Film-Tech Forum ARCHIVE: no low end on analyzer

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Posted by Mike Babb (Member # 1379) on 03-03-2004, 06:20 AM:

Here's a quick one because I am out the door...

Pink noise, analog, no low end on either channel from about 500hz down it drops off completely.

Christie P35C, jax lite, CP55. One clue, tested with 3- 240 cards, same result but it works fine when we put in a 240A from a CP65. We swapped solar cell, lens assy jax line amp and rechecked all wiring and it is a new run of wire. My main question would be is something different between the cards that would either explain a problem in the a-chain or with the card/jax lite compatability?

[ 03-03-2004, 08:56 PM: Message edited by: Mike Babb ]

Posted by Rick Long Jr (Member # 541) on 03-03-2004, 06:45 AM:

What kind of analyzer? My PC based one will not show any LF unless the sampling rate is set right. Did you confirm by ear that there is in fact no LF?

Posted by Sam D. Chavez (Member # 1841) on 03-03-2004, 10:40 AM:

I would guess your RTA is OK and you are missing a connection somewhere, perhaps on the 240 input. Solar cells require a low impedance load or the low end disappears. The 240A uses an electronically balanced input while the older 240 uses an actual transformer.

Are both channels bad? Try the projector 2 input as a test. It is likely that the 240 is defective

Posted by Bruce Hansen (Member # 281) on 03-03-2004, 04:56 PM:

If you have the phase reversed on one channel somewhere, that may cause the lows to cancel in the room.

Posted by Gordon McLeod (Member # 33) on 03-03-2004, 07:19 PM:

500K? very high if the room response is ending at 500Hz then I would think you have lost a driver since 500 Hz is the common cross over frequency

Posted by Sam D. Chavez (Member # 1841) on 03-03-2004, 07:45 PM:

I mispoke myself on my earlier response about the 240 being defective as you have tried 3 of them. Still the symptoms indicate the cell is not being properly loaded. I assume you mean 500 HZ and not 500K.

If you are using two pair shielded, the low sides should go to the L- and R- on the Dolby and both be tied to the black wire on the cell end. If it is 3 wires, the left minus needs to be jumpered to the R minus on the Dolby fanning strip.

Is the cable going directly to the cell wires or to some sort of interface plug or terminal strip. If so I would suspect the connector or terminal strip. As I remember, The Christie wiring is rather circuitous.

Posted by Mike Babb (Member # 1379) on 03-04-2004, 05:23 AM:

Thanks for the tips...to answer questions I tried two analyzers including different leads. The measurement is off the processor not in the house off the mic and yes, you can hear it audibly as well as on the RTA. It is indeed 500hz and not 500k, sorry. The wiring runs off the cell to the Jax line amp, then out to the processor. Negatives are tied together on the cell terminal strip and then go into the line amp. Wiring was checked over many times and wired the same as 9 other houses, including 8 USL and one CP65 processor, nothing similar happened. The run of wire is fresh. I suspect it has something to do with the difference between how the 240A handles the input as opposed to the 240 but I am not ruling out some as yet undetected wiring issue. For now the theatre is going to be using their spare 240A but of course I'd like to solve the riddle.

Posted by Sam D. Chavez (Member # 1841) on 03-04-2004, 08:08 PM:

I totally overlooked the issue of the Jaxlight line amp. Have you tried swapping it with another one?

Posted by Mike Babb (Member # 1379) on 03-05-2004, 06:03 AM:

Yes, swapped it with a another same type and also an unbalanced version.

Posted by Jon Bartow (Member # 241) on 03-07-2004, 02:22 PM:

Hi Mike, not to beat a dead horse, but you said that you tied the negatives together at the cell terminal strip then to the line amp then to the CP. Is the negative off of the cell tied to both JAX light line amp neg inputs? (i.e. L- & R- ). Then you have verified that the wiring is 100% corect from there on out?

Jonathan

Posted by Mike Babb (Member # 1379) on 03-07-2004, 05:37 PM:

Hi Jonathan!
Yep, two negatives tied together off the cell terminal strip negative, going into the L and R negative. Then out to the processor.
At this point there is a 240A they had and everything is running fine. So I'm just satisfying curiosity now.

A couple other things I noticed at another installation...on an older (I assume)P35C cell I had to wire the cell backwards into the line amp to obtain stereo, otherwise it was running mono. Also had to isolate the line amp from the projector frame to avoid hum, and finally, I wired negatives to neutral and ground to ground on a Mod 2 rather than the way USL recommended on their field bulletin. Lots of fun.