Why Is My STK 4191 Amp Burning a Resistor?

[NipunaG]

STK 4191 amp. burning resistor :(

I just built a STK 4191 amp. when i supply power a 100Ω resistor (1w) near the IC starts smoking. there're no short circuits across the ICs pins or anywhere on the board and i took all precautions when soldering the IC on (was done with heatsink + 40mm fan turned on and let the IC cool for about 1 min between soldering each pin) can anyone help me with this? thanks.

 

[davenn]

is it the 100 Ohm resistor going to Pin 9 on the left channel amp and via a couple of 3k3 resistors to pin 17 of the right channel ?

I hope you didnt also connect it to the -Vcc rail ?
if so, that would be one reason why you have let the smoke out

or maybe its the 100 Ohm resistor between Pins 13 and 14 ?

some more info from you would be helpful and maybe a close up pic of your construction
well lit and sharp please :)

cheers
Dave

 

[NipunaG]

no. it's the one across pins 4 and 7.
P.S- I just realized that it's working properly for about one second on turning on before the volume drops to the sound output level of a headphone.
BTW thanks for replying :)

 

[NipunaG]

This would be the schem. (minus tone control and lb1405 led meter)

I also noticed that the voltage across the smoothing caps is 21.x (transformer is 28-0-28) but i guess its that resistor dropping a large percentage of the voltage across itself.

 

[Dickfore]

P = U x I, R = U/I => P x R = U^2, P/R = I^2

So, with P = 1 W, and R = 100 Ω, the maximum voltage drop is U_max = 10 V, and the maximum current through it is I_max = 100 mA. You can measure these quantities. If the voltage drop is bigger than 10 V, you need a higher power rating!

 

[NipunaG]

Yup. it's doing 10+ volts all right. But, the resistor is already a 4w one and I'm using a factory made (or whatever they're called) PCB i.e.- not one I made. that came with a specifications sheet with everything like the transformer voltage, component values etc. mentioned. I just noticed another thing- the other 100Ω resistor is heating up too, but not as much. I just can't imagine where the problem is.

 

[Dmytry]

make double sure the PCB is correct and there's no shorts when soldering.

 

[jim hardy]

i'd check real carefully the schematic you used against the datasheet for the 4191.

the SANYO datasheet i found doesn't show that 100 ohm resistor from 4 to 7, instead a 50K coupled through a capacitor.

try to understand why your source shows the resistor,
and whether it makes sense.

but I've never used that IC...

 

[jim hardy]

never mind my last post.... i found a schematic closer to yours, with that resistor as shown.

if you've got a good schematic and the circuit you have isn't working,

then it's a safe bet the circuit isn't the same as schematic.

double check everything for a component wire put in wrong hole, IC soldered in backwards, or not-quite right part number on IC. last letter often represents packaging & pin arrangement.

your 21 volts DC at 'smoothing cap' ought to be closer to 35 volts, sounds like 'smoothing cap' is open or drastically overloaded. measure the AC there - should be like 2 volts or less.

also - try it with speaker disconnected. might be routing of speaker wires too close to input..

good luck..

by the way - where did you buy the circuit board? I'd like to try one of those...

old jim

 

[NipunaG]

All right guys! it's working :D. It was just a bunch of resistors in all the wrong places. lol. (I'm so sticking up the resistor color code on my wall now!). Thanks guys.
@Jim
I don't know where you could get one where you live (I'm from sri lanka), I got mine from some local shop long ago.
The savvys over here say that STK amps have got pretty bad bass though. So don't go for one if you're looking for high quality sound :).

 

[jim hardy]

I'm so sticking up the resistor color code on my wall now!

good plan. you want it to become as automatic as reading text.

when i was a student/ summer intern in an electronics factory a huge bin of resistors got spilt. i was assigned to sort them out and put them away.

seemed humiliating at the time but by mid-day i was reading them like ABC's. i forever thank the grizzled old foreman who gave me that task.

for another homebuilt tale, see thread at discovercircuits dot com forum under "circuit design" , "stereo" , by danud.