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Old 24th Jan 2020, 1:51 pm   #1
alanworland
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Default On a mission! Screened mains?

Following on from my post regarding my 401 neon that was acting as a spikey waveform transmitter I have been aware of him at highish volumes.
With that in mind I have repositioned wiring within the 401 plinth. This meant fitting a DIN socket very close to the SME arm base and as far away as possible from motor mains, excess SME cable has been coiled and tye wrapped around the arm's base keeping it contained in a very small area. Also shortened the deck to amp cable.
Mains now comes in and uses a very short length straight to the motor junction box.
Spikes now eliminated I am still aware of hum although we are now talking only high volumes. So I made up a DIN socket with the input connections shorted to earth and plugged in the amps input cable, quiet as a mouse!
It seems my hum is introduced when deck mains is 'plugged in' , turntable on or off. So I removed earth lead in the plug to deck, no different.
I seem to be picking up mains from just the cable.
I have (somewhere) a length of twin screened cable certainly big enough to supply the deck would this help contain the mains?

Alan
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Old 24th Jan 2020, 2:02 pm   #2
Electronpusher0
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Default Re: On a mission! Screened mains?

When coiling excess cable, especially audio cable you should coil non-inductively.
make the excess into a flattened loop so that the "outward" path is as close as possible to the "return" path. Now coil the loop.

Peter
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Old 24th Jan 2020, 3:17 pm   #3
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Default Re: On a mission! Screened mains?

Quote:
Originally Posted by alanworland View Post
... I seem to be picking up mains from just the cable.
I have (somewhere) a length of twin screened cable certainly big enough to supply the deck would this help contain the mains?
It is indeed possible to pick up 50Hz noise from mains cable. If there's no significant current flowing, because the load is not running, then there will be little magnetic field. But there will be significant electric field from the live conductor in the cable and screening the cable will reduce this.

When it comes to mains wiring I'm very much in favour of doing things by the book. So I would stick with properly rated and specified mains cable which I trust not to fail. I would then simply slide a braid sleeve over the outside of it and earth that sleeve as close to the mains plug as possible. I realise that that might be next to impossible with a moulded plug, and in the case of a rewireable plug it's very important to make sure that the connection to the braid can't come into contact with anything 'live' inside the plug.

Cheers,

GJ
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Old 24th Jan 2020, 3:31 pm   #4
dave cox
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Default Re: On a mission! Screened mains?

One thing to consider is that the 'mains' hum is actually being picked up via a 'mains frequency' modulated RF signal. In that case it could just as easily be the L/N wiring.

I would try an RF 'common mode' choke on the mains input, Ideally as close to the wall socket as is safe and practical, it might even work inside the turntable. Maybe try and use one of the 'all in one' IEC inlets with all the filtering built in. Avoid any old stock ones that use RIFA capasitors!

dc
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Old 24th Jan 2020, 7:46 pm   #5
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Default Re: On a mission! Screened mains?

This took me back! In the late 60s I had a hum problem. At the time I was doing a course in the engineering workshop at Bristol University. One of the engineers was justifiably proud of his ability to turn off perfectly regular curls of swarf. I scrounged a length and used it to screen my mains cable. Long time ago but ISTR it worked.
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Old 24th Jan 2020, 7:53 pm   #6
Craig Sawyers
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Default Re: On a mission! Screened mains?

It is a little pricey, but I fitted one of these to my 401 https://keystrobe.co.uk/product/keys...optibloc-401f/

Craig
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Old 24th Jan 2020, 8:32 pm   #7
turretslug
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Default Re: On a mission! Screened mains?

I recall how older (valved) Sansui hi-fi kit often screened runs of internal mains wiring going to and from front-panel switch etc. by running it inside lengths of close-coil steel spring (like a bigger version of curtain wire spring) whose ends and mid-points were secured to chassis with solder blobs.No doubt other manufacturers have done this sort of thing, too.
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Old 24th Jan 2020, 8:45 pm   #8
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Default Re: On a mission! Screened mains?

You really need to work out if the hum is caused by capacitive/electrostatic or inductive/electromagnetic coupling.

Braided/shielded cable (with the braid grounded) will solve the former, but the best answer to the latter is to increase the number of twists-per-inch of the live and neutral conductors, and the number of twists-per-inch of the conductors in the signal-cable so you get more 'field-reversals'. [this also helps with the issue of electrostatic coupling]

[look at what they do with twisted-pair Ethernet cables - there you have the transmit- and receive-pairs running alongside each-other inside the same overall jacket for up to 100 Metres: the number- and uniformity-of-the-twists is really important to avoid excessive crosstalk between TX and RX pairs]
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Old 24th Jan 2020, 9:30 pm   #9
alanworland
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Default Re: On a mission! Screened mains?

Interesting. This afternoon I disconnected the cartridge terminations and shorted the signal to earth leads, hum was non existent but once reconnected it returned. So at least I know my coiled up pickup lead under the arm is not a cause - it's being picked up by my V15.
Also then removed the plug fuse which greatly reduced hum as did fitting a clip on choke that I found in my parts box. So as I haven't been able to find my screened cable I have ordered a short length of Belden screened cable to replace the deck mains cable up to the motor junction box.
Does the forum consider wrapping all under deck mains wiring in a earthed copper screen would be beneficial?
If none of this works I could turn it into a clockwork deck!

Thanks for the strobe link but my strobe is now 'normally off' until I press it so won't be radiating any spikes.

Alan
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Old 24th Jan 2020, 11:57 pm   #10
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Default Re: On a mission! Screened mains?

Quote:
Originally Posted by Craig Sawyers View Post
It is a little pricey, but I fitted one of these to my 401 https://keystrobe.co.uk/product/keys...optibloc-401f/

Craig
There's a circuit somewhere to make one of these for just a few quid. I did and put it in a small plastic box. I use it with my strobe plattered 301 and it works a treat.
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Old 25th Jan 2020, 12:07 am   #11
qualityten
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Default Re: On a mission! Screened mains?

Here are pics of the strobe light in action. The platter is rotating in the first picture. I just switch it on to check or adjust the speed.

I'm sorry that I can't recall at present where I found the circuit.
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Old 25th Jan 2020, 12:36 am   #12
alanworland
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Default Re: On a mission! Screened mains?

Didn't the 301 have a strobe originally?
To be quite honest I have found the platter rim strobe is quite visible with room lighting!

Alan
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Old 25th Jan 2020, 6:46 am   #13
qualityten
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Default Re: On a mission! Screened mains?

I think the strobe platter was an optional extra for the 301, as there was no neon of course.
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Old 6th Feb 2020, 11:22 pm   #14
alanworland
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Default Re: On a mission! Screened mains?

Just thought I would let you guys know how I got on.
Eventually (after going missing) my screened cable turned up so I set to screening all under deck mains with a self adhesive copper wrap giving approx half it's width in overlap. Mains cable was replaced with the new screened one, this turned out to be massively over rated for my 16 watt motor as the conductor's are just over 2mm sq each!
The earthed inner screen cable was soldered to the mains earth at the motor with the screened earth also soldered toe the copper wrap.
The result is total silence, far better than I had hoped! The neon has been refitted and now makes no spikes or unwanted waveforms at all.
All in all, well happy, photo of the turntable underside showing the installation.

Alan
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