Casio CT-401 weird noise

DIY STUFF
1

Hey everyone!

I picked up an old Casio CT-401 and all of the functions seem to be working properly, but on the pot that adjusts the mix balance between the rhythm section and the melody, when the rhythm is turned up at all there is an incredible amount of noise that comes out. The rhythm section can still be faintly heard underneath the noise.

I’ve cleaned all the contacts and replaced the pot. I don’t know much about the inner workings of this type of thing to be honest, the only possible explanation I can think of is there could be an old cap that has gone bad and maybe DC is getting through? No clue to be honest.

Hoping somebody smarter than me could provide an explanation or some pointers on how to get it going properly. Thanks!

Here is a video demonstrating the issue:
https://drive.google.com/file/d/1jrGlT556gGYw2dnIPdHZTOQix1ckCiKB/view?usp=sharing

Here is the service manual if you are so inclined:
https://drive.google.com/file/d/1oND9ugvzNN9W4g83hsgiObp5lFSWdhh4/view?usp=sharing

2

Listening to the noise in the video I get the impression that it takes a while after switching the device on before the noise starts. Is that correct?

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3

Thanks for the response! The noise is always present, but it seems to peak at random intervals. In the video I was demonstrating that the noise is related to that balance pot as it only starts to become noticable when the balance is anywhere except 100% melody.

4

Hi, i bought a ct-401 today and have the same exact issue. Have you solved that problem yet? I cant get it to work.

5

Unfortunately I have not solved this issue yet. Funny timing on your post actually, I had my ct-401 back on the bench to try and figure this out again a few weeks ago.

This time around I recapped the entire synth and replaced the mixer op amps, but no luck. I started to try and trace the noise with an oscilloscope and it seemed to be leading to the CPU, but I got busy and wasn’t able to confirm.

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6

Okay thanks for the reply. I`ve found that my ct-401 sometimes stoped making that noise when i have it on for a while with the balance complet on the Melody side. When i turn the knob back, the noise stops more often than not. maybe something to do with a chip warming up or so. Sadly im not to knowledge about these things. Keep me updatet when you find something :slight_smile:

7

I can find some internet posts that mention the adapter as a possible source. Do you have the original Casio adapter that according to the manual should come with the synth? Did you try it with a differeent aadapter?

8

Yeah that sounds right. The problem exists in the accompaniment circuitry, so when the mixer knob is all the way set to melody the issue can’t be heard.

I’ll be sure to update this thread if I ever solve it.

9

Which adapter are you referring to? Mine did not come with any special accessories. The CT-401 is powered with a 3 prong IEC connector.

10

Sorry, I was looking for the term ‘transformer’. Does it have a transformer that changes the AC voltage to a significantly lower DC power for the synth?

11

Oh I see. Yes the CT-401 has a transformer. It is internal however.