NAD 705 Stereo Receiver Service Manual?

I am using my "old" NAD 705 Stereo Reciever mostly as Preamp and Tuner in conjunction with the PC and a "gainclone" as poweramp.

Some days ago, I changed two opamps (4558 and 2068) by OPA2134.

I wondered myself about not figuring out any bypass caps nearby these chips.

So I attached 470uF Panasonic direct at the voltage pins to ground.

Result: 100Hz buzz.

I tried different value of caps and tried to figure out another gnd plane.

But, still buzzing, with or without additional bypass caps. When I insert the original opamps:

Silence, as it has to be.

Now, I am looking for a service manual for this amp, but NAD manuals seems to be hard to find.

Any help?