8mm Forum: Elmo ST-1200 problem

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Posted by Barry Fritz (Member # 1865) on April 01, 2018, 03:05 PM:

Intending to reduce my collection of projectors, it was time for my minty Elmo ST-1200 to go. I had not used it for a couple of years so I threaded it up for a test. I noted that the Auto-threading lever had to be held down, and that was not right. It is supposed to stay down on its own and the pop up upon completion of threading. The projector ran the film perfectly and sound was good. The only problem was the auto-threading issue. In an attempt to fix this, I opened the front and back of the projector to see if I could determine what the problem was. I noted there was a solenoid that controlled the lever. I also noted that one of the two fuses in the back was blown. Aha. I replaced the fuse and plugged in the power mains with the front and back still open. I switched the projector to forward and pressed down on the auto-threading lever. It worked! I stopped the projector and was in the process of moving it around so the front faced me when a heard a pop. I saw a small bit of smoke coming from behind the mains socket on the back. I quickly switched off my power strip that the projector was plugged into. I started to unplug the mains cable from the projector when I heard a very loud pop and saw a bunch of smoke coming from behind the socket again. Looking closer, here is what I saw.

I do not know what caused this. It is an electrolytic capacitor that blew. When I was moving the projector, the mains cable may have had a bit of side pressure pulling on it and maybe behind the socket a wire moved slightly and touched another one… I have no clue. I’m done messing with it so I guess I will be selling it for parts. What a shame. It is mint, as is the case.

Posted by Winbert Hutahaean (Member # 58) on April 01, 2018, 03:40 PM:

Barry, capacitor may blow after a long period because of the humidity, etc.

I experienced the same with my Sankyo 700 and my repairman simply just changed the capacitor with a new one. The capacitor is easy to find in the market.

Posted by Barry Fritz (Member # 1865) on April 01, 2018, 04:50 PM:

Winbert. This is the first time I have had one blow. I've replaced many on old 16mm amps and never had that issue. My projectors are kept in a heated and air conditioned enviornment that is usually 35 to 50 % humidity so I don't think that is what caused it. You are correct, the caps are readily available and this amp is only held in with 3 screws, but I am not interested in messing with it. I'll sell it for parts or repair and be done with it. The projector still works, but no sound.

Posted by Thomas Knappstein (Member # 6134) on April 03, 2018, 10:17 AM:

Replace this elektrolytic Capacitor and the Sound will be working again.

Posted by Steve Klare (Member # 12) on April 03, 2018, 10:47 AM:

It's not a hard repair to do if you are good with a soldering iron. You'd need some copper braid to wick the old solder out of the mounting holes and one of those soldapullt tools would help too. (This is assuming it's board mounted, it's even easier if it's not.)

Of course not everybody can do this, but if you have a friend that can it's not a big favor to ask.

The big catch is getting the new part in mounted correct polarity. It'll blow up if you don't.

While you're at it, see if it has any neighbors that are the same part and replace those too. Very often the same part doing the same job in the same place will fail pretty close to the same time. (That's why they sell taillight bulbs two in a package...)

You should also check if these is a fuse for the sound circuit. This may have taken that out too.

Posted by Barry Fritz (Member # 1865) on April 03, 2018, 02:16 PM:

I've replaced many caps, but none on a circuit board. I don't want to mess with it. I have it listed for parts or repair on Ebay. It could also be usd to run silent film with no problem. There are three fuses..none of them blew.

Posted by Rob Young. (Member # 131) on April 03, 2018, 04:48 PM:

This is one of those shames as I don't get what has gone wrong and can't begin to decipher what the problem is or how to fix it?

Steve, what's up with this and how do you fix it?

Posted by Steve Klare (Member # 12) on April 03, 2018, 08:02 PM:

Basically an electrolytic capacitor is a sealed aluminum can of liquid. When they go bad they often start to leak current and heat up. When they get so hot the liquid boils the pressure builds up until you have a little bomb. (You can speed up this process radically by mounting one backwards.)

-as old as these caps are, I'm surprised we don't see a lot more like this. An old employer of mine considered an electrolytic bad after five years even if the thing still worked.

"No sound" kind of bothers me here. This is probably a smoothing cap for the sound circuit's power supply. If there was no sound and the fuse was blown, that wouldn't be so bad. The fact that the fuse is intact could mean something else has gone open circuit in its place. A bad cap all on its own would probably give you "bad sound": a lot of hum.

Posted by Rob Young. (Member # 131) on April 04, 2018, 05:40 AM:

Oh I see, thanks Steve, I wasn't clear from Barry's pictures what it was.

I think sometimes it's amazing that our 40 year old projectors are still working at all!

It is worrying that Barry is getting no sound at all, but hopefully he can fix things.

I must admit, I do kinda cross my fingers & toes when I power up the Beaulieu these days...

