Infinity ServoStatik - need help with subwoofer

I had a set of these. Traded on Infinity RS 11B. I used the old dynamo 80 tube amp for tweeter. Sweet sound. I had Phase Linear for muds. I just got tired of midrange panels going out. There are no longer available so be careful. I would not play extremely loud and of course have Good surge protection. At the time they were state of art. Woofer was mushy there was a panel on eBay. That might be good source for parts. Tweeters never gave a problem. There is original set on Audiomart for 4K. Problem nov replacement parts. Good luck. I have the infinity QRS now  sound is great. I have never forgotten the midrange and highs on system. 

I fixed the hum by plugging both amps into the same power strip.  The speakers sound great, and I'm partying like it's 1969!

Thanks again.

Yes, everything is working now!  It sounds great so far.  Thanks for the suggestions.

I plugged them into my Kappa 9 setup, which is a Parasound HCA-3500 for mid and Nakamichi PA-7 for high end.  I'm thinking that a Parasound 806 or 855 would be just fine for a permanent Servostatik amp, and I can experiment with some tube amps on the highs next time I have some $$ to burn.

The SS makes me think that my Kappa 9s have some tweeter issues since the high end seems more clear now.  The bass is a comparison between a single 18" with a dedicated amp vs the 4 12" woofers in the K9s.

There is still an annoying 60 Hz hum, though... there's always something!

Thanks for the suggestions!

After running through some of your tests, it looks like the main problem was 50 years of oxidation on the pins connecting amp and subwoofer. I used a file to scratch all of the pins and after that then I was able to measure ~5 ohms. I unplugged one of my Kappa 9’s and connected the cable to the sub and was able to hear audio... awesome!

The resistance varied quite a bit when I moved the cone. I was able measure both the signal and servo paths, though it looks like the servo resistance varied a lot more. I’m assuming this is a good sign.

Tonight I’ll hook everything up and everything should be working properly. Thanks for the hints.

I took the crossover/amp apart and it looks clean enough... nothing obviously broken. So I guess it is time to look at the subwoofer.

I have never tested amps like this, so let me ask what is probably a dumb question: should I be able to do a voltage test on the output to the sub while some bass-heavy music is playing?

Obviously I don’t want to short the signal and servo pins... but will the amp send a signal at all if the servo part isn’t connected? It isn’t easy to find known-good ServoStatik parts to use for testing!