Helfried

I don't think it's worth the effort to design a new power supply for these keyboards. It's certainly possible, but it's enough for me to know that there is no single problem with my device. With this knowledge I can live with the hum 🙂

Ah , thank you very much, that is enlightening ! Now I have great certainty that nothing is defective in my devices, but quite simply all CT6000 (and maybe some other Casios with built-in power supply) have this noise. It's not that dramatic, but once you've got it into your head that there must be a defect, you want to fix it 😉 I think I can stop researching now. Many thanks !

Hi CO-Windler, as far as I know, you also have a CT-6000 and know it well. Does yours have no hum like my two specimen? see mp3 example above, clearly audible in the speakers, but also in the headphone and line-out, where I recorded it.

oh, one more thing, the rhythm is not too quiet - in my example I only show how much of the rhythm is still audible when the Rhythm knob is set to zero. That's not the sound the drums actually make, it's a kind of crosstalk from digital chirp noise. And this happens even if the cable to the Rhythm Volume pot is disconnected, so this "chirp" comes into the power amp via other paths (ground, Power VC+, VB+, whatever).

thank you the two CT6000 behave in every respect exactly identical, sound exactly the same and also the hum is at the same level and identical. It is mains hum with predominantly the frequencies 50/150/250/350...Hz , so slightly distorted towards squarewave. I've also run the keyboard in different rooms in our house to rule out such things (and they are on different phases of our house power supply), no difference. And here in my studio no other device (and that's a lot of synthesizers , effects units a lot of audio equipment ) makes any hum problems. Ground loops through audio wiring I have been able to successfully fix with several DI boxes and the AC power seems clean. Protective earth is also connected correctly (3pin connector) and there have never been any problems with that here. My first guess was possibly a capacitor in the power supply of the device, but there I had already replaced some and it has not changed anything. It hums also already on a certain level, if the main volume control is on zero, so it is also already in the power amplifier - and can therefore only come over the power supply or from ground connection with voltage offset or similar. A ground problem within a device, and that in two devices identical? Or actually insufficient filtering/smoothing of the different supply voltages? Meanwhile I also have an oscilloscope here and may measure again - but my suspicion is that it may actually be a "property" of the synth. I know some organs from the 70s that hummed similarly and more, my old Hammond tonewheel organ and the Leslie also hum, more than the Casio, high impedance tube amps also hum - but I don't really expect something like that from a mid 80s synth with ICs and transistors 🙂

OK, thanks so far, I think I'll give up researching it for now. In the meantime I have a second CT-6000 here, also in very good condition and everything works. However, the noise (hum, crosstalk drums etc..) are EXACTLY the same. It seems now as if this is normal for this device - which I find surprising for an instrument from 1984, I know other devices from this time and from earlier, which meet significantly higher "audiophile" standards. But ok, if it is so, then it is so 🙂. If someone here owns a CT-6000 , which sounds cleaner or absolutely free of such noise and hum, I would still be interested to know .... Until then, all the best!

CT-6000 service manual Here is the service manual - it is indeed detailed, the complete circuit is included and many hints for measurements and adjustment.... However, the layouts of some boards are missing, but they have not been forgotten, as it looks like they are also missing in the original. In any case, it is very helpful.

hello, I have a question about the Casio CT-6000, so to all owners of this keyboard here 🙂 My specimen is in good condition and was obviously rarely used, inside and outside everything looks like new. It works perfectly, however I notice a relatively strong mains hum from the speakers. The hum depends on the settings of the volume controls of each section, it is strongest when I turn up the "Rhythm" control. It also varies in strength with different sounds, so it is also affected by the different filters and thus is generated before - or everywhere. I have already checked and measured the electrolytic capacitors in the power supply, they all have the specified capacitance (at least as accurate as can be reliably measured in their built in state) and look good. So I think, it doesn't seem to be a problem with the power supply and its hum filtering. All connectors seem to be OK, too. Could it be that this hum is simply normal for this device? And now I would be interested if someone can confirm this. Then I could stop researching for any fault 🙂 What else I noticed, I a leaking of the drums with closed "Rhythm"-slider, and indeed "chirping". Are maybe both effects related - or are both normal ? These would be my questions. If both also occur with other CT-6000, I would be appeased:-) It also comes from the line outputs and from the headphone output (there it is particularly disturbing, since it is already very loud with completely closed volume control). I'll attach an audio recording from the headphone output - first the basic hum , then drum leaking with the drum slider closed, some knob movement and some playing. Thanks! Helfried Casio CT-6000 hum etc.