8mm Forum: The Elmo Hum!

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Posted by Leon Norris (Member # 3151) on February 27, 2019, 10:28 AM:

I get customers telling me adout a hum! Well the first you check is the cancel coil! They are usely buy the sound!HeadYellow or black in color! To a just take a pair of needle nose pliers and turn it! Be gentle! On two track machines you have two cancel coil! I use headphones when adjusting.Its Ether bad or needs adjustment! Turn up the volume all way up! Use headphones to adjust! They do go bad!

Posted by Steve Klare (Member # 12) on February 27, 2019, 12:11 PM:

We don't often talk here about Elmo Hum...usually Eumig!

What I find is a 60 Hz. notch filter in the line out to the Amp does a really good job taming the hum from my machines. It doesn't exactly kill it dead, it's just in order to really hear it you need to crank the volume up to levels you'd never use if you have neighbors on the same block!

Leon, are you talking about Hum-Buck coils on the GS? I don't think I've ever seen any on the STs I have.

Posted by Leon Norris (Member # 3151) on February 27, 2019, 01:01 PM:

Steve, All Elmo's have cancel coils on them! They are always buy the sound head! You can't miss it! Mono have one. Stereo or two track have two. But be care full when adjusting. Because they can break! Go slow!

Posted by Steve Klare (Member # 12) on February 27, 2019, 08:38 PM:

You are correct!

They are tucked in very tight behind the sound head and hidden by the loop former lever.

(The Eumig 800 series coils are out flapping in the breeze for all to see!)

Posted by Leon Norris (Member # 3151) on February 28, 2019, 09:36 AM:

Also what can cause hum. Is a bad micro switch! Usely in the record section. The Elmo's use them in there machines! That's from lack of cleaning! Keep those micro switches clean! If you don't they will act up!

Posted by Paul Adsett (Member # 25) on March 01, 2019, 09:18 AM:

I think it's been said many times before on this forum, but worth repeating - the recording switches in the GS1200 should be periodically activated to prevent oxidation of the contacts. Same thing goes for the MAG/OPT switch and MONO/STEREO switch and all those little switches on the rear panel.

Posted by Jason Moffatt (Member # 4416) on March 01, 2019, 09:43 AM:

My ST180 has quite a bit of hum but i’m not seeing any coils near the soundhead, are they accesable from the front or back of the projector?.Also if i do find them can they be adjusted while it is on so you can hear it improve while being adjusted or does it have to be off ?.

Posted by Jason Moffatt (Member # 4416) on March 01, 2019, 09:55 AM:

My ST180 has quite a bit of hum but i’m not seeing any coils near the soundhead, are they accesable from the front or back of the projector?.Also if i do find them can they be adjusted while it is on so you can hear it improve while being adjusted or does it have to be off ?.

Posted by Steve Klare (Member # 12) on March 01, 2019, 10:00 AM:

I found mine beneath the head. When I pushed aside the lever for the loop restorer I saw them there: tiny yellow spirals, maybe too small for late-middle aged eyes! (This was one ST-800M and one ST-1200HD: it's what I'm running right now.)

I have literally stripped a couple of these machines down to the frame and never noticed these there!

This adjustment is done powered up, so you can listen to the hum while you "fine-tune" the hum to a minimum.

You can think of these kind of like "antenna": they are out in the same stray magnetic field as the head itself. The idea is if they produce the same hum voltage, they can cancel the hum picked up by the head because they are connected "backwards" polarity wise. (Electric guitars use this trick too.)

They can only do so much. There will be hum because of the stray magnetic fields around them, and they were designed to address this. There may also be hum because the filter caps are 40+ years old and have dried up and the voltage ripple on the supply rails has risen. They can't help with this.

We need to be a little careful here too. This is 1970s electronics and the hum performance never was as good as the modern stuff we are used to. Sometimes it's not good to listen with modern ears.

Posted by Leon Norris (Member # 3151) on March 01, 2019, 10:31 AM:

Usely on the ST180 models you have to take off the plastic guide cover. Once off you should see it! Its two screws to remove! That guide is next to sound head!

Posted by Bill Phelps (Member # 1431) on March 09, 2019, 11:19 AM:

I was talking with Steve O about this and he wanted to add that you should make sure you keep the power cord away from your speaker wires because that can cause hum as well. He said he sells hum coils that go between your projector output and amp that can help also. [Smile]

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