Vintage Synth Repair

Good Morning All

I am looking for some help on a Wurlitzer 630A that I\u2019ve been asked to repair. The problem is apparently simple enough but has defied all attempts to solve.

The fault applies to the upper and lower manual, but not the pedals or the small top keyboard.

Switch on with no stops pressed i.e. if you press any key on the upper or lower keyboard you should hear nothing. No note occurs but a healthy hiss is present. When stops are activated pressing the keys you do of course get a note but with this high level hiss also present.

I have tried to track this hiss back from the audio amplifier stages but without any result. As soon as the key is released the hiss ceases. It affects all 44 keys on both manuals. On an oscilloscope, looking at the input to the audio stage, the waveform present without the keys pressed is a slight ripple of 50Hz. Press the key and the ripple immediately has the hiss superimposed upon it.

Any help to track this problem would be most gratefully received.

I would add that I don\u2019t have the organ in the workshop it is too heavy for me to remove from the clients house so I am not working under ideal conditions at all. However I do at least have a schematic of the works.

Many thanks in advance.

Brian