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Wolfsong 12th March 2009 05:12 PM

Help Identifying a part

I have a Krell KSA 250 that will not fully power on. It had been driving a studio Grand Subwoofer by Apogee. When I got it one of the woofers had been bottomed way out and was open. The main power switch on the Krell will power the front power led and one ( right channel) relay on the protection board will close but the other relay (left channel ) stays open. Power does not seem to make it to the toroid and the only way the led will power off is to pull the power cord out of it. I pulled the face off the amp and found a small part that looks burned. It is labeled TP1 on the board. It looks a bit burned/melted. I am guessing it is a thermal protection device of some sort but not one I am familar with. I have photos but no place to host them. The part looks like a tiny white plastic ring with a small metal "pin head" that sticks out about 1/16" It looks like there is only one solder point that attaches this part to the board.
Any help would be very much appreciated.

Steerpike 13th March 2009 12:29 AM

Tp is usually a test point. Your description sounds like it too, a single pin in an insulating bush, where you can clip on a test probe.

Unlikely the cause of your fault - unless it touched something metallic on the chassis by mistake.

An non-closing relay means one of two things:
1) the protection circuit isn't getting any power,
2) the amplifier is putting out DC, due to a failed output transistor.

I have no schematic, so that's as much as I can suggest.

Wolfsong 13th March 2009 01:42 AM

I figured out the test point thing. Feeling kind of stupid. DC offset seems possible as the one of the woofers the amp was driving shows evidence of massive bottoming. A crushed Voice Coil and former. Any way to test the output devices while still in the circuit? I am a novice but have some experience with DIY and general troubleshooting. NOTE: I will not be stupid with this. In Know lethal voltages exist and will respect the power always.


Steerpike 13th March 2009 04:12 PM

Output transistors , yes you can test while in circuit, since the fault is likely a direct collector-emitter short circuit, so you're not looking for anything subtle.

The difficult part comes trying to figure out what else is dead, and where the trail of destruction started. Just replacing one short-circuit transistor might simply result in the new one dying instantaneously, if you haven't found out what killed it.

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