Author Topic: Audio Systems on 35mm Projectors  (Read 1648 times)

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Offline Nuckollsr

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Audio Systems on 35mm Projectors
« on: August 04, 2012, 07:57:04 pm »
Good day folks! I've just taken on the task of assisting one of my neighbors in the task of refurbishing some of the audio equipment in the family's movie theaters . . . one a sit down the other is a drive-in. He's still running 35mm projectors but has goals to go digital. In the mean time, I'm patching up some ancient RCA MI-12188 amplifiers (imminently rebuildable). He has no schematics on the pre-amps that read the photo-pickups for the sound track.

Wonder if anyone has ever run across exemplar equipment that would get me pointed in the right direction for understanding how these things work. Given modern electronics and fast-turn ECB fabrication, I can probably build new pre-amps faster than I can get details for chasing demons out of the old ones.


Bob . . .


Offline bilko

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Re: Audio Systems on 35mm Projectors
« Reply #1 on: August 04, 2012, 09:21:23 pm »
I used to service this gear for a recording studio in the '80s
Look up Westrex or try this link

Offline Nuckollsr

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Re: Audio Systems on 35mm Projectors
« Reply #2 on: August 07, 2012, 12:59:29 pm »
Thanks for the leads.

I've found some service manuals on vacuum tube hardware that read soundtracks on film. When the drive-in I'm working on closes for the season in a few weeks, I'll get a chance to pull some covers off, peek inside and take some measurements. I don't think it will be difficult to craft a solid state pre-amp to bias up the photocell and read it's output.

The power amplifiers in this system are RCA MI-12188 . . . parallel-push-pull 807's driving a 70v line. Found a website that features restorations and updates to this venerable beast. Turns out not to be very 'hi-fi' . . . which is forgivable when the load is a couple hundred 5' speakers on poles! I'm planning to replace the amplifiers with modern solid-state . . . found a good deal on 4 ohm step up to 70v line transformer on e-bay. I can fit him with dual amps for under $300. I think I might be able to sell some of the RCA's for more money than the new gear costs.

He has an FM broadcast transmitter too that was poorly installed . . . the antenna isn't even cut to the transmitter's center frequency. I'm going to build him a new antenna and put a wattmeter on the transmitter to make sure it's working. It 'sounds' okay but cars at the fringes of the lot don't get a really quieting signal strength. I'm betting that will be easy to clean up too.

Thanks for the heads-up!

Bob . . .

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