Decca hum on SL-110

[Tolstoi]

My London Decca hums on my Technics SL-110. It get's worse when the arm moves nearer to the spindle. It's not audible when a record gets played but it is not really something you want in your set up.
Just swapped the cartridge to my SL-120 and noticed that there is absolutely no hum. So there are two decks which are quite identical, I'd say they have the same motor unit and the power supply has nearly the same distance to the motor unit, spindle and so on.
I'm really curious where this comes from and of course I want the SL-110 to be as silent as the other deck.

[Tolstoi]

Of course I'm not 100% certain, but that's what I found on the internet.

SL-1200 MKI = MJL-9A
SL-110 = MJL-9A or MJL-12A

posted from someone on a German forum. Two motor versions for the 110 and one for the 120.

I have to take a look which ones are built into my decks.

[Tolstoi]

Well, the SL-120 has the MJL-9A motor and the SL-110 the MJL-12A one. Electronics between those two decks are different.

[Barry]

As Andrew suggests, it is almost certainly hum pick up from the motor and/or electronics (I assume the Technics TTs in question are direct drive designs).

If direct drive, the motor is concentric with the spindle, so both models should affect the cartridge in the same way. Therefore the question arises, does the platter of one better shield the cartridge than the other? BTW which Decca cartridge are you using?

One other question: is the arm the same on either deck and are they wired in the same way (with respect to the earthing of the metalwork of the turntable)?

[Tolstoi]

The 120 has an old London Acoustics Tonearm which is similar to the Jelco SA-750D. It was rewired with cables from the headshell to the plugs. The 110 has a 12" Jelco 750L tonearm.

[hifi_dave]

Could well be the answer to your problem.

[Tolstoi]

Guess I have to swap tonearms and check it out.

[Tolstoi]

It's a Decca Gold about one and a half years old.

[walpurgis]

I have found from experience, that Deccas and Grados can be prone to picking up hum. I've noticed this with direct drive turntables and those with steel top plates. But not in every case.