Thorens 150 mk1 w/ TP13 original tonearm left channel noise

[adozeneels]

I'm at my wit's end about this one and I need to ask for help. It's a minor issue, but still annoying. I have had a Thorens 150 AB ("mk1") with the original TP13 (not-kugel/ball) arm for a few months. It's wonderful! Did the lubrication and tinkering, tuned the suspension, installed VinylNirvana's motor protection rod, bought a Schiit Mani 2 pre. Sounds amazing with my AR-3a and Sansui 7070. It skips on some rough vinyl, but its a low-force cart and I think I have to live with it.

My problem is that there is a low-level ground hum on the left channel. I can only hear it when I am not playing a record and is very noticeable anywhere in the room. It is mainly an annoyance. The ground hum is only in the left channel; the right channel has no hum and only (quieter) white noise from the phono pre. The ground hum is below the music level because the cart I have has an loud output, but it might be around music level if I were to switch to a quieter cart. I redid the grounding from the stock chassis-ground-through-left-channel-shielding (yikes) and now have run two ground wires to the preamp. The first ground wire is from the chassis and subchassis. The second is from the tonearm (I twisted a stripped wire around the lower shaft of the tonearm where it emerges below the tonearm board and taped it to the underside of the tonearm board so it's not visible to the user). The two ground wires meet at the phono pre and are twisted together. I first ran one ground wire but I found two ground wires reduce the left channel hum by maybe 3dB-ish.

The hum persists. I think it is louder when the air conditioner or dishwasher is running, so I think there's definitely noise on my line voltage. The hum varies in volume over time. I bought a Furman PL-Plus Series II power conditioner but it did not make a difference.

As far as I can tell there is nothing electrically different between the right and left channel, so why there is a difference in ground hum is beyond me at this point. If I swap channels at the pre, it follows the cable. I have used the phono pre on my Sansui 7070 and it produces the same hum.

My two hypotheses:

1. The previous owner replaced the RCA cable. I am suspicious of the RCA cable, it appears to be a cheaper and repurposed cable with the connector cut off. Not sure if the shield on the cable is in good condition. There's definitely connection through the shield of course.
2. The tonearm wires are bad and need to be replaced but I'm wary of doing this. This doesn't seem likely either.

Everything ohms/beeps out from the headshell connector to the RCA output, and all grounding is continuous from the chassis or tonearm to the phono pre.

Any help or suggestions is appreciated! Thank you!

PS: contrary to popular opinion there is a height adjustment on the TP13. There is a set screw under the plastic cone at the base of the tonearm on the outboard (right) side.

 

[adozeneels]

Bump. Any ideas? Thanks

 

[tnsilver]

Hi, keep it pithy, and add pictures - you'll get more responses. It's TL;DR.

 

[JohnRomano]

A couple of things you may want to try; change our the cartridge. See if the same issue occurs. Also try using the Aux imput on the Sansui, as opposed the phono stage. The signal path from the turntable is pretty straightforward. My guess it is originating at the cartridge. Also, make sure all of your connections are cleaned with a contact cleaner.
Good Luck.

 

[Nat]

If you use the aux input, obviously, you'll need a phono stage.
If the cables have been altered, that would be the likely issue. As I recall, some (or all) Thorens didn't have a separate ground, so the ground was tied into one lead. This works well with some cartridges, but it might not for cartridges which use the little tab to ground the cartridge case to one signal ground - the interaction of that tab with turntable wiring seems to have been variable enough that cartridges came with instructions to try with or without the tab.
It might be worth trying changing the cartridge leads right and left.

 

[adozeneels]

Thanks for all the tips! @Nat you're the winner here, it was the left channel ground tab on the cartridge! Removed it and poof. There's still mains hum on that line but it's below the phono stage or amp noise, so there's something weird with the wiring that I should get to the bottom of sooner or later.

Thanks again!

 

[Nat]

Glad to have helped - sometimes it is useful to be old enough to remember the 60s and 70s. Hang on to the tab - you might want to put it back in if you use a different arm later.