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Vintage Audio (record players, hi-fi etc) Amplifiers, speakers, gramophones and other audio equipment.

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Old 24th Jan 2022, 4:51 pm   #1
sparkymike
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Default lenco GL75 hum problem

I have had an elusive hum problem on my Lenco GL75 and the other day had a thought that it could be the phono sockets that I fitted in the rear of the wood plinth. These are normal chassis mount sockets, open on the rear side. I am now wondering if I should mount these in a small metal box and have two more chassis mount sockets on the inner side of the box so that the arm can be connected via phono plugs and thus fully shielded. Alternatively, is there a female/female chassis mount phono plug available which would obviate the need of the metal box.
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Old 24th Jan 2022, 5:56 pm   #2
RojDW48
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Default Re: lenco GL75 hum problem

Not a Lenco, but I have been setting up an SME/Garrard 401 combo for a friend and I was scratching my head over an obvious grounding hum - tried everything sensible and then it turned out to be the connecting lead - good quality, competently soldered - very unusual - maybe worth a try?
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Old 24th Jan 2022, 10:05 pm   #3
Uncle Bulgaria
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Default Re: lenco GL75 hum problem

Have you followed the grounding ideas on the Lenco Heaven forum?
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Old 25th Jan 2022, 9:05 am   #4
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Default Re: lenco GL75 hum problem

I find that my GL75 needs to be earthed via a separate lead or it hums
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Old 25th Jan 2022, 12:16 pm   #5
DMcMahon
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Default Re: lenco GL75 hum problem

Not familiar with the GL75 so may be way off base but some record decks will have a dedicated earth connection at their metalwork which may need to be connected direct to the amplifier metalwork earth, there is often an earth connection on the back of amplifiers to connect the deck earth cable.

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Old 25th Jan 2022, 12:23 pm   #6
fetteler
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Default Re: lenco GL75 hum problem

On the GL75 there is a little wire on the signal terminal block (located under the platter) connecting the screen of the signal cable to chassis earth. This often causes hum problems which can easily be remedied by cutting the wire.

Here's a picture showing the wire (turquoise arrow) which I did for the LH website.

Steve.
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Old 25th Jan 2022, 5:35 pm   #7
sparkymike
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Default Re: lenco GL75 hum problem

If the mains cable earth and seperate thin amplifier earth are both connected it hums. If I leave either off then hum is reduced, so sounds like earth loop, but from memory I think I cut the extra earth on the tag board. However, the metal pickup arm is connected to the motor base plate, so maybe if that was isolated, things might be a lot better? Might be worth a try.
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Old 25th Jan 2022, 7:27 pm   #8
PaulR
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Default Re: lenco GL75 hum problem

The mains lead on my GL75 doesn't have an earth wire. It just has the "optional" separate earth lead. As far as I know it is all as it was originally so it might be worth reconnecting the earth on the tag board and connecting the earth lead to earth and see what happens
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Old 25th Jan 2022, 7:31 pm   #9
fetteler
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Default Re: lenco GL75 hum problem

Having the arm tube connected to the chassis is fine and will not cause an earth loop unless something else is wrong like a problem with the insulation to the internal wiring of the arm allowing the screen to contact the arm tube.

Perhaps before going any further it would be a good idea to measure the resistance between the signal screen and the chassis to see if there is continuity? If there is then track down where it is occurring. Disconnecting the signal cables from the arm at the tag strip is where I would start...

Also, have you checked that there is no connection to the headshell at one of the cartridge pins? Many Shure cartridges ( for example) have a little removable tag between one of the pins and the cartridge body.


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