need help repairing powered subwoofer

mbates14

Member
2002-06-02 2:14 am
non
Im not sure if this is the proper forum for this or not, but since it deals with solid state amplification, i thought I'd ask.

I have a KLH audio 765S powered subwoofer with serious problems.

my cousin complained that it had a loud hum and popped the fuse. and he said it pops it everytime you plug it in now, on or off.

I immidiatly thought bad output chip. (I thought it had one of those infamus STK chips at first).

I took it apart, and to my surprise it has discrete output circuitry. I was wrong. lol.

Anyway, I looked at it, it looks seriously heat scorched around all the high wattage resistors. (im assuming thats normal).

I thought one of the output transistors was shorted at first, so I looked at the emitter bias resistor network, and it was so seriously heat scorched, it just fell off the board when I picked the board up. (bad).

the resistor still statically checks good. I pulled the output, driver, and VAS transistors out, and tested each one, and they are ALL good! (odd?!?)

I noticed the bridge rectifier was bloated and cracked on the sides, which tells me its most likely shorted, which would explain popping the fuses.

I dont know where to go from here. the thermal cutoff relay contact one one pole was shorted, but the other is open. They are both supposed to be open when unpowered. hmmm.

Any ideas tips or hints? I could replace the bridge, but that wouldnt be the reason that caused the loud hum. What would cause the bridge to blow and look that bad in the first place? don't get me wrong, but i repaired several several amps in the past, but never seen one this bad.

filter caps arnt shorted, I pulled them out of circuit and did a simple ohm meter test, and they are normal as far as electrolytics go.
 
It is obvious from your description of the unit that it is running VERY hot. That will kill semiconductor devices quite well. It is likely that the designer did not take thermal engineering very seriously. The bridge failure could cause straight AC to go to the circuitry which would certainly be a cause of a lot of humm!

Anyway, replace the bridge and see if it pops the fuse again. Most relays have a NC and a NO contact... are you sure yours is faulty? If so, replace that as well and have a go. While you are in there, the capacitors may not be shorted but they have likely cooked from excessive heat if they are near that bridge... replace them as well, if you can; nearly-gone capacitors will cause humming as well.

Personally I would try to replace the resistors that are burnt with higher-power versions, especially any that have fallen off! Also, check the traces are not connected by carbon tracks from burnt PCB material...that would be conductive, as well. Also, see if any traces are lifted or burnt-open.

Hope that gets you off to a running start.
 
Quite often these 'plate' sub amps are on the limit in many component ratings as they're honed to a budget. One of those can be the bridge - not allowing for operation at sustained square wave clipping which may not be very audible due to the limited response of the speaker to reproduce the sharp clipping harmonics - so there is no audible feedback that it's overloaded. Only that it's very loud.

For a 200W 4ohm sub amp for example, driven from a bridge and split supply the current draw at full power sinewave is 3.18A! But if it's driven harder to produce square waves at the same peak voltage the current demanded through the bridge goes to 5A.

Could be enough, sustained, to blow the bridge and wreak havoc downstream.
 

mbates14

Member
2002-06-02 2:14 am
non
they are using .14ohm resistors for the emitter bias.

I thought the minimum was .22??


if you want my opinion on this, This is the very first time i seen a bridge blown EVER.

the only time i would think of seeing a bridge blow like that, would be if the ouput is shorted, and someone jumped the fuse. But the fuse hsant been jumped, at least not to my knowlege.
 

mbates14

Member
2002-06-02 2:14 am
non
ok, i did a bunch of resoldering, fixed a broken trace, and replaced the bridge.

The amp comes on, no more hum.

if I play audio through it, i have to crank it up all the way, and when the bass hits, it just crackles. the cone vibrates, and it works, but it just crackles. I think its weak and highly distorted.

one of the resistors color bands are burnd off beyond readability. (that may be bad, and i cant tell teh value).

Does anyone ahve a schematic? or have the same amp that would like to share resistor values?

the resistor is on the base of one of the driver transistors. It reads 28kohms.

The base of the other driver transistor has the same color resistor, but it has color bands and reads of 33ohms. (i belive its the base of the transistor.)

Both resistors are linked together with one foil line that runs across.

it does sound like one of the drivers arn't working. Who knows.
 

mbates14

Member
2002-06-02 2:14 am
non
all fixed.

it was that resistor.


i kind of figured, that both drivers have both resistors on both exact same pins, so I kinda figured they had to both be the same value, and I put a 33ohm 1/2 watt in, and BOOM. it worked.

I nearly blew my eardrums out, because when I started playing music through it, i forgot the volume was all the way up. OWW.