Problem: Advent 3 speakers sounded muddy, for lack of a better term. Very weak high end as well. Other speakers, using the same amp (Pilot mono tube amp) sound normal. Just for giggles, I ran a frequency test, comparing input signal & output signal on my dual-trace scope. The amp tracks perfectly from 40 - 18khz.

I measured the crossover caps (NP electrolytics) out of circuit with my Fluke DDM, and they were spot on, but I realize that's not a conclusive test. While the woofer DC measures 6 ohms, the tweeter is 1.2 ohms, using either a DMM or VTVM. There is a 4 ohm w/wound resistor in parallel with the tweeter cap though. Could the tweeter VC be shorting to itself, or is this normal? I was going to install new caps (16 uf and 3 uf), but I hate throwing parts at a problem willy-nilly. Your thoughts?

Many thanks,
Dan

I generally replace NP crossover caps in old speakers. Usually that restores function to the tweeters. NP electrolytics were used because mylar or film was not available in higher values without huge cost at that time. But it doesn't matter... NP electrolytic or a non-electrolytic equivalent will work equally well. You can of course use two electrolytics back to back ... that's essentially what's inside a NP anyway. Just make sure you get your value correct - capacitors in series value vs voltage calculations..

As a quick test, set the amp up for normal lower level listening levels, and then MOMENTARILY short out the capacitor feeding the tweeter. If your highs return, the cap is bad. Don't leave it shorted for more than a second or two, as you're allowing low frequencies to go to the tweeter and it won't be happy for very long, but at low power you don't risk burning it out for a few seconds.

Follow up on Barry's suggestion, you can turn the Bass way down and Treble up and that will help that quick diagnosis and avoid damage.

Thanks for the advice, gentlemen. I replaced the caps in one of the speakers this afternoon, and the difference is amazing. I'm going to do the other one as soon as the identical caps arrive from Parts Express, so that both speakers sound the same. (I had a couple caps left over from another speaker project.) I went with the old standby bipolar electrolytics; they were what were installed at the factory, and they're cheap to boot. My Fluke said both original caps were good, but obviously not in a real-world setting. One of these days I need to buy a "real" capacitor tester. A while back, I restuffed a filter can cap on a Stromberg Carlson, only to discover it was bad. Lots of work in a tight chassis that I shouldn't have had to do.

Thanks again.

Dan