Sizzling, Crispy Resistors.

[steveyraff]

I generally build all stripboard layouts. Thought this would be a comparatively easy build since its a bought in PCB. Nope - I've ran into problems.

So here it is. Fired it up and it sounded great - and then I smelled something burning. Realised part of the circuit was hot to the touch. Then seen that R20 and R21 were completely cremated looking. Strangely, it still works perfectly, but I am afraid to leave it on for long now.

Any idea what could be causing this? Schematic and layout in the link, and a photo of the burnt components.

http://pedalparts.co.uk/docs/SpectreVerb.pdf

[EBK]

I noticed that in the build doc photo, those resistors are hidden from view by an electrolytic cap laying over top of them, presumably to hide the crispiness beneath.   

More seriously, you must have accidentally put in the wrong value parts (way too low resistance).  I'd replace them. 

[vigilante397]

A couple questions:

1) Did you have a look at the bottom of the board to make sure there's nothing touching/shorting?

2) Obviously it's too late to check now, but are you at least pretty confident you used 10k resistors and not something else? Granted it would have to be a pretty low value for them to fry (like less than 80 ohms).

Those are the only things I can think of that could have caused a burnout there. R20 and R21 make the voltage divider to form your Va reference voltage for the op-amp. Technically if they both fried the same amount they are both equal in resistance, so the voltage divider could in theory still be working, and since it's just a reference voltage it's not really sourcing current, allowing the circuit to work I guess? I'd be interested to see what voltages you're getting everywhere, but particularly at pins 12/13/14 of IC1.

[steveyraff]

A couple questions:

1) Did you have a look at the bottom of the board to make sure there's nothing touching/shorting?

2) Obviously it's too late to check now, but are you at least pretty confident you used 10k resistors and not something else? Granted it would have to be a pretty low value for them to fry (like less than 80 ohms).

Sorry guys, posted too hastily as always. I already did check for accidental shorting, and after looking at the schematic I was going to replace those two 10k resistors just to be sure. While replacing them I rechecked their values. The place I bought them from has a sticker on the pack saying 10k. My fault for not looking at the colour bands - its actually 10R !!

I also see I've used 5 from this pack - and I am juggling about 8 different builds right now. So time for me to go on a treasure hunt.... 

Thanks all! Apologies! Doh!

[PRR]

4.5V in 10K is 0.002 Watts. Never get hot.

4.5V in 10 Ohms is 2 Watts. Gets HOT.

Yeah, you want to find the other "not 10k parts" before they make trouble too.

[steveyraff]

4.5V in 10K is 0.002 Watts. Never get hot.

4.5V in 10 Ohms is 2 Watts. Gets HOT.

Yeah, you want to find the other "not 10k parts" before they make trouble too.

Yup. And I only moved on to that batch because I've just used up my big roll of 10k's. Now I gotta wait for another delivery before I can finish these 

[EBK]

Sorry....
....
Apologies!
....

Why? 

Even simple problem solving posts are useful. 

Plus, the sight of those burned up parts is actually giving me an interesting (and crazy) idea....

[steveyraff]

Sorry....
....
Apologies!
....

Why? 

Even simple problem solving posts are useful. 

Plus, the sight of those burned up parts is actually giving me an interesting (and crazy) idea....

Oh just feel bad wasting peoples time! If I had have just done my usual basic checks before asking I'd have saved people a few clicks! Thanks anyway all!

... now I'm wondering what your crazy idea is lol

[bluebunny]

FYI, 10R resistors are often used as power-supply fuses in synth DIY designs.  Just sayin'... 

(BTW, mis-labelled resistors: been there, done that...  )

[Ben N]

I never trust my read of components, which is why I always run Rs, Cs, Ds & Qs through a cheapo M328 component tester before soldering them in nowadays.

[italianguy63]

A couple questions:

1) Did you have a look at the bottom of the board to make sure there's nothing touching/shorting?

2) Obviously it's too late to check now, but are you at least pretty confident you used 10k resistors and not something else? Granted it would have to be a pretty low value for them to fry (like less than 80 ohms).

Sorry guys, posted too hastily as always. I already did check for accidental shorting, and after looking at the schematic I was going to replace those two 10k resistors just to be sure. While replacing them I rechecked their values. The place I bought them from has a sticker on the pack saying 10k. My fault for not looking at the colour bands - its actually 10R !!

I also see I've used 5 from this pack - and I am juggling about 8 different builds right now. So time for me to go on a treasure hunt.... 

Thanks all! Apologies! Doh!

I've done it too!!
MC

[GibsonGM]

I think you're over-reacting.  Those resistors can take a HECK of a lot more heating than THAT...jeez...   

[vigilante397]

Oh just feel bad wasting peoples time! If I had have just done my usual basic checks before asking I'd have saved people a few clicks! Thanks anyway all!

Don't feel bad, you weren't the first person to make a mistake like this and you certainly won't be the last, so now that this thread is here, complete with answers, the next guy that puts in resistors multiple orders of magnitude too small and starts a fire just has to read through this thread to find his answer

[EBK]

But, if you ever experience a potentiometer getting so warm it glows and starts to emit smoke, don't make the mistake of searching Google for "lighting up and smoking pot".   

[bluebunny]

^^^ Eric wins the internet for today.   

[Ben N]

But, if you ever experience a potentiometer getting so warm it glows and starts to emit smoke, don't make the mistake of searching Google for "lighting up and smoking pot".   

There are worse solutions.