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Topic: ST-800 Speed Problem

Dan Lail
Film God

Posts: 2110
From: Loganville, Georgia, USA
Registered: Jun 2003

posted May 14, 2005 09:03 PM

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My ST-800 is running too fast. The Speed selector works. When the back cover plate is off there are two pots on the circuit board. The pot on the right will vary the speed a few degrees. I'm not sure what the pot on the left is suppose to do. When in the 24 fps mode the pot will not slow it down enough. It probably slows it down to about 28fps at the slowest adjustment. When in the 18fps mode I can adjust the circuit board pot down to about 25 or 26 fps. Since I don't have a meter this is just a guess on the fps. Is there a speed adjustment on the motor like in a cassette motor?

This machine has had this problem since I bought it used.

I can provide pictures if that helps.

Help!!! [Confused]

[Confused]

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Jan Bister
Darth 8mm

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From: Ohio, USA
Registered: Jan 2005

posted May 14, 2005 09:10 PM

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Have you tried the other pot at all?

My Chinon SP-330MV has three pots, all of which seem to affect the speed, though I seem to remember that one only changed the 18fps setting, one changed the 24fps setting, and the third one had an effect on both.

[Eek!]

[Eek!]

--------------------
Call me Phoenix. *dusts off the ashes*

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Dan Lail
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From: Loganville, Georgia, USA
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posted May 14, 2005 09:24 PM

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Jan, I tried the pot on the left in both 24 and 18 fps mode and couldn't detect any speed changes. Mayby there is another adjustment that I can't see.

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Jan Bister
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From: Ohio, USA
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posted May 14, 2005 09:52 PM

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Well, my knowledge of electronics is limited... but this sounds like one of two cases: 1. there is a resistor (or something else with resistive properties) in line with the pot and it has failed, thus letting the full voltage through now which means the motor is getting too much juice even with the pot turned to maximum resistance. Or 2. some other component, possibly a rectifier, has failed.
Or 3. someone tampered with the mechanics (drive belt, pulleys) and the motor really is running at the correct speed but everything else isn't? [Confused]

[Confused]

(Does the ST-800 use different sets of pulleys for operation at 50Hz and 60Hz AC voltages? Could it be set to 50Hz, if so?)

--------------------
Call me Phoenix. *dusts off the ashes*

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Dan Lail
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From: Loganville, Georgia, USA
Registered: Jun 2003

posted May 14, 2005 11:02 PM

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The mechanics, pulley's and belts looks normal. I couldn't find a voltage selector. I'll try and get a picture of the inside projector on soon. [Smile]

[Smile]

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Rick Skowronek
Expert Film Handler

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From: Marietta Georgia USA
Registered: May 2005

posted May 15, 2005 11:54 AM

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Just a guess since I don't have the schematic for this unit, but the one's I've seen, use DC motors with some form of negative feedback to a motor control circuit (where your two pots are).

Can only guess that the feedback system (tach/generator) on the motor has gone bad, or the motor control IC or circuit has a failed component. 50 or 60 hertz line frequency makes no difference to these types of circuits. Probably why the manufacturers went to them since it's a whole lot cheaper not having to provide separate motors for all the different worldwide standards of frequency and voltage.

Again, if you or someone has a schematic I'd be happy to refine the guess a little.

Rick

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Dan Lail
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From: Loganville, Georgia, USA
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posted May 15, 2005 12:53 PM

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Thanks Jan and Rick, I'll try and find a schematic on line. [Cool]

[Cool]

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Kevin Faulkner
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From: Essex UK
Registered: Jun 2003

posted May 15, 2005 04:47 PM

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Yes I would agree with all that has been said. Normally on the Elmos they have just two pots one for the 18 and one for the 24fps.
If the machine is running fast in both settings then I would probably go for compnent failure. As rick said it will be in the feedback circuits. If you can let us see a pic I can tell you if its the same circuit as the 600 which I do have a schematic for.

Kev.

--------------------
GS1200 Xenon with Elmo 1.0...great combo along with a 16-CL Xenon for that super bright white light.

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Dan Lail
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From: Loganville, Georgia, USA
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posted May 15, 2005 11:43 PM

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Hi, Kev, here is a close up photo;

web page

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Kevin Faulkner
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From: Essex UK
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posted May 16, 2005 03:17 AM

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Dan, Ok that pic was great. Can you do me another favour and tell me if there is an Integrated Circuit on that board or just transistors. If possible do they have numbers on the IC or Transistors. I think this is looking similar to the ST600's speed control.
Kev.

