Farfisa Compact noise

[melvin]

After tuning up my Farfisa and modifying the preamp from the SE-15 to the SE-52 it’s sounding great BUT I have become obsessed with the background noise, which isn’t bad but I think could be better. (I like to play soft sounds like just the 8’ flute) I have discovered a couple of things that help but was hoping to hear other people’s experience- like how much background noise people have and what they’ve found helps.

The noise I’m talking about is all the notes constantly in the background. It’s not loud and probably normal but I’m sure it can be improved. I thought it was a power supply issue but I tried bypassing the filters with no improvement and my preamp is all tube now so it should be quieter as far as this particular noise is concerned as the supplies are separate. Plus, all the filter caps were replaced several years ago along with most of the other electrolytics. However, I have NOT yet replaced the 1uF coupling caps on the generator boards and possibly elsewhere. Would it help?

The things I found to help: try star grounding where possible. Where the ground wire runs along the top of the switchboard (treble section?) and connects some circuitry and a lot of shielded cable ends back to the power supply ground I have divided into two and ran another wire separately back to the power supply ground terminal 3. It helped.

Shielding. If I remove the switchboard and swing it out away from the rest of the circuits a lot of the noise goes away. Making some form of shielding to mount on the back of this board between it and all the oscillator boards and their wires which are extremely close (!) seems to help a lot.

One thing I cant figure but would help increase the signal to noise is the voltage levels listed in the schematics are much higher than what I measure. I do not get 130mVpp with just the bass tab on coming into the preamp. I can’t figure out why. I am going to have another look at the oscillator measurements because I’ve traced everything else. It’s strange.

[David Anderson]

What you say about grounding and shielding makes sense. I think we have to remember that these combo organs were designed to be portable for gigs, so they weren't built to last decades or to be ultra-quiet. They probably ran grounds where it was convenient and used the least amount of wire. And where circuits spread out on circuit boards, they become more vulnerable to picking up local noise or even radio signals.

The 1uF capacitors at the tone outputs are unlikely to have an affect on noise. They are simply there as coupling capacitors to block voltage from the divider circuits. Where they tend to fail is via increasing ESR (internal resistance). And they fail unevenly from one to the other, so this can result in uneven tone volume. It's tedious to replace all 73 of them, but cumulatively, it will lift a bit of haze from the organ. And tone output may become louder overall. That might have something to do with your last question.

It's also the case that those Italian electrolytics will go bad, one by one. Do you want to have to keep opening it up and replacing them one at a time?