problem with little gem

[Talmir]

I´m building a little gem amplifier. But not sure what I did wrong, I turn it on, and after 40 seconds or so it dies.. The speaker gives a whiny sound for half a second and the led goes dim. Does it mean the electricity isnt flowing correctly or something?

[joecool85]

Does it work for the 40 seconds it is on?  It sounds like you have a short somewhere.

[Talmir]

Here´s a video of how it behaves.. Gonna go over it with a spyglass, if there´s a short somewhere I´ll find it

http://youtube.com/watch?v=q6EnenWPrgs

[joecool85]

I would say almost definetely a short.

[Talmir]

I´ve gone over it.. Cant find a short, I´ve tested disconnecting everything and reconnecting.. Must be something I put on wrong..

When you have upright capacitors with two pins out the bottom (100uf and 220uf), the negative side is the one with arrows pointing down, right?

[teemuk]

You mean a radial electrolytic; yeah, the stripe with arrows inside marks the side which should have a negative polarity (often there are minus signs inside the stripe instead of arrows).

Just for reference, here are my notes about the different polarity markings:

Identification = Meaning
---------------------
Plus = positive
Minus = negative
Side with stripe (radial) = negative
Groove (axial) = positive
Direction where arrow points (axial) = negative
Longer leg = positive
White side of circle (PCB)  = negative
Dot or stripe (SMD) = negative
Black block (schematic) = negative
White block (schematic) = positive
Curved or "cupped" (schematic) = negative

The original schematic shows no LED, which leaves me to wonder if you have calculated a proper current limiting resistor for it. Maybe you are shorting the thing by killing the LED with too much current. It seems to be very bright for a red LED but then again, the video quality is not that great.

[Talmir]

A resistance value of a resistor before the led is correct. I tried disconnecting the led and listen for it to die again, which it did. So, it has nothing to do with the led.. I´m coming slowly to the conclusion that I should just forego this creation of mine, and start over. I think there´s a short somewhere that I might have missed. (my soldering skills are oh so crappy)

[joecool85]

Sometimes it isn't a bad idea to start over and try again, I had to do that on my first guitar project, a fuzz face.

[Talmir]

I redid it, strictly according to the schematic. But after I play it a bit, it starts sounding muddy, and eventually dies on me. Lasts longer than 40 seconds at least this time Wonder what I'm doing wrong.

Is it ok to connect all the ground connections (the speaker, input jack) to the ground connection of the amp? Or is that what is killing it?

[Talmir]

ahh, nevermind. Turns out the coating of the table I was playing it on when it died tonight was conducting electricity a little bit causing a pretty big short in the amp... It works great as soon as I start playing it on my worktable. The old one still fails though.