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Vintage Korg CX3 with noisy 50hz

 
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bortolo



Joined: 02 Mar 2020
Posts: 2
Location: Italy

PostPosted: Mon Mar 02, 2020 3:17 pm    Post subject: Vintage Korg CX3 with noisy 50hz Reply with quote

Hello everyone!
I'm new to the forum and I'm not too strong with English.
I am an Italian boy passionate about music and vintage instruments.

As a title I own a Korg CX3 that works very well but has always had annoying 50Hz in the output.

Here I found the schemes:
http://robertweatherford.com/bx3/schematics.htm

I have already disassembled the keyboard and changed all the electrolytic capacitors in the KLM-244 and KLM-246 boards (except c35 which looks good) which were visually damaged and also replaced C50, C51 and C52 in the KLM-245 board.
The hum has decreased but still quite present, I think it is generated before the "over drive" stage since if I turn the potentiometer the noise increases.

I would exclude a failure on the KLM-245 board as the noise persists even when it is disconnected.

Would anyone know how to help me or have they already dealt with similar keyboards?
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voip
Platinum Member


Joined: 27 Nov 2014
Posts: 3758

PostPosted: Mon Mar 02, 2020 8:00 pm    Post subject: Reply with quote

A little bit of mains hum is almost a part of older audio equipment. Mains hum can have several origins.

Power supply ripple: Use an oscilloscope to check the supply rails do not have too much ripple on them. Although various capacitors may have been changed, there is always a chance (rare) that one of them may have an issue, or the soldered joints may not be fully wetted with solder, or the circuits are drawing more current than originally designed, causing increased power supply ripple.

Grounding: Ensure the places that should be grounded do actually have good continuity with ground. In the CX-3, this includes the PCB tracks that are designated as ground, any potentiometer grounds, panel and metalwork grounds, output jacks and send and return jack ground connections. Also ensure that the property itself is properly grounded. I recently helped to solve a mains interference problem, where there was 125VAC between a metal kitchen sink and metal kettle nearby, because of an earth fault in the property.

Hum loops: Keep amplifier power leads on the same mains outlet as the CX-3, and keep the mains leads close to each other. Route audio signal cables close to, but not touching the mains leads.

Electromagnetic fields: Ensure no external equipment nearby is generating strong stray electromagnetic fields. This could be the mains transformer of another piece of audio equipment on or near the CX-3, or a nearby washing machine, or aircon unit, or possibly high voltage mains distribution cables near the building.
.
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bortolo



Joined: 02 Mar 2020
Posts: 2
Location: Italy

PostPosted: Wed Mar 11, 2020 10:01 am    Post subject: Reply with quote

Now I have changed all the capacitors and also the 12v regulator.
Noise is less but still present, including white noise.
I noticed that if I move the board away from its position the noise decreases, as if it were the transformer passing part of the noise.

I ask myself this problem because I have a korg m500 of about the same years and it is very silent in comparison
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voip
Platinum Member


Joined: 27 Nov 2014
Posts: 3758

PostPosted: Wed Mar 11, 2020 6:57 pm    Post subject: Reply with quote

Could the transformer be moved and remounted to a position where it causes least hum, but is still practical and safe? A bracket might need to be made up to hold it there.
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