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Topic: ST-800 Speed Problem

Kevin Faulkner
Film God

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From: Essex UK
Registered: Jun 2003

posted May 24, 2005 04:03 PM

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Dan, in the circuit diagram of the ST600 the transistor that could be giving trouble is the one I think you called an A/33. On the ST800 board the control transistor is marked as a 2SA733. This forms part of the feedback control circuit from the motor govenor and also the variable resistors which control the speed. BTW when you change the switch to 24 fps it shorts the 24 fps pot accross the 18fps one. Its strange this is not working but with the circuit upset anything could happen.
I would put my money on this transistor first.
If you have a meter it would be worth checking the continuity through the 2 zener diodes. High resistance one way and low the other. The other transistor to give trouble is probably the one mounted off that board somewhere. If it has gone leaky then you will not get much control from the control circuit. My diagram shows it to be a 2SD525.
To be quite honest with you I would just check all those components if you have a meter to hand as there really is not much associated with this circuit.
I will take the manual to work tomorrow and get it scanned so you and Rick can have a look at the circuit. Its pretty typical stuff really and what you would expect.
On the 600 there is a capacitor from that control transistor to earth and effectively accross the pots. If it were to go leaky then again you would get bad control or no control from the pots. In your picture I suspect it will be the green/blue one laying on its side with 473 marked on it. It is infact a 0.047uf/50V poyester or similar cap. To double check you could unsolder one of its legs and see what effect it has on the problem if any.

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 04:18 PM

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That makes sense on the third transistor. Cross reference lists it as
2SD525
Type Max V Max Amp Power W Gain
n-p-n 100 5.000 40.00 40

That sounds like a wild circuit and I think your diagnosis is right on.

And thanks for getting the schematics to me.

Rick

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Dan Lail
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From: Loganville, Georgia, USA
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posted May 24, 2005 04:21 PM

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WOW! What great forum. As far as reading the meter at the zener diodes, I haven't a clue what that means. Should I just replace the transistor A/33 and maybe the transistor located off that board? [Confused]

[Confused]

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Rick Skowronek
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From: Marietta Georgia USA
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posted May 24, 2005 04:26 PM

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Dan,

Since we're close (you're in GA and I'm in S. Fla.) I'll make the same offer I did to Mike Clark. That is if you'll pick up shipping in both directions, I'll do my best to troubleshoot and fix it. You would also be responsible for any parts that are necessary with your approval. I have the necessary testers and expertise to check this out but, of course, will need the schematic from Kevin.

Rick

PS. While there's not a great many parts to replace you could still further hurt the machine by just willy-nilly replacing things without knowing exactly where the problem is coming from. Just a suggestion.

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Kevin Faulkner
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From: Essex UK
Registered: Jun 2003

posted May 24, 2005 04:37 PM

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I agree with Rock on this one, if you dont know what your doing leave well alone. You could end up with further probs on your hands.
Rick, I doubt if those transistors are available but I'm sure you will find substiutes. It will also be worth traceing the wires from the motor to check the govenor windings are not open circuit. All will be clear from the circuit.
I would suggest that we make contact via direct email so that I can send the scan direct to yourself and Dan. Message me privately.

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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posted May 24, 2005 04:48 PM

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Thanks Kev,

I was just checking the specs on the 2SA733. It's pretty much a vanilla PNP silicon transistor. Relatively high gain at hFE of 200 avg. and a low signal type with a max dissipation of 250 mw and max. voltages in the 50 volt range. Is typically used as an audio amplifier (in the early stages) because of it's excellent linearity.

Shouldn't be any problem finding a substitute though several companies still say it's available. Same with the rest. I'm sure this isn't requiring exact specs in this type of circuit. Primarily close gain and current/voltage matches.

Rick

PS. Kev, you have my email from previous contact but, if not, I'll be happy to email you again so you have it.

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Dan Lail
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From: Loganville, Georgia, USA
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posted May 24, 2005 04:59 PM

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Rick, you've got mail! [Cool]

[Cool]

Thanks, Dan [Smile]

[Smile]

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Rick Skowronek
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From: Marietta Georgia USA
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posted May 24, 2005 05:54 PM

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Kev & Dan,

I think I just ferreted out why the second pot doesn't do anything when going from 24 fps to 18 fps. Obviously you, Dan had the pot adjustment all the way down to try and slow it in 24 fps. All the way down means the pot is set at 0 ohms resistance. If, as you say Kevin, the 18 fps setting just puts the second pot in parallel with the 24 fps pot then it wouldn't do anything since you can't go lower than 0 ohms, right. Hence no further impact.

God, sometimes I just amaze myself (or not) [Razz]

[Razz]

.

Rick

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Kevin Faulkner
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From: Essex UK
Registered: Jun 2003

posted May 25, 2005 05:33 AM

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Think you may be right. I have just had a thought. There may not be a problem here.
Dan put the machine in the 18 fps setting and now adjust whichever pot will effect the speed. Only one pot will change the speed. If that works change the machine to 24fps and adjust the other pot.
If that procedure doesnt work there is definitely a problem with the circuit or motor govenor.

Rick, The 18 fps has a resisitor between the pot and earth so you wont present a 0 ohm situation to the circuit. The 24 fps pot is then switched between the wiper of the 18fps pot and earth so its important to set up the 18fps speed first. I'm just wondering if someone has mal adjusted the 18fps speed which has meant that there is insufficient adjustment on the 24 fps pot.

