Beovision MX 7000

7th October 2008

This set gave no results and the standby lights were not illuminated. The cause of this was quickly traced to the BU2508DF line output transistor 4TR33 being short circuit. A replacement was fitted and at the same time C100 (220uF) was also renewed in case it had contributed to the failure. The line driver transformer T4, the tuning capacitor 4C102 (6.8nF 1.6 kV) and the pins of the socket on the scan coils were also all re-soldered, all these things can cause 4TR33 to fail. The set worked normally after this except for occasional picture disturbances that took the form of a single random bright line sometime accompanied by a brief loss of colour and a “snick” noise from the line output stage. After a few hours use 4TR33 failed again during one of these disturbances. It was first thought that an arc somewhere in the line output stage was responsible but replacing 4C101, 4C102, the EHT lead and the focus control assembly made no difference, the effect continued and was obviously threatening the second replacement line output transistor. The defective component in the end turned out to be 4TR31, the line driver transistor. This is a field effect device and is therefore difficult to test with a meter, a replacement cured the fault however. [Beovision MX 7000]