Re: Replacing SP-30N(US) Adapter for Ext speakers
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!