Vintage Synth Repair

AFAIK a cause for ground loop is using of different outlets in the  
room (different line circuits). Using only one outlet for all  
equipment can hardly be a reason for such ground loop.

Ground loop can be also avoided by ground lift on some equipment but  
it can be dangerous.

I had small hum problem in Japan because here grounding (earthing) is  
missing on power outlets. So I had to make my own grounding and now  
it's OK.

Frequency of hum caused by ground loops depends on net frequency, it  
can be 60 (USA} or 50 Hz (Europe}, Japan has 50 or 60 in different  
regions. Your 120 Hz looks like the second harmonics.

Daniel Forro


On 17 Nov 2009, at 10:29 AM, Alan Probandt wrote:

> Do you have several pieces of equipment plugged into the same  
> outlet?  This might be causing a ground loop that increases in  
> strength after the equipment has been powered on for a short period  
> of time.  Does the hum remain when no voices are being sounded?   
> More likely a ground loop.  Try plugging the other equipment into a  
> different outlet across the room with an extention cord.
>  Is the hum only present when certain instrument voices are played  
> and is not present when other voices are played?  More likely to be  
> a voice programming issue.  The organ voice is most likely of all  
> the voices to be a sine wave (or close).  A hum is usually a 120 Hz  
> sine wave.  That is about the B or B flat in the octave below  
> middle C.  Does playing that B flat note on organ voice sound  
> either louder or softer than the other notes?  Does that B flat  
> have a 'LFO-type' waving sound that the other organ tones don't  
> have?  All signs of a ground loop.
>   Do you have an oscilloscope?  Do you have a sound-card  
> oscilloscope program?  Download a scope program and feed the hum  
> from the synth into the PC audio line input.  Run the scope program  
> and see if the hum actually is 120 Hz.  If not, it's a synth  
> hardware issue. If it is, then it is a synth power supply problem  
> or a ground loop.