Vintage Synth Repair

If lifting the ground changes anything, it is more likely that you have ground loop noise, common to all unbalance output circuits. 

Try a direct box. 

I typically do this for my unbalanced gear:
Calculate or measure the equivalent series resistance of the output circuit (R series limit).  I then take a TRS balanced cables and on the keyboard end I put a TS jack on the other end.  Tip to tip, ground shield to ground and a resistor equal to the equivalent resistance from the cold lead to ground.  I hide the resistor under the sleave of the mono connector.  This will balance the paths to ground of both sides and allow the common mode rejection(CMR) of the balanced inputs of your DAW or mixer to now work as intended.  Mark these cables for only this JD-990. 

You can also look at the schematic to see what value Roland selected for a output limit resistor or you could make up a test cable with a pot as above and tune or null your ground loops down to zero, measure and make.  Be sure to match as close as possible. Some Rolands stick a cap in series too and this will not work as outlined.





On Monday, March 3, 2014 8:36 AM, 65 Lotus <Lotus@...> wrote:

 
>I have a JD-990 plumbed into a DAW that's picking
up computer noise whine. No 60 cycle hum.
> 
>It has the usual EIC 3-wire plug. The whine goes
away if I lift the 120 V ground which I don't like to do.
> 
>Looking at the schematic, there are five 1000 pF
and two .01 uF ceramic caps across the analog ground to case
ground.
> 
>With everything unplugged, the resistance from the
case to the analog ground is 0.3 ohms, basically the resistance of the
meter leads. Shouldn't the case to analog ground
be nearly open if they're putting capacitors across it? What good would that
do?
> 
>I'm thinking one of the caps is shorted. I
checked the two 0.01 uF caps and they're ok, so it would have to be one of
the 1000 pF parts.
> 
>It has the original caps on the power supply
fwiw...
> 
>Thoughts appreciated!
> 
>Scott in Ohio
>
>
>