Cupcake smoke

[Ang3lus]

Hey all,

I've just finished a cupcake build, no parts repalcements except i'm using OPA2604 as an IC.
I also didn't hook up the LED on the board, I've used the standard diagram with positive going into 9V and negative to the 3pDT.

when i turned it on it didn't work and made a very nasty sound, then after a few seconds R12 started to smoke and is now completey burnt.

checked all connections, pinouts etc and all seemed fine, any idea why R12 would smoke ? could the led not being connected on board (I left the LED connections disconnected) cause this and the circuit not to work ?

[madbean]

Looks like the OPA2604 is pulling too much current for the 10R. Go ahead and jumper R12 and then measure your voltage on pin8 of the IC. It should sit around 9v.

[jimilee]

What he said

[Ang3lus]

thanks for the advice.

I tried it with the OPA (jumpering R12).

I took it out and put a TL072 and put R12 back in and i still get a really loud hum noise.

I put some pics in case you guys spot anything, I tried everything i could think of but i get no sound

[Ang3lus]

2

[Ang3lus]

3

[jimilee]

I don't see any solder coming through those holes. I'd  reflow everything.


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[mgwhit]

Voltages on all the IC and JFET pins wouldn't hurt, either.

Also, your R3 and R4 resistors look like they might be 47R instead of 470K.  Is that multiplier band orange or gold?

Oh, and R6, too, which if it is 47R would totally blow out your R12.

[Ang3lus]

it's orange, they are 470k.

I will post voltages on Saturday night.

I don't think there's power coming to the chip though, i tapped on the lugs and i get no disturbance in the sound. my Multimeter decided to died on me today

[Ang3lus]

Took some readings with the multimeter

from pins 1-8:

3.34v
3.3v
3.315v
0v
5.93v
5.78v
5.73v
0v

that don't seem right ?

checked R6 and R7 values, they seem  fine at 390k and 470k, c9 is a filtering capcitor ? tayda has a weird way of writing values on the capacitors, so either this is 100n or 220n, would it matter if the value is 220n?

[mgwhit]

Took some readings with the multimeter

from pins 1-8:

3.34v
3.3v
3.315v
0v
5.93v
5.78v
5.73v
0v

that don't seem right ?

checked R6 and R7 values, they seem  fine at 390k and 470k,

That is strange.  First off, I would expect to see somewhere between 8-9V on pin 8.  If that's really 0V, the op amp is not going to work.  Second, pins 1-3 look a little low.  I would expect to see something between 4-5V.  Third, I wouldn't expect to see any voltage on pins 5-7.  I recommend reflowing all the joints on those IC pins and double checking for shorts (to ground or elsewhere) with a multimeter. 

With the power disconnected, confirm pin 5 has continuity to ground.  Confirm pin 8 has continuity to the positive leg of C8.

c9 is a filtering capcitor ? tayda has a weird way of writing values on the capacitors, so either this is 100n or 220n, would it matter if the value is 220n?

It wouldn't matter enough to cause any problems.  What is written on it?

[Ang3lus]

22UJ100

but the package i got it in says 100n

I'd have to measure them to be sure

[midwayfair]

Your voltages are correct -- for a battery that's only putting out 5.93v. Use a fresh battery or a power supply and take voltages again.

[mgwhit]

Your voltages are correct -- for a battery that's only putting out 5.93v. Use a fresh battery or a power supply and take voltages again.

Jon brings up a good point.  If you were using a battery when you blew through R12, you may have significantly depleted the battery before the resistor fried...although I'm not sure if a 9V battery would provide more current than a 1/4W resistor could handle.

Jon, would he see any voltage on pins 5-7?  That op amp is grounded and unused.  (Sure, I could breadboard it, but I'll bet good money you've built this one.  )

[RobA]

Your voltages are correct -- for a battery that's only putting out 5.93v. Use a fresh battery or a power supply and take voltages again.

Jon brings up a good point.  If you were using a battery when you blew through R12, you may have significantly depleted the battery before the resistor fried...although I'm not sure if a 9V battery would provide more current than a 1/4W resistor could handle.

Jon, would he see any voltage on pins 5-7?  That op amp is grounded and unused.  (Sure, I could breadboard it, but I'll bet good money you've built this one.  )

Pin 8 should be seeing close to the supply voltage and not 0V. Pins 6&7 could actually be at something other than ground even though pin 5 is at ground. Some types of op-amps will latch up in a single supply setting with the input at ground. For those op-amps, it's best to set the input pin of an unused section at half supply. I haven't seen it be a problem with a 4558 and I haven't tried it with an OPA2604, but if it did latch up, that could explain the high current draw. Put still, I'd think that pin 5 should still be at ground.

If you aren't using a battery, it could be that you've got a short or stray connection somewhere that's drawing a lot of current and causing your supply to sag.

I did some messing around with this circuit some time ago and to me, op-amps with bipolar inputs worked better than FET input op-amps. I didn't try an OPA2604 at the time, but I did test an OPA2134. In the end, I went with the 4558 and didn't even bother to socket it.

I'll pull out the board and take some voltage measurements.