Matthews Posted January 23, 2016 Share Posted January 23, 2016 Meeting the seller of a pair of La Scala's in a McD's parking lot in the morning. I want to check the drivers with my ohm meter before handing over the cash. I performed this operation on my current pair with mixed results. The tweeters and woofers ohm'd out to their proper readings at the crossover. The mid driver did not ohm properly until I disconnected it from the crossover and measured directly at the driver. Is this the normal procedure for measurement? Please advise... Thanks, Matt ♪ ♫ ♪ 1 Quote Link to comment Share on other sites More sharing options...
WMcD Posted January 23, 2016 Share Posted January 23, 2016 Yes. I believe you did well to disconnect the leads to make the resistance check. You can look at AL2 and AL3 schematic in the pinned crossover link. I think earlier LS used an A type crossover. In the A the mid is fed from the autotransformer "output". Therefore unless disconnected, you're measuring the parallel resistance of the autotransformer windings (which I'll guess is about 2 ohms) in combination with the voice coil of the mide driver (which has a resistance of about 10 ohms I think). Therefore you'll be getting a reading down around 2 ohms or less unless you disconnect a wire to the driver. The AL2 and AL3 have an inductor across the leads in addition and that should lower the connected resistance even lower. WMcD. 1 Quote Link to comment Share on other sites More sharing options...
Matthews Posted January 23, 2016 Author Share Posted January 23, 2016 I think earlier LS used an A type crossover. Thanks "WMcD" - the pair I am looking at in the morning are AA crossovers. Quote Link to comment Share on other sites More sharing options...
Matthews Posted January 23, 2016 Author Share Posted January 23, 2016 Lets say a tweeter is bad. What type of reading will show on my meter? Will it even sound off for continuity? Normal resistance is around 6.9 on a K-77, give or take. Quote Link to comment Share on other sites More sharing options...
jimjimbo Posted January 23, 2016 Share Posted January 23, 2016 It will read open. 1 Quote Link to comment Share on other sites More sharing options...
WMcD Posted January 23, 2016 Share Posted January 23, 2016 (edited) Lets say a tweeter is bad. What type of reading will show on my meter? Will it even sound off for continuity? Normal resistance is around 6.9 on a K-77, give or take. I think I understand the question. But maybe not. The issue is that you can set a multimeter to continuity test so that it makes a tone when the test leads are touched together, and the resistance to current flow is very low. And if you're testing a wire for continuity you use this. But is this good for testing the continuity of a voice coil generally and tweeter voice coil specifically? It must be that there is a threshold of resistance. The tone sounds when there is zero or near zero ohms. But how about 1 or 2 or 3 ohms and how about 10 ohms of a tweeter voice coil. I don't have my multimeter here to experiment. However one site discusses the issue in a reverse sort of way. If we're testing wire, how low does resistance have to be before wire and the connector is said to be continuous or "good." http://www.cirris.com/learning-center/general-testing/guidelines/102-setting-practical-resistance-specs-for-continuity-testing If you require 0.01 ohms for "good" that is more than is needed and is a tough test. If you let 10 ohms to be considered "continuity" or "good" that is not a tough enough test for wire and the connector on the end. So Cirris is saying that the threshold for the test should be about 1 or 2 ohms. So any resistance lower than that is good. That doesn't mean that the threshold for every multimeter sounding off is 1 or 2 ohms but it is probably close. This also means that a voice coil with a d.c. resistance of 10 ohms will probably not trigger the tone on most multimeters. But that (no tone) doesn't mean that the tweeter voice coil is "bad" or "open"? As mentioned by Jim, it necessary to look at the resistance reading on the meter. A 10 ohm reading indicates a good voice coil and I'll say that 20 ohms indicates a problem. Generally, the voice coils will melt in a small section when overdriven and stop working all together. So you get 10 ohms (good) or (if bad) a reading comparable to what you see on the meter when the leads are not touching each other (said to be an "open" circuit. On some meters this is "OL" on the display using seven-segment digits. (Called this because the numbers are made up of seven bars -- for the figure 8, all seven segments are triggered.) Further to my earlier post. Yeah, the A and AA have that issue with the autotransformer connected to the mid and you have to disconnect one wire for a valid test. Also, you'll see that the tweeter is connected to the rest of the crossover through a capacitor. (That symbol with two parallel lines -||- .) The d.c. test current used by the multimeter can not pass through the cap and therefore you don't have to disconnect a wire to get a valid reading. WMcD Edited January 23, 2016 by WMcD 1 Quote Link to comment Share on other sites More sharing options...
Matthews Posted January 23, 2016 Author Share Posted January 23, 2016 I think I understand the question Thank you so much, "WMcD" !!! - My meter, when set to continuity automatically reads the resistance (ohm). I tested it on some spare K55 and K77's at various settings. It reads 10.9 and 6.5, respectively, as well as sounds off for continuity. Hopefully it performs as expected during my field testing this morning. If it reads much differently than my at home results, will have to do some renegotiating with seller. Your help is much appreciated Matt ♪ ♫ ♪ Quote Link to comment Share on other sites More sharing options...
MookieStl Posted January 23, 2016 Share Posted January 23, 2016 I looked for a shaking his head emoji, but could not find one, and I think there is one of those cleaver acronyms about shaking your head but cant remember it. I just wanted you to know that I am reading this and shaking my head. It would have been much cooler had I been able to do it the cooler ways, but..... 1 Quote Link to comment Share on other sites More sharing options...
Bacek Posted February 7, 2016 Share Posted February 7, 2016 Unfortunately resistance check is not enough. But of course better than nothing. When I bought Lascalas one of the woofer was not blown but incorrectly rewired/repaired. Resistance of bot was quite similar. But impedance me sure mentioned where way off and inductance was really crazy for one of the woofers. As result they had similar efficiency above 200hz but below there was about 6dB difference. Ended replacing them with Eminence Kappa 15c and k33 waits for professional fix. 1 Quote Link to comment Share on other sites More sharing options...
Matthews Posted February 7, 2016 Author Share Posted February 7, 2016 Unfortunately resistance check is not enough. But of course better than nothing. All good, "Bacek" - They tested fine in the parking lot and sounded even better at home. The guts are now in a different pair and sold. Quote Link to comment Share on other sites More sharing options...
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