Connecting a clip lead from the earth terminal on Nick's phono to a screw head securing the Thorens top plate resulted in the disappearance of said hum.
Now this concerned me. Sure the hum was gone but it told me there was nothing earthing the metal parts of the turntable except Nick's phono stage!
I grabbed a manual for a Mk1 TD150 off the vinyl engine and sure enough, the manual said that the turntable was earthed via the tonearm third wire, there being only a ratty grey two core lead on the mains side.
All fine and dandy unless you have a Rega arm. Then there is no earth at all and a fault condition could potentially put live mains onto the subchassis, top plate, platter and possibly, even the tonearm itself.
I remember MJ in "Building Valve Amplifiers" talking about classic stuff from the 50s and 60s being all "without exception, outrageously dangerous" in terms of mains connections and earthing arrangements etc. Well here was the proof? Maybe, but I reckon there must at one time have been some bit of wire attached to the top plate at least and going to the original integrated arm base. Thorens can't have been that stupid!
So I spent the morning making the TT safe. I fitted a proper 3 core lead, taking the earth wire to a bolt, held by shakeproof washers onto the top plate. Then ran a very flexible wire from a tag I had bolted to the subchassis, back to the top plate terminal. I then tested every metal part for continuity to the earth pin on the plug.
Here is the mod:
Now I may be paranoid here but after being thrown on my back by an earth fault on one of my earlier amps I'm a bit wary of this sort of thing.
One or two other things had also alerted me to this problem with the TT; one being a loud crack through the speakers when I took a record off the platter after cleaning it with a carbon fibre brush, due to the resulting static charge having no way to get to ground.
Anyway after carrying out the mod, hum on Nick's phono is a thing of the past. The Rega arm earthing method via one of the braids in the lead works properly, as there is now a 0V ground reference for the signal from the cart, that does not depend on a bit of flimsy black wire with a spade connector on the end, connected to the phono stage.
That this is the method they once used for signal and electrical earthing of metal turntables is quite frankly alarming. My LP12 used a 3 core lead and the same earthing method as in my picture. I never had any trouble with hum from anything I fitted to it, but then the LP12 was bought in 1988, when safety standards were a bit more sensible than in the mid 60s.
As we all like playing with old Garrards and Lencos it might be a good move to check out how these things are earthed electrically and if necessary take the appropriate measures to ensure safety.
Steve.
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