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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posted May 25, 2005 08:37 AM

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Kev,

Sure sounds like that's worth a try before all the back and forth. Certainly feasible with the weird way they're hooking these up. Kinda figured they wouldn't let that pot take the circuit to pure earth but if it's set really low it'll not let the switched in pot do much of anything, and as you said, all speeds would be mucked up.

Typically these circuits are designed so that the adjustment pots are somewhere near the the mid-range of their travel. A starting point may be to put both pots in their mid-point (about the 12 oclock position) and then follow Kevin's guidelines,

Dan, it's in your court to try that out.

Rick

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Rick Skowronek
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From: Marietta Georgia USA
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posted May 25, 2005 06:51 PM

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Kev,

Got your schematic and this is by far and away one of the weirder but, as you said, simple circuits. It is basically nothing more than an early comparator circuit matching actual voltage (from the tach generator) to a preset voltage set by the speed set pots. I am in the process of calculating voltages based on the voltage divider networks comprised of R205, R207 amd VR 201 (the 18 fps pot) and VR202 when in the 24 fps mode. It appears that the regulated voltage across this divider network is set by Zener diode ZD201 at 6.2 volts. Not absolute but the part # points that way. The tach generator places an opposite voltage on the base of the first drive transistor based on the direction it's going. Appears that it allows the motor to go much higher speed in rewind or reverse speed (but not sure). Don't know what that little center tap device on the tach winfing does but both ends of the tach go through + diodes to the same place.

Anyway, will feed more as I calculate my way through the circuit. It's very clear it's a very high gain circuit. Note the fact that the drives from the first two transistors feed only the bases of their respective next stage. No emitter or collector resistors on either of the two first stages. Only base current provides the neccessary bias on the previous stage. This usually translates into maximum gain in what is commonly called a Darlington pair.

Anyway, don't want to get into electronic design in an 8mm forum.

Let ya know.

Rick

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Dan Lail
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posted May 25, 2005 08:30 PM

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Hi, guys, I just read the new trouble shooting threads. I did try to adjust the left pot in 24 fps but nothing happened. I'll give the twelve o'clock positions a try and report back. [Cool]

[Cool]

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Kevin Faulkner
Film God

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From: Essex UK
Registered: Jun 2003

posted May 26, 2005 03:15 AM

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Dan, you must try to set the machine up at 18fps first. Thats very important. Once you have done that with whichever pot changes the speed set the machine to 24fps and try to adjust that with the other pot.
Rick, As you can see there are a number of possiblities on that circuit but is should be very simple to find. An equiv for the 2SA733 would be a BC212L. I think though that it could be that capacitor gone leaky accross the input to that transistor in question if there is a problem at all.

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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posted May 26, 2005 07:08 AM

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That'd be the first check for sure. If it had gone leaky it sure would screw up all the tach/set point voltages and the tach would not be able to be sensed at least at it's full voltage, telling the circuit to "speed up" trying to compensate.

Let us know, Dan how that works out, or not.
Thanks,
Rick

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Dan Lail
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posted May 26, 2005 08:36 AM

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Rick & Kev, I had no luck getting anywhere close to 18fps or 24 fps. The machine runs too fast no mateer how I adjust the right pot. The left pot still doesn't seem do vary the speed at all [Frown]

[Frown]

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Rick Skowronek
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posted May 26, 2005 11:15 AM

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I kinda was a bit suspicious it might not work as it would have responded better even totally mis-adjusted. I guess it's on to plan B. Let me know when you want to box it up and ship it. I'll put the test equipment on it to see where the hole is. Let's just keep our fingers crossed that the tach generator on the motor [Eek!]

[Eek!]

is good. It's a strange configuration and not sure how it compares to other Elmo similar models.

I should have my St600 soon and was assured it runs perfectly so I should be able to do a one on one comparison with yours.

Rick

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Rick Skowronek
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posted June 02, 2005 04:52 PM

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Thought I'd take a moment for anyone that followed/contributed to this thread to update with a final resolution.

I received the projector yesterday from Dan and troubleshot the problem. Ended up being a shorted output transistor. That's the transistor that directly drives the motor under control of the feedback circuit. With it shorted it was placing full supply voltage on the motor no matter what the control circuit said.

Interesting failure and not a usual one from what Kev says.

Anyway, just wanted to add to the storehouse of knowledge on the forum.

Rick

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Dan Lail
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From: Loganville, Georgia, USA
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posted June 02, 2005 07:42 PM

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Thanks, Rick, can't wait to experience the ST-800 in my living room. I'm so used to watching films downstairs in the screening room, it will nice to relax on the sofa and get out the 5'x5' tripod screen. [Cool]

[Cool]

Really enjoyed talking about tube amps on the phone. Let me know if you find that illusive MacIntosh Hi-Fi. [Eek!]

[Eek!]

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Dan Lail
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From: Loganville, Georgia, USA
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posted June 13, 2005 12:01 AM

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Just wanted to post a public Thanks You to Rick Skowronek for doing a super repair job on my Elmo ST-800. Also to Kev Mr Elmo Faulkner for providing the schematic. What a smooth machine! What a great couple of guys! [Smile]

[Smile]

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Kevin Faulkner
Film God

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From: Essex UK
Registered: Jun 2003

posted June 13, 2005 06:44 AM

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No worries Dan and thanks. This is what this forum is all about, helping others and I'm glad that Rick and I were able to help get your machine sorted.

Kev. [Smile]

[Smile]

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

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