2 pole motor hum? Replaced ceramic cart with MM Shure SC35C

monkboughtlunch

Super Member
I installed a Shure SC35C in an 1961 Zenith idler/belt hybrid drive turntable that was originally equipped with a Euphonics U2 ceramic cartridge. This version of the Zenith turntable predates the MicroTouch 2G tonearm. Instead it has Cobra arm apparently designed to track at about 3.5 - 7 grams.

The Euphonics U2 was reportedly good in its day, but even when I installed a NOS replacement stylus, the cart still performed poorly. I think it's due to rotted rubber yokes.

So I installed a moving magnet Shure SC35C, which tracks at 4-5 grams. This cart sounds amazing! But there is a problem.

While the sound quality, tracking, sibilance and IGD performance is far superior with the Shure MM cart, there is a constant motor hum being picked up by the SC35C that wasn't present with the ceramic Euphonics cart.

If I lift the tonearm as high as it will go, the hum decreases dramatically. When I bring the tonearm back down to the rest position, the hum increases dramatically.

And the hum is only present when the motor is turned on.

I'm thinking it might be the Alliance motor which appears to be unshielded. I suppose this is a two pole motor. But it also has two permanent magnets, making it a synchronous induction hybrid motor.

The underside of the Cobra tonearm is open (i.e. not in an enclosed arm tube) so the wires are exposed underneath. I wonder if that is part of the problem, as it was mentioned the cart itself may be well shielded.

Any ideas on how to eliminate the hum without pulling out the Shure SC35C and installing a low quality ceramic cartridge?
 
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Could very well be induced hum. Is the cart body tied to one of the grounded leads? If not, do that and see if it helps any. I don't remember if my SC35C has a metal body with a ground strap or not honestly.
 
Could very well be induced hum. Is the cart body tied to one of the grounded leads? If not, do that and see if it helps any. I don't remember if my SC35C has a metal body with a ground strap or not honestly.

It does - Its on the RG (-) tab, green on bottom.
 
On the original ceramic cartridge, Zenith used a wire to bridge the left and right ground pins of the cart. I used that same wire to bridge the left and right ground pins of the SC35C.
 
hum is induced as a form of EMI and I'm not convinced this is hum - although it's possible. It might as well be motor vibrations being picked up by the more compliant Shure cart. My suggestion is to try to compress the motor on it's mounts or suspension (I'm not sure what's in there) while auditing the change in interference amplitude. If it's for the better, perhaps seek a way to improve the motor isolation. If it's real hum, we're back in square one - no harm done.

The motor has brand new motor mounts.
 
Here's a photo of the arm taken before installing the SC35C. There are three twisted tonearm wires with plastic insulation. The two cartridge ground pins have a jumper and thus there is only a single tonearm wire (black) for the two ground pins. In the rear of the tonearm, there is a copper braided wire visible. I assume that is used to ground the tonearm.

IMG_9324.jpg
 
re seat the connections to rule that out .. another thought is the output higher with the problem one ?
 
No pete, ceramic carts like the original output at least 300mV, some nearly a full one volt. The current Shure cart in use outputs 5mV.
 
No pete, ceramic carts like the original output at least 300mV, some nearly a full one volt. The current Shure cart in use outputs 5mV.
rules that out then . unless i am missing something here ? . like maybe which has more detail ?
 
I don't suppose you're missing anything. The way I see it, if the interference isn't mechanical, and isn't induced by line freq. then the background noise is constant while the signal is at least 60 times weaker. This calls for verifying it isn't a grounding issue, or getting the noise level considerably down with shielding - or both.
 
yes, I can't really see the braid on account of a very low res photo...

Here are some close ups (braided wire in rear of arm and the cart pin wiring). These were taken before I installed the SC35c, but I wired the SC35C the same way as the original Euphonics ceramic (i.e. bridged the two ground pins) and used the existing three twisted tonearm wires.

wire.jpg

pins.jpg
 
On BSR record changers the ones with a ceramic cartridge came with a two-pole AC motor and the ones with a magnetic cartridge came with a four-pole motor, which was necessary because the two-pole motor caused too much hum in a magnetic pickup.

Unless you can retrofit a four-pole motor into the Zenith, you may have to stick to using a ceramic cartridge, such as the Pfanstiehl 226, which is TVOM's recommended substitute for the Euphonics U2:

http://www.thevoiceofmusic.com/catalog/part_detail.asp?PNumberBase=P226
 
I use Shure M44-7s and and an AT5625. For mono consoles I combine the channels at the cartridge. Nothing special with the wiring. These old units have a certain amount of hum to start with, I have a 1960 JVC stereo record player wired to play records or plug into a phone with a source switch, and the hum is only slightly louder on phono. I use $14 Pyle phono preamps with these.

Here's a vid of a 1950 Webster with a M44-7 and 78 stylus.

 
Go over to VOM's website. He has info on ceramic to magnetic cart issues on older changers: http://www.thevoiceofmusic.com/cartridges_help_substitution.html#MagCar .

I've also found antique radio forums to be helpful when dealing with these types of issues.

Thanks, the VOM website states:

"An example is shown in the pictures where a ground wire is installed that connects the left and right ground connections from the cartridge together, and then ties them to a metal portion of the record changer base or mechanism."

On my table, the left and right ground from the cart are tied together and tied to the metal base of the turntable chassis. Hum is still present.
 
Additional photos.

Below: SC35C installed in Zenith Cobra arm. Note that I bridged the two ground pins on the cart because the original Zenith tonearm wiring uses a single black wire for the combined ground -- and the original Euphonics U2 ceramic cart also bridged the ground pins.

The SC35C appears to have a plastic shell. Is there supposed to be a metal part of the cartridge that connects to the tonearm? The green ground pin of the SC35C has a metal tab (see photo). Is that supposed to connect to something? What is its function?


sc35c.jpg

Below: Under the chassis, Zenith used a 3 tab phenolic solder lug terminal strip. The tab that is for the ground is connected to the screw that mounts to the chassis. Thus, it appears Zenith grounded the cartridge ground wires to the chassis.

groundwire.jpg
 
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