How do I test SDA 1.2tl before installing replacement tweeters?

zumbini

My college age son was recently gifted a pair of 1.2tl's that needed some work. I am in the process of restoring them to original condition before performing any of the many fine upgrades that are discussed on this forum. So far I have repaired rips and tears in about half of the mid woofer surrounds and replaced the grill and side panel cloth. (see http://forum.polkaudio.com/discussion/comment/2163033/#Comment_2163033)

The next phase of my project is to replace 4 non-working tweeters (T2 and T3 in both speakers) so I recently acquired 4 SL3000's in a trade with a forum member. Before I install the replacements I thought it prudent to determine if there is a problem with the crossover or wiring that took out the tweeters.

To that end I removed the dead tweeters and measured the static resistance of the circuit across the tweeter leads. T2 reads 14.0Ω (left speaker) and 14.2Ω (right speaker) which seems reasonable based on what little I understand of the circuit. Am I correct?

T3 reads 5-6 MEGAohms (left) and 7-8 MEGAohms (right) and the readings keep rising slowly over time. I know what a crossover does but I don't understand how each component affects the circuit electrically. I assume that the battery in my DMM is charging C5 and that's why the readings increase, but I'm concerned about the high resistance. Is that "normal" or does it indicate an issue with the crossover?



At that point I removed one of the crossovers and did a visual inspection. I noted a powdery white substance on some of the block resistors and some dull solder joints which I reflowed.









Then I removed or lifted one leg of each of the components are measured them:
The 22.5Ω resistor reads 23.5Ω and C5 (12mF) reads 13.4uF which seem OK

The parallel 750pF mica cap reads 0.41nF (several orders of magnitude too high)

The series 750pF mica cap reads 0.44nF (several orders of magnitude too high)

I can't get a consistent reading on the 12uF mylar cap. It either reads 0 or somewhere between .1 and 25nF (several orders of magnitude too low)

I rang out the coils (not open) but they seem too simple to fail in a low voltage application like a crossover.

If I read the schematic correctly the series mica cap and mylar cap affect all 4 tweeters. If they are "bad" why didn't T1 and T4 blow too?

Does any of this make sense?

Am I an idiot for trying to fix this myself?

NOTE 1: I used a Fluke 77 to measure the initial circuit resistances but it doesn't read capacitance so for that I used a cheap Vichy VC99. I know it reads about 5% high on resistance (versus the Fluke) but it's probably not worth the cost of recalibration.

NOTE 2: I know many of you will suggest that I upgrade the crossovers but I just don't have the money right now. I just want to install the used tweeters I picked up without frying them.

[Deleted User]

Hello,
When I worked directly for Polk CS and a customer would send in a crossover that needed checking I would connect a pink noise generator to an amplifier (Dynaco ST70) and I had made a 4 ohm resistor in a box with alligator clip leads to use as a load. Then I would look at what each pair of leads was producing on a handheld spectrum analyzer. This would show me the frequency spectrum produced by each portion of the crossover.
My point of this was to point out that only by involving an amplified AC signal can you examine what the crossover is doing. You could come close to this by using an AC generator, amplifying it's signal, then connect a 4 ohm load at the tweeter/driver terminals and using a "true RMS" AC volt meter (most volt meters get inaccurate when dealing with frequencies above 100Hz). Then as the frequency goes up or down you can chart the response at the actual load.
I hope this is useful information.
Cheers, Ken

zumbini

Thanks Ken. That's great info for a service tech but I'm just an electronics noodler who doesn't own a pink noise generator or a spectrun analyzer. All I have is a DMM so I can only test the static resistance of each of the crossover sections (T1, T2, T3, T4, M1, M2) and measure individual components to see if they are in spec.

At this point I am just trying to determine if it's safe to install the new (to me) tweeters. Perhaps someone with a stock 1.2tl would be kind enough to remove T2 and T3 and measure the static resistance of each circuit so I have a point of comparison.

zumbini

Sacrificial tweeters? D'oh! Why didn't I think of that? I might have some in my junk pile that'll do the trick. I'm overthinking as usual. Thanks nbrowser!