Posted by Barry Fritz (Member # 1865) on April 05, 2018, 10:52 AM:

There seems to be a fair amount of interest in it on Ebay. It has 5 watchers. I will be happy to sell it to someone that will fix it and get good use out of it or just use it to scrren silent family films. I am in the process of reducing my projector collection and have a good deal of other projectors on hand. After that cap blew, I was reluctant to use the amp. Maybe I will thread a sound film and see if there is any sound at all. With previous bad electrolytic caps, all I heard was a loud hum.
UPDATE: Welll, I'll be darned. I just ran a sound film and the sound seems just fine! Should have done that to begin with. There isn't even a hum. Still, something obviously is not right with it.

Posted by Steve Klare (Member # 12) on April 05, 2018, 11:16 AM:

OK, that's good!

What that probably means is there is a second (or third, or...) cap connected in parallel with the one that blew and they are doing the job, at least well enough.

Whoever gets the machine should replace em' all.

Posted by Barry Fritz (Member # 1865) on April 05, 2018, 02:08 PM:

Sold. Thanks for the responses. I would not have tried the amp again if not for the insightful comments.

Posted by Steve Klare (Member # 12) on April 05, 2018, 02:16 PM:

Sure!

Might have been tempted myself if not for all the other projects I have!

I have a very virginal Elmo ST-800. Legend has it was put on hold brand new for a customer in a camera shop back in the 1970s (there is a name scribbled on the box...) and never picked up.

My project is to gradually bring this thing up in voltage and try not to blow up the entire neighborhood in the process! (OK..I exaggerated a little...)

My hope is these old caps will form up again and I'll have a projector that should be usable for decades.

-Or maybe a soldering project if they don't.

(You think YOU had capacitor trouble!)

I heard a story of a New Old Stock sound board being installed recently and the hum being awful, yet gradually improving as the caps became capacitive again. I'm hoping for similar luck.

Posted by Rob Young. (Member # 131) on April 05, 2018, 02:50 PM:

Hi Steve, what does "form up again" mean?

Posted by Steve Klare (Member # 12) on April 05, 2018, 03:02 PM:

Well, as I said, an Electrolytic capacitor is basically a can of fluid. On one terminal you have a metal plate. When a voltage is applied, a very thin insulating layer forms between it and the conductive electrolyte. This layer between the conductive fluid and the plate forms a very good capacitor. Most of the largest capacitors for power filtering and energy storage are of this type.

When the cap is allowed to sit unenergized, this layer gradually breaks down and the capacitance is slowly lost.

If you apply voltage backwards, the layer breaks down, the thing conducts like crazy, overheats and "BOOM"! (Actually smells like a fishing boat before the Mate hoses down the deck...)

My hope is by starting out low I can tickle that layer into forming again, or maybe do no worse than popping the fuse.

Posted by Barry Fritz (Member # 1865) on April 05, 2018, 03:09 PM:

There are articles on reforming caps but I never tried it. I know the old paper and wax caps went bad but I did not think the newer ones sealed in aluminum did. Guess I was wrong.

Posted by Steve Klare (Member # 12) on April 05, 2018, 03:27 PM:

Nothing is forever! (except maybe vacuum tubes!)

Sometimes the electrolyte just dries out too and there's no coming back.

We shall see.

Thing is I'm going to have to bring this to work and do it after hours. It'll be fine, but I still don't want to start some kind of...incident.

(I kind of NEED this job!)

Posted by Barry Fritz (Member # 1865) on April 05, 2018, 03:54 PM:

This cap was clearly not dry as there was some oil obvious.

Posted by Rob Young. (Member # 131) on April 06, 2018, 03:37 AM:

Interesting! So this approach seems a sensible one for any projector that has been out of use for some time.

Steve, how do you physically start with a lower voltage and raise it to the projector? Just use a transformer?

Posted by Steve Klare (Member # 12) on April 06, 2018, 09:49 AM:

I'd use a variac: a variable transformer. What's nice about it is there is a slider to adjust the output voltage and it goes all the way down to zero.

Very typically I'll just crack it open for a couple of volts to start and see what happens. If there's a short at that low voltage level all that will happen is the fuse will blow without frying anything else. There is a lag before a fuse blows, so if you hit it full blast and there's a short you may do much more than just blow the fuse.

-after that I slowly bring it up and pay close attention. I'd probably linger a while at some low voltages and give those electrolytics a chance to form up again.

What's also nice is even at a low voltage, a variac operating into a short will start to hum ominously as you bring it up: kind of an early warning you're getting into trouble and it's time to back off. (This has saved me a lot of grief over the years...)

Posted by Barry Fritz (Member # 1865) on April 06, 2018, 09:43 PM:

With older projectors with amps, using a variac is a good practice for the first time you want to fire it up. Short of that, a dim bulb tester is a good practice. Google has a lot of articles on reforming caps and also about making your own dim bulb tester.

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