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Author Topic: Lamphouse buzz getting in magnetic playback Don Furr Jedi Master Film Handler Posts: 509 From: Sun City, Ca USA Registered: Nov 2002 posted 04-14-2005 10:02 PM                     I've just finished installing a 4 track mag system. Everything was working really well....channel balance, etc. until I struck the lamps. Now I have a low level hum in all speakers. I've used good shielded cable, floating the shield on the heads and tying down the shield with the black on the MPU side. Any ideas? BTW, the cables from the heads are NOT running close to the AC line feeding the lamphouses. (ORC 1000's) Don  |  IP: Logged Sam D. Chavez Film God Posts: 2153 From: Martinez, CA USA Registered: Aug 2003 posted 04-14-2005 11:18 PM                       Don, I have a question about wnere you are tying the shield. It should be on the earth terminal as the Dolby MPU has a plus, minus and earth for each input. Are you saying you tied it down to the black wire on the minus? The Dolby preamps are very subject to hum fields. How far is the MPU frame for the lamp or rectifier?  |  IP: Logged Don Furr Jedi Master Film Handler Posts: 509 From: Sun City, Ca USA Registered: Nov 2002 posted 04-15-2005 07:17 AM                     I'm sorry Sam....I wasn't very clear. On the INPUT side of the MPU the shield is tied to Earth ground, not with the black. On the OUTPUT of the MPU the shield is tied to the black. From there the output goes to a 25 pin plug that goes directly into the digital input of a Smart Mod2C for a pass-through to the amps. The audio rack is now about 6 feet from the lamp/PS. It's an ORC 1000 self contained. Also the cable (Belden) is NOT in a conduit. I'm not sure that would make any difference. Thanks  |  IP: Logged Gordon McLeod Film God Posts: 9532 From: Toronto Ontario Canada Registered: Jun 99 posted 04-15-2005 09:23 AM                     Try unplugging the mag head and see if the hum is still there If so it is an issue between the MPU/cable/ processor connection If it goes away then it is the head itself that is picking up the noise Make sure the head sheild is properly installed and that the penthouse and lamphouse are extremely well grounded  |  IP: Logged Don Furr Jedi Master Film Handler Posts: 509 From: Sun City, Ca USA Registered: Nov 2002 posted 04-15-2005 09:27 AM                     I'll disconnect the heads and check that out. The mag heads are well grounded because this is a direct bolt-down to the top of the XL projector and the entire base is grounded to the audio rack with several conduits.  |  IP: Logged Gordon McLeod Film God Posts: 9532 From: Toronto Ontario Canada Registered: Jun 99 posted 04-15-2005 09:38 AM                     Grounding via the bolts and conduit can be not as good of connection as one might think I always run an insulted #6 ground wire from the base and the lamphouse back to the main panel On mag systems I often pull a bonding ground wire as well  |  IP: Logged Sam D. Chavez Film God Posts: 2153 From: Martinez, CA USA Registered: Aug 2003 posted 04-15-2005 10:08 AM                       Gordon, Where can I purchase some of this insulted #6 wire . Is that like Kosher food?  |  IP: Logged Tim Reed Better Projection Pays Posts: 5246 From: Northampton, PA Registered: Sep 1999 posted 04-15-2005 10:11 AM                       quote: Gordon McLeod I always run an insulted #6 ground wire from the base and the lamphouse back to the main panel  |  IP: Logged Gordon McLeod Film God Posts: 9532 From: Toronto Ontario Canada Registered: Jun 99 posted 04-15-2005 10:14 AM                     insulated is that better  |  IP: Logged Don Furr Jedi Master Film Handler Posts: 509 From: Sun City, Ca USA Registered: Nov 2002 posted 04-15-2005 09:43 PM                     Well what ever black magic I worked has helped with the hum. I did find one head that the right channel was open causing a good bit of the noise. I believe in time I can get rid of most of the remaining problems.....EXCEPT, these ORC 1000m's are still a source of the worst of the noise. I've done the heavy gauge wire to every ground in the house, back and forth but nothing seems to help. Does anyone have any other lamphouse tricks (for the ORC1000) before I change the lamps out to the ORC 1600's with seperate supplies? Thanks.....  |  IP: Logged Mark Gulbrandsen Resident Trollmaster Posts: 16657 From: Music City Registered: Jun 99 posted 04-15-2005 10:17 PM                     quote: Sam D. Chavez Gordon, Where can I purchase some of this insulted #6 wire . Is that like Kosher food? Sam, Gord once took me to a Kosher Deli in Toronto somewhere. Great place and they definately had insulted wire and great Kosher pickles to go woth it . Don, The 1600's may work out better as they are either a straight high reactance unit or if its the older version just a bunch of pass transistors on a big heat sink. You still risk the possibility of induced hum and of rf being radiated from the arc, but these lamps always tended to be alot quieter with mag than the 1000's scr regualtor circuit ever was. Hope you get it wrung out, mag is alot of fun! Mark  |  IP: Logged Frank Angel Film God Posts: 5305 From: Brooklyn NY USA Registered: Dec 1999 posted 04-15-2005 11:53 PM                        Did I ever tell you about the time I was trying to get the change-over THUMP out of my mag system and I did everything I could think of to eliminate it but to no avail. I pinned it down to the change-over zipper coils as being the cuplrit. They were radiating that 60hz magnetic field right into the penthouse. I was getting really frustrated trying to figure out a way to stop it. Finally I decided I had to do something drastic. I went to search for a supply of lead so I could make a lead shield to wrap around the penthouse and the zipper unit. I found a supplier but he would only sell me this huge bulk roll of lead weighing in at 100lbs. It was very thick, malleable stuff that I could mold around the change-over unit and the back portion of the penthouse. OK, so I had to buy a lot of it; who cares since it was going to stop that hum in its tracks. I was every proud of my handiwork, lead wrapped around anything that didn't move. Even though it didn't look very pretty and I probably absorbed lots of lead in the process, I was convinced this was going to do the trick. Then came the test only to find, to my great dismay and utter disappointment, that the thump was still there, audible as ever. I was truly bewildered. I mean, I covered every inch of the casing of the coils and thoroughly covered the penthouse with a few layers of lead. I even glued a layer on the door. You know that feeling you get when you work on a problem for hours and hours and at the end it just laughs at you? It's not a fun feeling. I gave up and left the booth and found one of the older techs in the sound studio and poured out this story to him over a beer. Throughout my lament, he looked at me with this perplexed and awed expression. When I was done explaining how the 60hz hum was still able to get through the shield I had fabricated, he ask in a slow, deliberate voice, "So what made you think that lead will stop a 60hz AC induced hum field? I thought for a minute, and couldn't really come up with anything other than, "Well, EVERYONE knows that lead stops all kinds of radiation." He just smiled and said, "Frankie, you are thinking of Superman's X-ray vision....Superman can't see through lead because it blocks his X-Ray vision; it has no effect on AC hum fields." I was stunned. And quite terribly humbled. The huge roll of unused lead hung around the back stage area for months only to prolong my pain. In my defense, I was only 22 yrs old. If you heard my story before, I apologize, but it still is a good lesson of what can happen when you make assumptions while troubleshooting.  |  IP: Logged Sam D. Chavez Film God Posts: 2153 From: Martinez, CA USA Registered: Aug 2003 posted 04-16-2005 12:19 AM                       That's why they call it cut and paste. You try something and then test it, but you should not wait till showtime. There is a metal that does block these frequencies. It's called mu metal. Perm-alloy works also. There is a piece of mu metal on the back of the JJ Zipper douser. Works wonders. The heads on the this penthouse should have mu metal shield.  |  IP: Logged Steve Guttag We forgot the crackers Gromit!!! Posts: 12814 From: Annapolis, MD Registered: Dec 1999 posted 04-16-2005 07:31 AM                     Yeah, most people don't get the Mu-metal thing. They seem to think they are dust shields or something. Simplex's 35/70 forgot completely about the mu-metal shields in the 80s (don't know about the originals). A way of dealing with the hum at changeover time is to eliminate the hum...that is...run your changeovers "DC". Ever noticed that Essanay offers DC coils? I've done this several times. However, things that may have worked okay in AC may fail in DC...check the rating of your switches/relays before making the switch. You will often have to make your own power supply too (needn't be anything fancy).  |  IP: Logged System Notices Forum Watchdog / Soup Nazi Posts: 215 Registered: Apr 2004 posted 11-29-2012 03:00 PM                 It has been 2784 days since the last post.  |  IP: Logged

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