One of the later series of top models, manufactured from 1984 to 1986. The ‘X’ indicates the X-balanced circuitry, essentialy four power ampliers, two per channel, one of each is 180 degrees out of phase with the other. The in-phase output is connected to the positive output terminals, the out-of-phase output to the negative output terminals.
Adjusting the DC-offset and bias is a delicate operation, no less than eight adjustments have to be done, in the right order. I prefer to replace the original trimmer potentiometers for multi-turn types to make adjustment a little easier, but the owner of this unit wanted only basic service to be carried out.
Cleaning the unit it became obvious that the previous owner was a smoker. Generous amounts of oil based cleaning solvent made the unit as good as new again. The main power capacitors measured ok, still having full capacitance. Soundwise I recommended replacing all the small electrolytics, which I am confident will make the amplifier sound much more transparent and dynamic.
The owner told me the unit suffered from signal dropout. Usually this relates to corroded contacts of the speakers relays. Removing them in order to clean the contacts is quite an undertaken in this model; the entire back panel has to be removed and the speaker terminal block has to be removed from the circuit board before the relays can be removed from the circuit board. The contacts were indeed corroded, confirming the complaint. Because of its massive toroidal power transformer this model has a soft-start circuit, reducing the otherwise substantial inrush current on switch-on of the unit to manageable proportions. The relay for this circuit sits on the same circuit board as the speakers relays, so it is little extra effort to clean the contacts of that relay as well.