Electronic repairs on Elmo st-800

[Jon Byler]

Because other projectors I have were showing signs of trouble with their electrolytic capacitors, I also took a look inside my st-800. Attached is why this needs addressing sooner than later for electronics this old. Not sure when this was made, but it's got to be close 40 years ago.

I've only had a couple of items that showed this failure mode. Glad that I caught this before it got worse!

[Jon Byler]

Most of this is pretty easy stuff. But every so often there is a strange part. This has 9 leads, but looks otherwise like an electrolytic capacitor. Anyone know what this is? I don't have a service manual, just for the ST1200. that manual shows a couple of IC's that are 9 pin, but I can't tell that they are round like this.

[Phil Murat]

Hello Jon,

From what I can see on first picture above, there is not too much damage on copper track (to be confirm of course...).
I suggest you to thoroughly wip off "green varnish" localy and untill copper appears (using Isopropyp Alcool, scotch brite and nitryl glothes).
Then, coat copper track with a good electronic tin solder to protect and increase track conductivity.
Second Picture : Unusual component shape . Try to report all marking information(s) and symbols marking (As +; -, AC symbol, values, etc...) which look to be hidden

[Jenny List]

Most of this is pretty easy stuff. But every so often there is a strange part. This has 9 leads, but looks otherwise like an electrolytic capacitor. Anyone know what this is? I don't have a service manual, just for the ST1200. that manual shows a couple of IC's that are 9 pin, but I can't tell that they are round like this.

I would give good odds on that being an inductor or transformer of some sort. It reminds me of the bias oscillator coil in some tape recorders, does this projector have a recording feature by any chance?

Post a picture of the top of it, I'm guessing it may have a ferrite core adjustment slot.

[Jenny List]

This page has a picture of one from a 1980s Sony video recorder. https://www.palsite.com/950tech.html

This is the picture from the Sony VCR. https://www.palsite.com/pics/bias_erase_transformer.jpg

[Jon Byler]

I would give good odds on that being an inductor or transformer of some sort. It reminds me of the bias oscillator coil in some tape recorders, does this projector have a recording feature by any chance?

Post a picture of the top of it, I'm guessing it may have a ferrite core adjustment slot.

No slot. this machine does have a record feature.

[Jon Byler]

This page has a picture of one from a 1980s Sony video recorder. https://www.palsite.com/950tech.html

This is the picture from the Sony VCR. https://www.palsite.com/pics/bias_erase_transformer.jpg

Thanks! that does look very similar.

[Jon Byler]

well, I finally got this done, I thought! But the main fuse goes poof when I try to turn it on I need to dig out whatever schematic I can find and try to trace the problem. I took pretty careful notes, and replaced one part at a time... tested every part I installed before installing. Hopefully I can sort this out, or perhaps I can find another set of boards. The machine worked pretty well until the power supply capacitors died. I don't have many sound films, so it's not a huge deal for me not to have this working, but I'd love to make it work again, just because it has been a good projector when it worked, and so much better build quality than others I've seen.

If anyone has a dead projector and the two circuit boards are still good, I'd be interested to purchase or trade something for them.

[Steve Klare]

Hi Jon,

I wish you the best of luck finding ST-800 schematics, as a matter of fact much better than MY luck finding them! I've been working on that particular Holy Grail for about 20 years now. I wrote to ELMO on the odd chance they microfilmed the ST-800 service manual years ago, but they told me "no".

Just maybe I have an idea for you. There is always some kind of rectifier between the transformer secondary and the filter capacitor(s) -there has to be something. When the caps short, the excessive current through the rectifier can cause it to overheat and after that, it shorts too.

This can be a single 4 pin IC, or either two or four two terminal diodes. If you follow the PC board traces between the capacitor(s) and the connection to the transformer secondary, you'll find it (or them). A little quick work with an ohmmeter should tell the tale.

You may not be able to find the exact part this many years later, but the odds are there is a modern equivalent.

[Jon Byler]

Hi Steve,

I will start trouble shooting this at some point. The power board is not super complicated. I may even try to draw up a schematic for posterity. It certainly helps to understand a circuit when one has done that. Fortunately I have other machines, and not many sound films, though I'd really love to watch the sound films I have. Some time ago, I ended up with a small pile of new old stock shrink-wrapped heckle and jeckle cartoons that have sound, and they are possibly in german, which might be amusing to hear.

[Jon Byler]

Looking at the picture I posted of the circuit board, it looks like one of the power supply capacitors may be shorted but the board where things corroded, and were subsequently repaired. I'll at least have a good starting point for tracking down the problem.

[Steve Klare]

Hold On!

-It looks like Van Eck can help us out!

The Holy Grail at Last!

(I do believe I want this .pdf.)