Power Filtering Enigma!! Can you solve it??

[blanik]

Sorry for the catchy title 

I have an old analog keyboard i'm fixing up, i tuned all it's 12 oscillators to perfection but there's a constant hum on the output it gets louder as i turn up the volume to the point of being really annoying...

there's the enigma: I found a place where the output goes from the PCB to the output jack and the ground in also taken from the chassis and goes to the output jack ground, when i play around with the wires in that area the hum changes in volume but still there so i tried to put a 220uF 25V cap between the output wire and Ground wire and miracle! the hum stopped, the sound was loud, clear and perfect but then after a couple of seconds the volume lowered to the point of silence, i removed the cap, hum was back so was the sound, so i figured the cap was loading to the point of shorting...
i also tried a 1M resistor in parallel, no success... i measured voltage at the ground-output and there's 0.326 V  AC  so i guess the 25V cap is enough (the synth has a two prong cord, seems to run on AC (although there's a big power transformer ans a smaller one inside)

what can i put there to make sure the hm stops without shorting the sound out???   

[Rob Strand]

I'm not sure from the info you gave but I suspect a ground loop issue with the output jack.

Assuming there's no other faults, like bad filter caps.

First check for a possible ground loop:  disconnect the output jack wires from the jack and check if the the PCB ground still connects to the chassis?   

-If it does then there's a good chance you have a ground loop.   This is very common is equipment with grounded jacks.  I'd try wiring the the wires to a new jack or unbolting the current jack, then try using the unit and see if the hum is gone. 

Easy solutions are disconnect the ground lead that goes to the output jack, or replace the jack with an isolated type.

- If it doesn't then it could be a loop formed between the keyboard and the thing you are connecting it to.

In this case you can use an isolated output jack.  But you should also connect the chassis to the PCB ground via a parallel combination of a 10ohm resistor, a 10 to 100nF capacitor, and two 3A diodes in parallel pointing in each direction.


Beyond those solutions it gets down to specifics of tracks and where the ground point take-offs to the chassis come from.   One stupid thing I often see is the chassis ground point is on the track between the rectifier and the filter caps, the large pulsating current in that track causes humm on the ground - the ground point should be after the caps.