First dealings with SMD...

[patrick398]

Bought a pedal off the bay which was not working and thought i'd fix it up to resell. I'm not sure why but for some reason i really wasn't expecting it to be all SMD inside.

I had to sign a NDA to get a copy of the schematic so i probably shouldn't go into too much detail but i had a quick probe around inside earlier and noticed that there's 18v at the DC in (it's an 18v pedal) then on the other side of a 100R resistor it drops to 3.9v. That side of the resistor connects to a 220uf cap and a 1N4001 both going to ground. Then it's the input of a 15v regulator.

Question is, which is the most likely to have failed? My first thought was the 220u cap, but could also be the regulator no? There's 2.5v on the output of the regulator so it is dropping some voltage.

Or could it be that something has failed further down stream that is causing this massive voltage drop?

[EBK]

Is that 100R resistor toasty?  That would be nearly 2W power dissipated by it, wouldn't it?

Was 3.9 a typo? 

[GuitarPhil]

Diode could have been toasted if the wrong power supply polarity was connected? If it shorted out then that would explain the huge voltage drop across the 100 ohm resistor!

[patrick398]

Is that 100R resistor toasty?  That would be nearly 2W power dissipated by it, wouldn't it?

Was 3.9 a typo?

No, not a typo 
Trouble with smd is if i put one of my sausage fingers in there to check if it's hot i'll end up shorting about 11 components together haha.

Diode could have been toasted if the wrong power supply polarity was connected? If it shorted out then that would explain the huge voltage drop across the 100 ohm resistor!

Yeah my first thought was that it had wrong polarity power. The guy i bought it off was pretty vague, just said it stopped working one day.
I guess the worry is that if it was the diode that got fried there's a good chance there's more damage downstream 

[j_flanders]

Trouble with smd is if i put one of my sausage fingers in there to check if it's hot i'll end up shorting about 11 components together haha.

Perhaps your DMM has a temperature function. Even my chinese cheap one does.

[Rob Strand]

Removed crap post.

[tubegeek]

SMD tool EVERYONE needs to know about:

http://www.frantone.com/designwritings/design_writings5.html#hold

[EBK]

Is that 100R resistor toasty?  That would be nearly 2W power dissipated by it, wouldn't it?

It's only 150mW.

(18-3.9)²/100?

[Rob Strand]

(18-3.9)2/100?

Ah, I've mis-read the first post.

to 3.9v.

"TO" 3.9V

So what I've said in my previous post is crap.

As it stands we can't "derive" where the fault is without intervening.

[EBK]

Show some pics?  No NDA for those, right?

[patrick398]

Looks like D1 was bust, i've lifted it and now getting voltages through to the regulator. I've got about 16v in at the DC jack and the regulator has got about 15v on every pin so i think that's fried too. There's 15v on pretty much all other IC pins so i'll try and replace the regulator to see if i can get some normal voltage around the board. I don't really want to give it juice until the regulator is replaced through fear of doing even more damage