see how the switching of the LED works. Apparently somehow Q1 is involved, the marking is "MX" on this SOT-23 device. Maybe a faulty transistor? But then what is this transistor with the misterious "MX" marking?

Any help or ideas are welcome and appreciated, thanks!

bmxguitarsbmx · Post by **bmxguitarsbmx** » 23 Jun 2023, 20:18

Likely a footswitch issue

CheapPedalCollector · Post by **CheapPedalCollector** » 23 Jun 2023, 22:09

Footswitch or bad led.

drd · Post by **drd** » 24 Jun 2023, 07:13

Thanks for your replies, but unfortunately not

The LED functions ok, so does the footswitch, I have measured both.

Intripped · Post by **Intripped** » 24 Jun 2023, 13:36

Here's the complete schem:

viewtopic.php?p=26198#p26198

Lani · Post by **Lani** » 24 Jun 2023, 14:09

Marking MX is a BS250 P-Channel mosfet
https://www.hotenda.com/marking-code/se ... S250F.html

drd · Post by **drd** » 25 Jun 2023, 07:20

Intripped wrote: ↑24 Jun 2023, 13:36 Here's the complete schem:

viewtopic.php?p=26198#p26198

Thanks, I'll check it out. I'm very much inclined to leave the whole transistor thing out for the LED switching. I guess it is some kind of anti-pop measure, but honestly I never had LED popping issues, especially not with such mid-gain OD's like the OCD. Reworking the whole thing for standard LED switching might be actually easier, than dealing with potentially failed SMD parts and a damaged dual layer PCB...

bmxguitarsbmx · Post by **bmxguitarsbmx** » 25 Jun 2023, 18:26

I still think it's the switch. Assuming the switching is the same as in the schematic.

Think about it: The LED stays on... even when shorted out by the switch? This "bad LED" must have super powers, if it can light up when shorted out.

drd · Post by **drd** » 25 Jun 2023, 18:55

It was the BS250 mosfet, as I suspected, see the LED switching drawn by Bernardduur in the already linked OCD tracing thread. At some point it got either a voltage spike or a static charge, and it was open all the time, letting voltage through to the LED. I have simply rewired the LED so that the footswitch itself does directly the switching, the anti-pop arrangement with the mosfet is quite unnecessary in this application I think. And so I didn't have to re-create the wiring and replace a SOT23 part either...

Intripped · Post by **Intripped** » 26 Jun 2023, 10:28

I agree with bmxguitarsbmx: probably a defective switch that didn't make a good contact between the 2 pins used for shorting the LED.
With the new arrangement, now you're using the other 2 pins of that switch (the other side of that row of 3 pins) , and these are making a good contact it seems.

Anyway, the LED is doing its job now, so no more problems!