Farfisa Fast 4 noise problem

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Farfisa Fast 4 noise problem

Post by pabloevaristo »

Hi there! I'm looking for help. what could be happening to a Farfisa fast 4 with a continuous high pitched noise?.
Once you power it up, it's always there. All the notes and buttons are working but the noise is there.

https://drive.google.com/file/d/18guvsY ... sp=sharing

I'm good with electronics so I'm wondering if I could repair this.
Tthnks in advance!!!
pabloevaristo
Posts: 3 Joined: Tue Nov 30, 2021 9:54 pm

Re: Farfisa Fast 4 noise problem

Post by Funkyflash5 »

Someone more knowledgeable will hopefully chime in, but looking at the schematic for it, it should be fixable but may take quite a bit of troubleshooting. I would start by probing with an oscilloscope, beginning with the output and working back to find where in the circuit the noise is being introduced.
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Re: Farfisa Fast 4 noise problem

Post by Wonks »

I expect all the electrolytic capacitors need replacing, especially those in the power supply. Replace them with a good quality brand, as they should then last a couple more decades. Avoid using cheap Chinese capacitors.

The schematics can be downloaded from here if you haven't already got them.

https://www.synthxl.com/farfisa-fast-4/

Check the DC supplies for the level of ripple current. Obviously, the lower the level, the better.

But I'd also move the organ to another room, just in case it's just external interference like a dimmer light switch (or turn the lights off and see if the noise disappears).
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Re: Farfisa Fast 4 noise problem

Post by pabloevaristo »

yes, capacitors is what everyone always say... ;-)

I would not think that is an interference, I too loud for that.
pabloevaristo
Posts: 3 Joined: Tue Nov 30, 2021 9:54 pm

Re: Farfisa Fast 4 noise problem

Post by Hugh Robjohns »

It sounds to me like crosstalk leaking through from all the tone oscillators into the output circuitry at high level -- it's as if you were holding down all the notes at the same time.

All keyboards of this era suffer capacitive crosstalk like this to some extent, but this is clearly an excessive amount.

You can clearly hear the crosstalk takes on vibrato (which acts directly on the master oscillators) when the switch is activated, so it's definitely an internal noise, not external interference.

Looking at the schematic, my first guess would be that an effective (muting) earth is missing from the busbar switches, quite possibly due to corrosion, so I'd be looking carefully there first.

If you have access to a 'scope you should be able to track down where the crosstalk is breaking through, as it may be on just one or two busbar links.

I definitely wouldn't be randomly messing about with swapping capacitors at this stage as the instrument is clearly working in all other respects and that kind of surgery is unlikely to cure this problem but may well cause other problems along the way.

So fix the one fault before delving in deeper...

Instruments of this era are fairly simple and straightforward to service by any competent electronics technician. There's nothing complicated in there...
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Re: Farfisa Fast 4 noise problem

Post by pabloevaristo »

thanks a lot for your thoughts and for sharing your expertise!
If I finally buy it, I think I will start with the output of the power supply.
(maybe I will be around in the next weeks...) ;-)
(another video in THIS LINK https://drive.google.com/file/d/12DcG9b ... sp=sharingfor all the notes playing)
pabloevaristo
Posts: 3 Joined: Tue Nov 30, 2021 9:54 pm
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