[Djard]

I need to replace the power adapter for the external speakers--SP-30N(US)--that came with an eMachines T3990. I'm probably safe getting a 9VDC, 200 to 500mA rated transformer, but what the core of the connector...is it (+) or (-)?

[carnageX]

Can always use a multimeter to find that out.

[Djard]

Thanks for the idea. I have a digital multi-tester but only know how to measure the output voltage and current on a working adapter; and as I mentioned, I don't have an adapter for these speakers. Are you saying there are contacts on the motherboard that I can use?

I contacted eMachines' (Gateway) live tech support and was told that the company does not have any info about the speakers's power adapter's specs. Wow!

[carnageX]

Ah, oops, guess I misunderstood what you meant, sorry. Is there a tip polarity symbol on the speakers anywhere?

[Djard]

Unfortunately no info is stamped on the speakeres regarding the polarity. But apparently you were correct: I spoke with an electronics engineer and he told me to plug in a 1/8" stereo connector into the speaker jack (without cable attached), clamp or touch a ground source on the case with the common (black cable) probe, and then use the hot (red cable) probe to touch the part on the 1/8" plug that represents the tip with the plug. If you get a negative reading, the tip is negative.

Another method is to plug in an adapter with an output of 9-12 VDC (no higher than 24 VDC), about 200 mA for a pair of small speakers, and listen for 60 Hz hum. If no hum then the polarity of the adapter for the self-powered speakers is correct; otherwise reverse the wiring on the adapter plug.

He said you can't do any harm., and he has used the latter method for years to test phasing etc. I hope this info will help others who may face the same problem. Now all I need to do it get a battery for my multi-tester, which died in storage. Aaargh!

[carnageX]

Glad you figured it out .