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So, I just bought a new record turntable (Audio-Technica AT-LP60BK) since… Hey! I guess I still like records. This is the base model AT-LP60BK turntable that only has RCA connectors for output; no Bluetooth or USB connection. Put analog connections happening for audio output.

Anyway, this turntable has a built in phono pre-amp. It amplifies well for true line level output, but there is a catch… Sometimes when the phono pre-amp is switched on I can pick up what sounds like CB (or maybe even HAM?) radio signals from my neighborhood. Possibly from a local car service? Definitely not a baby monitor since it’s clearly the sound of a conversation into a microphone.

No full voices are discernable but that kind of mumbling that sounds like an adult in the Peanuts/Charlie Brown cartoons. It’s incredibly annoying. FWIW, these rogue transmissions are sporadic in nature and there are times of day I (thankfully) hear nothing but the sweet tunes I am playing on my setup.

If I switch it to normal phono level output the levels are obviously lower, but then there are utterly no mumbling radio interference nonsense. Tested with it hooked up to my Sony CMT-SBT100 receiver as well as a stand alone speaker so I am 100% positive the issue is isolated to the turntable and only when the phono pre-amp is hooked up. I can hear this stuff if I simply have the turntable powered, not spinning and plugged into a speaker… All I have to do is switch on the built-in pre-amp.

I went ahead and opened up the bottom of the case to see if I could see anything obviously “off” about things; see picture below. Like most modern electronics, it all seems like a single-board setup and simple enough. But one thing comes to mind: Perhaps the issue is because there is utterly no shielding around the circuit-board?

I’m at a loss at what to do: Maybe get an external phono pre-amp and just use that? Or something else perhaps? Maybe add some simple shielding to the setup?

enter image description here

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  • \$\begingroup\$ A simple metal-on-paper shield should fix it. Just find a grounded screw and attach it there. \$\endgroup\$ – Ignacio Vazquez-Abrams Oct 29 '16 at 4:27
  • \$\begingroup\$ @IgnacioVazquez-Abrams So you mean shielding the circuit board pictured, correct? So simply getting a shield that could cover the board—without interfering with anything else—and mounting it via one of the screws you see pictured should help? \$\endgroup\$ – JakeGould Oct 29 '16 at 4:29
  • \$\begingroup\$ The screw needs to be grounded, because the shield should be grounded. It will be far less effective if it isn't. \$\endgroup\$ – Ignacio Vazquez-Abrams Oct 29 '16 at 4:30
  • \$\begingroup\$ @IgnacioVazquez-Abrams Ahh, okay… I know where ground is hooked up on this thing so I would have to rig something up. \$\endgroup\$ – JakeGould Oct 29 '16 at 4:30
  • \$\begingroup\$ I do have to admit that I'm surprised that AT wouldn't have already done that. But then again, I'm surprised that they sell anything for under $100. \$\endgroup\$ – Ignacio Vazquez-Abrams Oct 29 '16 at 4:32
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Serious radio amateurs know how to fight RF interference.

Just adding a shield and taking care of proper earth grounding might help but is probably not enough. Frequently the RF signal is picked up by the incomming mains cable and so measures need to be taken in that area.

In your case the power supply is realised with 4 diodes and a capacitor. (Top right on the picture. To make the apparatus insensitive to incoming RF signals a little change of the simple power supply could be effective.

Try to place (have placed) small capacitors over the 4 diodes with values between 10 - 100 nF. The schematic added is for 230VAC but is also valid if the incomming mains is only 110VAC. Even in situations where there is no mains transformer involved.

Why?

In many situations the problem is caused due to the increased sensitivity of semiconductors for HF and VHF signals. Even a diode as 1N4007 is capable to demodulate such signals.

Simple schematic explaining how to place capacitors.

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  • \$\begingroup\$ Thanks for sharing this! But when you say, “Try to place (have placed) small capacitors over the 4 diodes…” do you mean—looking at your mini-schematic—soldering 4 100nf capacitors to the anode and cathode of the existing diode on the power supply? \$\endgroup\$ – JakeGould Oct 29 '16 at 15:02
  • \$\begingroup\$ Meaning something like this? Or as explained in this video near 4:04? \$\endgroup\$ – JakeGould Oct 29 '16 at 15:33
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    \$\begingroup\$ Yes, that is the idea. Hope it works for you. \$\endgroup\$ – Decapod Oct 29 '16 at 16:14
  • \$\begingroup\$ So here is an update: I scored some 100k 100 capacitors which equate to 100nF; equivalent of a cheap 104 ceramic disc capacitor. Carefully soldered all four to the 4 diodes and… It works! My turntable is no longer some bizarre receiver for some neighbors weird CB or HAM radio transmissions. That said, my receiver still faintly picks that junk up but I guess that will be another project. For now, the goal of listening to records without some mumbling nonsense in the background has been achieved! Thank you so much for the advice! \$\endgroup\$ – JakeGould Oct 30 '16 at 2:24

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