F1nut

If they are "bad" why didn't T1 and T4 blow too?

The tweeters are a progressive point source. IIRC, the firing order, so to speak, is T3 (main), T2, T1 and T4. Therefore, T3 and T2 would be the first ones to cook when sent a clipped signal. Since the same tweeters in both speakers are cooked, that would be the likely reason.

vmaxer

Any progress on the speakers? I hope it works out for you.

zumbini

F1nut wrote: »

If they are "bad" why didn't T1 and T4 blow too?

The tweeters are a progressive point source. IIRC, the firing order, so to speak, is T3 (main), T2, T1 and T4. Therefore, T3 and T2 would be the first ones to cook when sent a clipped signal. Since the same tweeters in both speakers are cooked, that would be the likely reason.

Ahhhh. I remember reading about "progressive point source" in the SDA manual but didn't fully understand the implications. Thanks for connecting the dots for me.

[Deleted User]

If it's any comfort, while not impossible, it is certainly very difficult for a crossover to damage a tweeter or driver. At least in my experience when there's a problem with a crossover, usually, something doesn't play. No tweeter/driver output, or a reduced output is usually what happens.

zumbini

Kenneth Swauger wrote: »

If it's any comfort, while not impossible, it is certainly very difficult for a crossover to damage a tweeter or driver. At least in my experience when there's a problem with a crossover, usually, something doesn't play. No tweeter/driver output, or a reduced output is usually what happens.

So it's unlikely, but not out of the realm of possibilities. I like those odds but I am going to take nbrowser's advice and find some "sacrificial tweeters" to test with. I'll let y'all know how that goes.

[Deleted User]

Ah, a belt and suspenders man I can tell.

zumbini

Kenneth Swauger wrote: »

Ah, a belt and suspenders man I can tell.

Only once that I recall and that was in 1969 when I played Potemkin in a high school production of Celebration.

[Deleted User]

I hope that was enough champaign to go around.

zumbini

UPDATE: I tested the T2 and T3 circuits with 2 junk tweeters I pulled from an old pair of Infinity RS2001's. They sounded fine in the left speaker but were barely audible in the right. I'm going to replace the 12mF mylar cap and the two 750pF silver-dipped mica's in the right crossover that tested out of spec and try again.

vmaxer

Good luck Dom. I hope the tweeters I sent work out. If not, let me know.

They will sound great when you get them right.

zumbini

I tested all 4 of the tweeters you sent me in the left speaker and they work fine. THANKS BILLY! I'm holding off on installing them in the right speaker until I replace those out of spec caps in the crossover and recheck it.

vmaxer

zumbini wrote: »

I tested all 4 of the tweeters you sent me in the left speaker and they work fine. THANKS BILLY! I'm holding off on installing them in the right speaker until I replace those out of spec caps in the crossover and recheck it.

Sounds like a good plan. And you are very welcome. I feel like I got the better end of the trade and still owe you.

zumbini

vmaxer wrote: »

zumbini wrote: »

I tested all 4 of the tweeters you sent me in the left speaker and they work fine. THANKS BILLY! I'm holding off on installing them in the right speaker until I replace those out of spec caps in the crossover and recheck it.

Sounds like a good plan. And you are very welcome. I feel like I got the better end of the trade and still owe you.

We both got what we wanted so no worries mate.

vmaxer

zumbini wrote: »

vmaxer wrote: »

zumbini wrote: »

I tested all 4 of the tweeters you sent me in the left speaker and they work fine. THANKS BILLY! I'm holding off on installing them in the right speaker until I replace those out of spec caps in the crossover and recheck it.

Sounds like a good plan. And you are very welcome. I feel like I got the better end of the trade and still owe you.

We both got what we wanted so no worries mate.

Thanks a lot. I really hope everything works out for you.

zumbini

I replaced the out of spec crossover caps and installed vmaxer's used SL3000 tweeters. There is still an almost imperceptable drop in output from T2 and T3 on the right speaker that I haven't been able to track down but no one but me notices it. Meanwhile I'm saving up to replace all the active drivers and upgrade the XO's.