8mm Forum: ST 800 switch problem

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Topic: ST 800 switch problem

John Capazzo
Master Film Handler

Posts: 421
From: Hillside, NJ USA
Registered: Jan 2004

posted March 29, 2017 08:04 PM

I have an ST 800 where as the sound is stuck at 24FPS. the actual switch moves freely from 18 to 24 and 24 to 18; but sound nor the function of the machine won't drop to 18 as it should. I am aware of the two speed pots and I have moved with the machine on with film and they have no effect. I've tried electronics cleaner but the way the switch is designed, there's no room for anything to penetrate. Anyone have any suggestions? If it were a defective switch, why would the two speed pots have no effect? Thanks--John.

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"the image is about 30 feet ahead of us."

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Janice Glesser
Film Goddess

Posts: 3468
From: Sunnyvale, CA USA
Registered: Sep 2011

posted March 29, 2017 11:46 PM

Hi John...just a thought... I had that problem with a Sankyo 600. The power transistor on the speed board had gone bad. After replacing it the speed pots and the 24/18 switch worked again.

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Janice

"I'm having a very good day!"
Richard Dreyfuss - Let It Ride (1989).

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Steve Klare
Film Guy

Posts: 7016
From: Long Island, NY, USA
Registered: Jun 2003

posted March 30, 2017 04:59 AM

It's an excellent thought, Janice! If this transistor was shorted you should see 24 FPS. (and more!)

The alternative is some motor controller on the power board has lost its mind. Let's hope not: this is much easier to fix.

John, if you play a sound film, I give you odds the voices will be a little bit from Munchkin Land, whether it's the Wizard of Oz or not!

I think this is the large power transistor mounted on the bracket. (I'm not 100% sure because mine hasn't ever failed.) The original part is out there, probably obsoleted so quite expensive, but there are equivalents available much cheaper.

John, if you poke around the connections to the motor switches on the control knob and this device is at least one of them, then we can be surer.

(There are days when I would gladly swap an ST-800 for an ST-800 schematic!)

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All I ask is a wide screen and a projector to light her by...

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John Capazzo
Master Film Handler

Posts: 421
From: Hillside, NJ USA
Registered: Jan 2004

posted March 30, 2017 05:34 AM

Thanks for the suggestions. The 24 FPS voice is fine. It doesn't play faster nor slower. The speed pots have no effect.I guess if one only wanted to play sound films and not silent, this machine would be for them since it plays steadily at 24 FPS.

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"the image is about 30 feet ahead of us."

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Steve Klare
Film Guy

Posts: 7016
From: Long Island, NY, USA
Registered: Jun 2003

posted March 30, 2017 05:39 AM

That's pointing away from the transistor. If the motor operated reliably at a steady 24FPS with the gas pedal fully to the floor (shorted transistor), then you wouldn't need a 24FPS pot.

After that it's some motor control circuit. The problem there is even though it's probably a pretty simple circuit I have no idea what it's like (once again: schematics).

It's still strange that the 24FPS pot has no effect. I've messed with FPS adjustment on these machines and 18 and 24 don't happen by accident.

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All I ask is a wide screen and a projector to light her by...

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Steve Klare
Film Guy

Posts: 7016
From: Long Island, NY, USA
Registered: Jun 2003

posted March 30, 2017 09:39 AM

OK,

-how about if I stop thinking ideally and think how the real world works instead.

24 FPS is not cast in stone as far as the human ear goes. An old friend told me that most people can't tell the difference in pitch in a range +/- 5% of the real thing. More than 5% is only the beginning of the range where you would tend to notice. One of my machines was operating at 22.something FPS and it was still tolerable, especially if it wasn't operating next to a true 24 FPS set machine. (Everybody sang bass!)

What if the transistor really is shorted, but not an ideal Zero Ohm short (of course)? What if there is enough resistance to absorb enough voltage to sneak the FPS down close enough to 24?

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All I ask is a wide screen and a projector to light her by...

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Janice Glesser
Film Goddess

Posts: 3468
From: Sunnyvale, CA USA
Registered: Sep 2011

posted March 30, 2017 10:06 AM

John I'll take a look at my Elmo ST800 and see if I can find equivalent components to what was on my Sankyo 600. In the mean time check out the thread with pictures of the Sankyo board and transistor. At least I had a schematic for the 600.

http://8mmforum.film-tech.com/cgi-bin/ubb/ultimatebb.cgi?ubb=get_topic;f=1;t=010987

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Janice

"I'm having a very good day!"
Richard Dreyfuss - Let It Ride (1989).

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Steve Klare
Film Guy

Posts: 7016
From: Long Island, NY, USA
Registered: Jun 2003

posted March 30, 2017 01:30 PM

This is the device I'm thinking of:

It is Toshiba 2SD110. I'm seeing them on E-bay for about 18 bucks, but NTE162 is being called an equivalent for a little over six.

NTE162 at Allied Electronics

Something that's always useful to try is to stick an ohmmeter on that metal case and on the bracket underneath and make sure there's some real Ohms going on. There's supposed to be an insulator in between, but they are known to fail, especially if a little metallic crud got in there during assembly.

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All I ask is a wide screen and a projector to light her by...

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Janice Glesser
Film Goddess

Posts: 3468
From: Sunnyvale, CA USA
Registered: Sep 2011

posted March 30, 2017 01:53 PM

Thanks Steve...That's a totally different package than the transistor I replaced. I don't think I would have recognized it as a transistor [Roll Eyes]

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Janice

"I'm having a very good day!"
Richard Dreyfuss - Let It Ride (1989).

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Steve Klare
Film Guy

Posts: 7016
From: Long Island, NY, USA
Registered: Jun 2003

posted March 30, 2017 01:59 PM

This is a smallish power package (I sometimes work with transistors that weigh a couple of pounds! -not ever in a movie projector, though!).

It's fairly typical of the packages you often see in the last stage of an audio power amplifier, often on a large heatsink with fans blowing cooling air across it so the transistors won't burn up. (Now THAT'S an amplifier!)

What I like about working with them is the collector of the transistor is the case itself: better access than most when you are trying to troubleshoot!

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All I ask is a wide screen and a projector to light her by...

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