drifting impedance

[valhallax]

why do drivers change their impedance? now ive to different drivers that have changed from 8 ohm to 16 ohm loads? i can understand burning up a driver and the coil going south, such as a 418b i have that got driven way too hard and now has an impedance of 2.8 ohms. now ive also a 515 8g that is 12.8 ohms and a 403a that is 20.4 ohms....

[valhallax]

by the way, the 403a was in a 840 cab..6 drivers at 8ohms wired in sequened and parallel to end up at 12 ohms. the 515 8g was in a 828 cab, bi-amped

[bfish]

If you're measuring with an ohmmeter, and the resistance is signifigantly different at room temp than some previous test, you're frying coils. A dying coil can go up or down in DCR depending on the failure mode. Going down is caused by shorted turns when the varnish melts/burns away, and up is from one or more spots where the wire got red hot but cooled still intact. Cold joints on the tinsel read high too.

Typical 8 ohm Altecs like your 515 should read around 6.7 ohms DCR.

Spring terminals can be a tricky read. Go around them and measure from the tinsel leads to be sure.

[valhallax]

fairly sure the sensor wont allow me to voice the explitives in regards to fried voice coils..my own fault anyway.

[bfish]

You have my sympathies. It's a long shot, but before you condemn them, it's worthy of mention...

...If you're using a digital VOM, make sure you measure where it's very quiet. Big, efficient, and compliant drivers make fairly effective microphones, in that the voice coil produces voltage when moved. Even a few millivolts applied to a DVOM set on ohms will change the reading considerably. The effect is usually noticed by a reading that just won't settle down and keeps changing, but if you took a quick reading or were using the hold feature...

Fingers crossed...

I'm guessing though that you heard something out of the ordinary that caused you to measure in the first place. In that case, the measurement's probably right.

[GM]

Yeah, how you measure can affect Dcr readings immensely, so how were all these done?

GM

[bfish]

Sounds fishy to me. AFAIK, GHPs only came in 8 ohms. I might be able to tell with your cone #s, I have some of each to check against, including some of Bill's Gs. May or may not help, depending on the visible numbers, as the only difference between 8Gs, 16Gs, and 8GHPs is the voice coil.

It really sounds to me like you've got one of each (8&16) rather than a burnt coil. Did the seller ship them to you or did they come from GPA? A savvy but honest seller would have GPA drop ship to you to save $ and time to match the story you got.

[valhallax]

weighing my options, im not sure if the frieght back and forth to great plains audio, and the potential repair costs, and the probability of not being the 515-ghp's i need...perhaps id be further ahead by just saving up for the ghp's from bill? tho the issue of the driver difference still needs to be resolved
the seller, i believe, said he picked them up in person, and shipped them from his location. tho they did arrive in gpa boxes

[bfish]

Being quiet is just a start too. HVAC vibrations on a driver on a floor or bench can be enough to whack things sometimes.

I highly recommend using some clipon type extensions on your meter leads. My faves are the push type that slip right on the probes (radio shack). Alligators not so much. Clip them on and step back till the reading stays put or just varies 1 digit.

Then yell HEY, and watch the meter go nuts.

[valhallax]

yep..im using a dvm. i learned a long time ago about ambient noises and measurements. if you recall, i was setting up my 210's and couldnt figure out the jumps in the ohm readings. crazy readings, from 8-24 ohms! it was the airconditioner...lol
the first clue about the 840 cab was a total impedance reading taken off the jack, shouldve been 11.8 ohms or there about, it wasnt, so down it came. these were originally loaded with 403a's, unlike the 839 cabinet that were loaded with 755's, wouldnt that be a nice find? by reading each speaker individually i was able to isolate the bad unit. i replaced it with a 409-t
the 515-8g is another story. i finally got my woofer tester 2 up and working, with the driver update, and was going through my drivers. when i got to these all parameters were very close, except the re. one was 5.8 the other 11.8. i couldnt believe it,as i only ran these drivers in my 828 cabs for about two weeks!
the woofer tester on a bench in a quiet room, and the dvm also in a quiet area
heres a pic of the 840's..theyre the blonde cabinets, also the 1217a's that got me started in altecs, with some 8156 clone cabs up front, and a pair of 418-8h loaded into a winisd sim cab way in the back
small altec cluster
these are located 8ft off the floor
thanks for the reply