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Sx-990 burning emitter resistors

Carbsandcaps

Carbsandcaps

Super Member
Working on a pioneer sx-990 that keeps blowing the amp fuse on one side. While checking everything out I replaced the fuse again and the 2 watt emitter resistors started heating up. Then I realized I put a 5 amp fuse instead of the 2 amp it calls for. So I turned it off right away but I would guess this might at least point me in the right direction. All of the resistors and transistors are testing good out of the board. The stabistor diode is testing good. The outputs were replaced with NTE (ya I know) 130’s.

Anyone got any ideas or guesses? I’ve got it so I can power the board outside the receiver for testing. IMG_0132.jpegIMG_0133.jpeg
 
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Have you checked the bias voltage? It should be about 15mv.

Also, with the power off, measure across the bias trimmer of that channel. I believe it is a 50 ohm so it should be around 30 ohms. These fail with some regularity.
 
Bruno Primas said:
Have you checked the bias voltage? It should be about 15mv.

Also, with the power off, measure across the bias trimmer of that channel. I believe it is a 50 ohm so it should be around 30 ohms. These fail with some regularity.
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You sir are the man. I measured it in the chassis and got megaphms. Removed it and still got megaohms. Replaced it with a 100ohm pot because that’s the lowest I had and no more overheating resistors or blown fuses! Thank you so much for the recommendation. IMG_0145.jpegIMG_0146.jpeg
 
Good news!

I've wrote this recommendation on more than a few threads, (some still open ones.) This is a very common problem on the earlier Pioneer SS units and should be replaced EVEN if presently working.
 
Bruno Primas said:
Good news!

I've wrote this recommendation on more than a few threads, (some still open ones.) This is a very common problem on the earlier Pioneer SS units and should be replaced EVEN if presently working.
Click to expand...
Ya I’m definitely replacing every one of these I come across now. Got it all back together and had a channel out. Finally found a spot where I soldered a transistor to itself. Man these tracks are close together. Anyway now I have sound coming from both channels. Thank you so much, I put this thing away a while ago because it was driving me nuts. Now I’ve got one more project off my plate. Thanks again!
 
Was immediately thinking bias pot too. My Allied 495 uses the same one, it failed and killed one of the outputs.
 
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