quick STK404 140S question

[ilyaa]

hey peeps -

long time no post!

quick question: im fixing an acoustic B200. everything looks good until the last power stage, where i get no signal on the STK404 output.

i'm getting +/- 50VDC on pins 12 and 13 (output pins). looks like they are spiking up to the rails. clear sign that the IC needs to go? should be 0VDC there, right? just not 100% sure i understand the layout of this particular chip....

thanks!

[Enzo]

So I assume we have good 50v rails on the power pins of the IC.

And do you mean there is +50v on pin 13, and -50v on pin 12?  That would mean 100v across R15,16 pair.  They sum to 0.44 ohm.  100v across less than half an ohm gets me over 20,000 watts.  Unlikely, so are those resistors open?

Are pins 11 and 12 shorted together?  Are pins 10 and 13 shorted together?  Internally, I mean?  Is the low voltage reference present on pins 6 and 7?

[ilyaa]

okay clarification:

there is -50V on pins 10, 12 and 13.

and the low voltage reference on pin 7 is only 8V, half of what it should be.

pins 11 and 12 are not shorted together, but 10, 12 and 13, are, at least when tested in circuit - but there is nothing i can see that'd be shorting them externally, so im guessing internal short.

new chip!

[Enzo]

Sounds like it, but you can google the part number and read tha data sheet, which includes internal circuit.

So pull the STK, and see if the 15v rails come back to correct.  That needs to happen before a new STK will help.

[ilyaa]

okay new STK chip works -

only issue is the relay/protect circuit was maybe also fried in the process of the output section going.

i bypassed the relay and amp works....bad idea?

[Enzo]

Not a good idea anyway.  It is a safety device.   You could remove the seat belts from your car, the car still works, but is less safe.

Any problems with the amp, and no relay there, your speakers can literally catch fire.

[ilyaa]

why relay in this amp and not other similar SS ones?

is there something particularly volatile about the STK chip?

[Enzo]

Why three airbags in some cars and just one in others?  Some designers want speaker relays, others see them as expense they can eliminate.