--------------------
GS1200 Xenon with Elmo 1.0...great combo along with a 16-CL Xenon for that super bright white light.

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Dan Lail
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From: Loganville, Georgia, USA
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posted May 20, 2005 03:12 PM

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Okay Kev, I finnally got a chance to get a picture on of the circuit board;

web page

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Dan Lail
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From: Loganville, Georgia, USA
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posted May 22, 2005 10:52 AM

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Kevin, I have posted a close up of the circuit board. I don't see any ic chips unless those two black things to the left of the two capacitors are chips. Any clues? [Confused]

[Confused]

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Rick Skowronek
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From: Marietta Georgia USA
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posted May 22, 2005 12:01 PM

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Good picture, Dan.

Those two black items next to the two capacitors are bridge rectifiers. Probably supply DC to the sound and and other boards. The motor control looks like all discrete components. I see 4 diodes probably hooked up in a bridge configuration for motor power and it looks like two transistors and an SCR for motor control at the back of the board.

On a positive note, that would be fairly easy to repair as these are all standard available over the counter devices. Just ahve to be able to troubleshoot. The only baddy might be if the tachometer generator itself, on the back of the motor, is bad. Not typically repairable.

Good luck.

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Dan Lail
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From: Loganville, Georgia, USA
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posted May 22, 2005 01:53 PM

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Thanks, Rick! I can't read a schematic, but I do have an ohm meter and know how to remove something from the circuit board and istall another part. If the problem is the motor, is this motor exchangeable(is that a word?) with the motor from another Elmo? I have a couple of junkers for spare parts. One is an ST-180 and the other I'm not sure of. It has a small screen on it and can project on a wall too. [Cool]

[Cool]

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Rick Skowronek
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From: Marietta Georgia USA
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posted May 22, 2005 02:08 PM

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Dan,

If by some chance it is the motor tach combination, then I'd have to say it probably is the same or at least similar to the ones in the like models. The key is whether Elmo may have made major changes to it from model to model.

But, the schematic is the key to understanding what and where to check. For that I'm sure Kevin F is the greatest resource. If he can provide a downloadable schematic of that circuit, I'm sure all the troubleshooting gurus including him can get involved and help you trace it to see what has failed.

Rick

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Dan Lail
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From: Loganville, Georgia, USA
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posted May 23, 2005 10:19 AM

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Thanks, Rick, maybe Kevin F will be able to shead some light on the this problem. [Smile]

[Smile]

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Rick Skowronek
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posted May 23, 2005 03:08 PM

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I'm hoping Kevein is checking for you. From what other members have told me in the forum the ST 180, ST 160 and ST600/800 were all pretty much the same units with slight modifications and different audio configurations. I'm sure improvements were made along the way as well.

As to circuit operation, it basically comes down to the fact that the circuit, for whatever reason, is not seeing the tachometer feedback from the motor. The fact that you can adjust to a small extent means that most of the speed control is still attempting to do it's job. However, when the circuit doesn't see this feedback voltage (should be about 7 volts DC per 1000 RPM, but I don't have a clue at the normal projector motor rpms)it tells the circuit to apply a maximum balance attempt to get that feedback voltage up (increase motor voltage and speed up to a maximum). The pots will still have an effect on that maximum voltage, hence why you're still able to get some small degree of control.

Since it sounds like you have an ohmeter/voltmeter you should be able to easily find the tach connections on the back of the motor and connect and run the projector with the voltmeter in the DC voltage setting of at least the 25-30 volt range connected to the two wires coming from it. If it is indeed putting out a voltage somewhere in that area it's pretty certainly a circuit failure. If not (it's little or nothing) guess what and I'll let you fill in the blanks.

It's off to get Kev's schematic however if tach voltage is there.

Rick

Oh lordy, I just realized I'm now cassified as a film handler. Now if I just had something that actually handled film, besides my hands, I'd feel more deserving. [Big Grin]

[Big Grin]

Someday.

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Kevin Faulkner
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From: Essex UK
Registered: Jun 2003

posted May 23, 2005 03:59 PM

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Thanks for that close up pic Dan. It does look like the simple 2 transistor design as used in the 600. I will have a look at the 600's diagram for you and get back to you with any ideas.
Dont suppose you noticed any numbers on those transistors?

