8mm Forum: ELMO ST1200 No mag sound but Optical sound is fine? HELP!

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Posted by Mark L Barton (Member # 1512) on April 24, 2019, 09:53 AM:

I recently bought, separately, two elmo st1200 MO projectors. And they both have the same fault, optical sound plays the audio perfectly, strong and loud with little distortion, even from the built in speaker. BUT, magnetic sound playback using both machine is none existent, absolutely no audio heard at all. In mag playback mode, one offers a high pitched squealing sound, the other is just rumbling audio, and no audio.
Any thoughts please? Its obvious the amplifier is fine, but what of the mag heads, could they be a problem? If yes how do you fix?
Cheers

Posted by Steve Klare (Member # 12) on April 24, 2019, 10:58 AM:

I had an ST1200HD go this way once. Track one was fine, so the large amplifier IC was good, track two developed this really irritating whistling tone.

I don't have any audio useful test instrumentation at the house, so I had to guess (Might be hard to explain working on a projector at work, too!).

There is this pre-amp IC that Elmo is very fond of (Toshiba 7139P) that they use a lot. Both track one and two have one of these (and if it was an optical I bet that would have one too). There is another one for the PA input and a last one that blends tracks one and two (or the PA) for the monaural output.

My guess was the pre-amp IC for track 2 had gone bad. I pulled the board out and replaced all four. They are obsolete but common on Ebay.

Was I right? Turned out I was. (-pleasantly surprised, too!)

When was I sure? When I put it all back together and tried it for real. (It was a tense moment!)

Of course before you go this way, it's always good to try out the switch trick. It's no money and little effort: a great investment!

The Switch Trick

Posted by Mark L Barton (Member # 1512) on April 24, 2019, 05:22 PM:

Re the switch trick. Steve, you are a steely eyed projector man. It worked immediately on one of the st1200, booming clear sound was heard from a projector that l thought would never play again from a mag stripe. The second one has the whine and no mag audio, l cleaned the sockets but noticed that when l pushed in the red record button the whine went, with a Mic plugged in my voice wad heard thru the speaker. So, lm guessing the red rec button is at issue here. I have used servisol contact switch cleani g spray but Zippo yet. Any thoughts my gifted friend? Oh and cheers for all your advise.

Posted by Steve Klare (Member # 12) on April 24, 2019, 08:56 PM:

quote:
-a steely eyed projector man.

I LIKE that! Where can I get a T-shirt made? (It's SO Apollo 13!)

-although Leon Norris probably has much steelier eyes than me! Where projector repair goes I'm basically a hobbyist who happens to be an Engineer.

Regrettably, your Whiner sounds more like the one I had which only got better when I replaced the pre-amp ICs.

I think when you depress that switch you disconnect the amp from the head. Maybe the whine really is still there, but you aren't hearing it any more.

Posted by Mark L Barton (Member # 1512) on April 25, 2019, 04:16 AM:

Thanks Steve, yep guess I'll look at the ICs.
Cheers from Bristol, England.

Posted by Steve Klare (Member # 12) on April 25, 2019, 08:27 AM:

Greetings from Long Island New York, Mark!

I did some research and I found the service manual for the non-HD ST-1200.

St-1200 Service Manual

On page 35 it shows the schematic for the sound board. It looks like the Pre-Amp associated with magnetic sound is Sanyo LD3120.

It is of course obsolete, but then again it's still findable.

Step one is to take a look and make sure it's on the board in the first place. Your machine and the service manual aren't necessarily of the same revision.

Posted by Leon Norris (Member # 3151) on April 25, 2019, 08:34 AM:

I think it the amp IC ! STK 015! I fixed many and turn out to be that IC. For the magnetic side! That's most likely the original one that's still in there. Over 40 years old! Good luck!

Posted by Steve Klare (Member # 12) on April 25, 2019, 08:37 AM:

The only thing is since Mark is getting optical sound, I think the STK-015 main amp is OK.

I'd be more likely to suspect that it's dead if mag/opt were both dead.

It's kind of like "If the kitchen sink won't flow, then the kitchen drain is clogged"

-or-

"If the bathroom sink won't flow, then the bathroom drain is clogged"

-but-

"If neither sink flows, then the line out to the septic tank is clogged"

(Watch OUT!) [Eek!]

(I HATE plumbing!)

Posted by Maurice Leakey (Member # 916) on April 25, 2019, 09:37 AM:

Many years ago I bought an Elmo ST-1200, the previous model to the HD (with an odd round mains input plug). The optical sound was very loud (if necessary) but the magnetic sound was hardly audible.
My engineer said the magnetic head had "gone soft", so he fitted a replacement.
That's some 20 years ago and all is still working as one would expect.

Posted by Leon Norris (Member # 3151) on April 26, 2019, 08:53 AM:

Remember, the AMP can still be bad! Even though the optical side is good! That main ic controls both! Good luck!

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