Beovision MX 4000

11th November 2008

This odd fault took ages to trace and could happen to any of the Unity II chassis models. The symptoms were erratic changes in height, width and geometry which occured randomly but at a roughly continuous rate of a couple of times a minute for the whole time the set was switched on. Some of the events were quite violent in nature but despite this during an extended test run none of the power devices were damaged.

A check on the power supply output showed that all the various lines remained roughly constant and the fault was not mechanically sensitive so attention was turned to 13IC2 (TDA8432), the deflection processor IC. Checks showed regular glitches on the vertical and EW correction outputs, so clearly the output stages were being incorrectly driven. Substituting PCB13 with a known good one from another set made no difference but doing the same with PCB4 (power and deflection) cleared the fault completely. The various supplies and inputs to 13IC2 were all carefully checked but nothing serious was found amiss. Various componets on PCB4 were also changed but again to no good effect.

This chassis uses a rather odd layout in respect of the earthing around the vertical output stage. The circuit is powered from its own winding of the line output transformer and all the earth connections are kept seperate from the others on PCB4, instead they are returned to PCB13 where they are tied in with the rest of the set. A meter check showed adequate continuity throughout this arrangement but joining the earths together on PCB4 by linking pins 7 and 9 of the line output transformer provided a complete cure of all the symptoms with no apparent side effects. [Beovision MX 4000]