Kev.

--------------------
GS1200 Xenon with Elmo 1.0...great combo along with a 16-CL Xenon for that super bright white light.

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Dan Lail
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From: Loganville, Georgia, USA
Registered: Jun 2003

posted May 23, 2005 06:44 PM

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Thanks, Kev. [Smile]

[Smile]

The transistor on the left is; Tx C609 GR5J. The transistor on the right is; NFC A/33 P25Y.

Here is the close up pic again; web page

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Kevin Faulkner
Film God

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From: Essex UK
Registered: Jun 2003

posted May 24, 2005 07:49 AM

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2SC609 & 2SA33. The 600 looking at the manual does in fact have 3 transistors. Is that another one at the RHS of the 2 obvious ones.
Anyway the circuit shows that one pot does adjust the 24fPS and the other the 18fps. One does nothing at all I noted from your earlier post. I will have a further study tonight. The circuit is very simple indeed using a tacho in the motor to control the speed via some feedback circuitry. Its the strange thing about one pot not working which puzzles me. I wonder if you simply have a faulty switch.
I will get back to you.

Kev.

--------------------
GS1200 Xenon with Elmo 1.0...great combo along with a 16-CL Xenon for that super bright white light.

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Rick Skowronek
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From: Marietta Georgia USA
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posted May 24, 2005 11:01 AM

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Hi Kev,

That third device next to the two transistors may be a Silicon Control rectifier (SCR). They are typically used to vary the duty cycle of a DC motor for speed control without losing any drive torque. It would look like the attached picture on the schematic.

Let me know if I can help.

Rick

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Dan Lail
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From: Loganville, Georgia, USA
Registered: Jun 2003

posted May 24, 2005 02:13 PM

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Kev & Rick, I really appreciate the help. Here is more information. The pot on the left has no effect in speed variation in 18fps mode or 24fps mode. The pot on the right does affect the speed variation in 18 or 24fps. The speed selector switch works. When I move it from 18 to 24fps the projector speeds up or slows down depending on which speed you select.

Below are more pics. One pic is a diagram of what is printed on top of the black item behind the two transistors.
The othe photos are close ups of the transistors and another view of the board. [Smile]

[Smile]

web page
web page
web page
web page

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Rick Skowronek
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From: Marietta Georgia USA
Registered: May 2005

posted May 24, 2005 03:37 PM

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That's really interesting. That black device is another bridge rectifier. Converts AC to DC and nothing more. Kevin will have to look at the schematic but it definitely is a bridge rectifier.

Note the symbols both from your picture and your diagram - ~ is an electronic symbol for AC or alternating current and the + and - are the DC output of the bridge. I'd be really surprised if they're driving the DC motor off of one those small transistors but it sure appears like that is the motor power rectifier and a pretty beefy one at that.

Pretty hard to tell without the schematic so I defer to Kevin with the program in hand.

I'll be glad to help if he has a way to transmit it electronically. Otherwise, I'm just shooting in the dark.

Rick

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Kevin Faulkner
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From: Essex UK
Registered: Jun 2003

posted May 24, 2005 03:45 PM

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Rick, I think this looks very similar to the 600 but I think there should be a third transistor. It lloks from the diagram that it will be a power transistor and it is probably mounted on the frame of the machine connected by 3 wires from that board eiter directly or via a plug and socket.

Kev.

--------------------
GS1200 Xenon with Elmo 1.0...great combo along with a 16-CL Xenon for that super bright white light.

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Rick Skowronek
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From: Marietta Georgia USA
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posted May 24, 2005 03:51 PM

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Kev,

I agree. It may be a power transistor mounted to the frame (for heat sinking) elsewhere in the machine. See my above post. The item I was looking at from the original picture turns out to be a beefy bridge rectifier. Most probably is the actual motor drive rectifier since it's located so close to the two transistors.

As I said, if you have a way to send the schematic, I'll sure be glad to throw my two cents in. Otherwise, I couldn't tell you.

On a positive note, I just bought an ST-600 in supposedly great shape (we'll see) so I would like to get those schematics just for my own use as well. I'd be happy to reimburse copying/post if no other way.

Meant to say something to Dan. Great pictures Amigo. Certainly does help!!!!

Rick

PS. Ain't it wonderful trying to troubleshoot electronics via remote control. Sorta like diagnosing a tumor over the telephone. Best guess is all you get. Thanks for being there.

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