Infinity ServoStatik - need help with subwoofer

Hello, I'm trying to resurrect an old ServoStatik I and the midrange and tweeters are working.

However, there is no sound whatsoever from the subwoofer.  I did a continuity test on the cable and it seems fine.  Is there an easy test I can do to tell whether the problem is in the amp or the subwoofer?

I tried testing for resistance in the connectors to the bass commode (with no luck) but now I'm trying to decide whether to take apart the amp/crossover or the commode.


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!

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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.

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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!

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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.
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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.