LED not working on Boss Guitar Effects Pedal (LED does work externally)

[coffee_king]

Hey all
I picked up this Boss HM-2 from a car boot for £6 the other day, but the LED doesnt light on it.
Ive desoldered the LED and connected it to a 9V battery externally and it works OK.

Any suggestions?

Here are some images. I can post more if necessary.


TA

 

[(*steve*)]

An image of the top side of the larger board would be good. These other pictures are useful, but that one is probably the most interesting.

What does the LED signify? Is it a "power on" indication, or something else?

Does the unit function at all?

 

[coffee_king]

Thanks for responding.
Yes, the LED being on signifies that the pedals sounds are being used.
Yes, the unit works perfectly apart from the red LED turning on when the pedal is pressed.
Without the red LED lighting up its hard to tell if the units effect sounds are on.

here is another image.
Thanks

 

[(*steve*)]

OK, given that the LED works, as does the rest of the circuit, I would suspect the connection between the boards.

Use a multimeter on a resistance range (and with the power off on the pedal) to trace from the pins where the LED was connected back to the flexible connector, and then to the other side of it.

If the resistance goes up dramatically, you've moved beyond the fault.

Also, note that the LED is a polarised component. Did you note which way it was on the board?

What appears to be D13 and R46 are in series with the LED. You could also check these components.

 

[coffee_king]

Cheers for your advise.
Here is where I stand now. I checked continuity and everything seemed ok, I connected the new LED up again and it worked. So god knows what was going on there. So that parts sorted and soldered in, so thanks.
OK, now new problem. Someone has obviously messed about with the before, because even with the guitar leads unplugged from the unit you can still turn the unit off and on by pressing the switch pedal. I am assuming the whole unit turns off and on because the LED lights up. Any help?

 

[BobK]

I would think that would be normal behavior. It would take extra circuitry to prevent the unit from being powered when there was no guitar plugged in.

Bob

 

[(*steve*)]

I agree with Bob.

 

[coffee_king]

I would think that would be normal behavior. It would take extra circuitry to prevent the unit from being powered when there was no guitar plugged in.

Bob

Hi Bob
Sorry but thats not right. Cheers for your help everyone, but Ive sorted the issue with the pedal being able to to switched on even without a guitar lead going into it.
For some reason previously the negative from the battery was soldered onto the output instead of the input. (This was not me that did that)
I found out what was incorrect by opening up one of my other Boss pedal and also I found these schematics

Cheers everyone, problem resolved.