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Author Topic: magnetic sound issues Jeff Else Expert Film Handler Posts: 125 From: Detroit, MI, USA Registered: Nov 2006 posted 11-21-2006 06:16 PM                     I am attempting to rehab my theatre's long-deglected 70mm setup for a few midnight movies next summer. We will be bringing in a tech before we handle any prints to make sure everything is good, but i'm doing whatever I can first, so here's my issue... The projector is a century JJ, ampex sound heads, dolby MPU-1, and a smart mod V. with the current setup, there is just a bit of buzzing, more in the suurounds.. but the center channel sounds a bit weak (we have one test reel), and we have two brand new heads so I thought I might as well replace it. The replacement is a Teccon gold head, and when it is plugged in, the smart level meters read almost red for the front channels, although there is less audible buzzing in the theatre than when the head is disconnected in the penthouse, when the meters read almost nothing. We also have a brand new ampex head, and it produces the same result when plugged in. so... my questions are... are the gains in the MPU just cranked for an old messed up head? why is there (louder) audible buzzing when the sound head is disconnected in the penthouse? could these heads just need to be de-magnetized? help!  |  IP: Logged Gordon McLeod Film God Posts: 9532 From: Toronto Ontario Canada Registered: Jun 99 posted 11-21-2006 08:38 PM                     Mag systems because of the high gain involved tend to be more prone to noise and hum I recomend something like Belden 9452 for the cables between the MPU and the head Also be sure that the shields are only connected at the MPU on the head cables and that the output of the MPU the shields are earthed at the ModV You may have to make mods to the cards in the MPU depending on there revision for the gain and loading of the heads  |  IP: Logged Scott Norwood Film God Posts: 8146 From: Boston, MA. USA (1774.21 miles northeast of Dallas) Registered: Jun 99 posted 11-21-2006 10:21 PM                        I know that this is off-topic, but if you want to get rid of one of your spare mag heads (even the old one...preferably an Ampex), let me know. One of our JJs is missing its 70mm mag head.  |  IP: Logged Dave Macaulay Film God Posts: 2321 From: Toronto, Canada Registered: Apr 2001 posted 11-23-2006 08:46 AM                     It was a long (20 years?) time ago, but when we changed from the iron 10-track 35/70 Norelco heads to 6 track 70mm Teccon we had to install a load resistor right across the head's output terminals to get them to work properly with the MPU. Sorry I don't recall the resistor value but try a 10K as a trial. Use a carbon resistor, NOT wirewound! Your hum with the head connected is either a grounding issue, poor cable choice, electrical interference, or actual magnetic interference. I would check the cable first, Gord has given you the way to wire it. If you find the wire is single conductor shielded cable you really need to upgrade it to shielded twisted pair like 9452 and connect the shield like Gord says. You have to degauss the heads, any magnetism will slowly erase the film's magnetic tracks. Use a proper head degausser and follow the instructions, basically turn off the MPU, plug in the degausser, and slowly bring its tip right up to each head gap and slowly away again. Do not turn the degausser on or off within 2 feet of the head cluster. Magnetized heads shouldn't cuase any hum but don't run film until you're sure they are degaussed. Set the MPU gains all the same; turn the adjuster trimpot all the way CCW until you feel a clicking, no more than 20 turns (I think they are 10 turn but may be 20 turn) then go CW about 5 turns. This should get you at least similar level on all 6 tracks. Check that the MOD-V input gains (if they are adjustable?) are set the same too. I'm not familiar with 70mm on a MOD-V. How does it manage the Dolby A noise reduction for 6 channels?  |  IP: Logged Tim Reed Better Projection Pays Posts: 5246 From: Northampton, PA Registered: Sep 1999 posted 11-23-2006 12:43 PM                       quote: Dave Macaulay we had to install a load resistor right across the head's output terminals to get them to work properly with the MPU. In a related thought, the mag track oxides also improved over time, and allowed for progressively hotter recording levels on the prints. I remember installing header resistors numerous times to bring MPU levels down. When 70 was resurrected with the Star Wars movies, the oxide formulation in use at the time was the same (and by the same company) as it was in the early 60s.  |  IP: Logged Steve Guttag We forgot the crackers Gromit!!! Posts: 12814 From: Annapolis, MD Registered: Dec 1999 posted 11-23-2006 08:43 PM                     To give more details.... It was not the Teccon head that caused one to need to alter the loading...it was the tracks as Tim noted. It was the fact that the track width increased from .35" to .050" and that the film was finally recorded referenced to 185nWb/m. This was a notably higher level than before. With this hotter recording, which was allowed due to improved oxides. (The company Tim was searching for was Magnacraft). Buy having the improved oxides and higher recording levels, the dynamic range was increased and noise floor reduced. Anyway. If one were to put a 330-Ohm resistor across each head (+/- terminal on the MPU), you will allow the original Cat 92 MPU cards to function. There was a Field bulletin on modifying Cat 92-4s for dealing with not only the gain but the fact that the new Teccon and similar heads had improved HF response which was not well matched by the Cat 92-4 and could cause the card to oscillate if the signal was strong enough. Part of the mod, as I recall involved installing resistors on the headers of the Cat 201 of about 33K to further reduce the level since the CP200 worked on 100mV for mag. I believe Sam was instrumental in the introduction of the Cat 92C magnetic preamp card. This is a fantastic magnetic pre amp....probably the best I've worked with. One can normally and easily achieve a rather flat response and deal with either older recoding levels or the current ones. Also the card has "head fringing" compensation filters (needed if you use the .050 heads to play back a .035" track or you will have an exaggerated bass response). As to wiring your heads...I agree with Gord that Belden 9452 is some particularly good wire for mag heads. It has many more twists per inch than typical 2-cond shielded wire and it also has two different types of shielding. It has proven successful where wires like 8451 would not. Another alternative is to use "Star Quad" cables available from Belden, Canaire and probably others. In this wire you have 4-conductors in addition to the shield (often an aggressive french braid type)...it is wired like 2-conductor shield i that you take the two blue wires and twist them together at each end and likewise with the two white wires. Due to the wires configuration in the cable, it is incredibly immune to noise being impressed upon it. Star Quad is often used for microphone cables too. Steve  |  IP: Logged Jeff Else Expert Film Handler Posts: 125 From: Detroit, MI, USA Registered: Nov 2006 posted 11-26-2006 06:59 PM                     Thanks, guys. we'll see how it goes... Scott, as soon as I test this new setup and it seems to be working we'll talk about sending you a head. jeff  |  IP: Logged

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