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]
Beocord 2000
11th November 2008
No sound and the level meter showing some segments lit all the time is a common problem with this model. The cause is lack of the 7.5V bias supply which comes from the LM1131C Dolby processor IC. The chip itself is rarely at fault however, the culprit is most commonly one of the two 220uF electrolytic capacitors C208 and C408, they tend to go short circuit. Fortunately the chip usually survives this. The Beocord 3300, 3500 and 4500 use a similar circuit (but with only one capacitor) and so can also suffer with the same complaint. [Beocord 2000]
Beocord 5500
30th October 2008
This recorder would sometimes jam when asked to perform a particular function, often record or rewind. When this happened the mode control motor could be heard to be spinning furiously but nothing was moving mechanically. Soon after this the machine would shut down.
The faulty component was the worm gear on the end of the motor, it had split. The remedy was to clean and degrease both the gear and the motor shaft, rough up the shaft with some oxide paper and make some indentations on it with some large serrated pliers. The worm gear could then be bonded on firmly with cyano-acrylate glue. Once the mechanism was reassembled and relubricated it functioned normally. The same fault could of course afflict the Beocord 3500, Beocord 4500, Beocenter 9000, Beocenter 9500 and Beocenter 8500. [Beocord 5500]
Beocord VHS 82
30th October 2008
This machine had a tape stuck in it. The clock flashed on and off but nothing else worked. A mechanical fault was suspected (it is hardly unusual!) but disconnecting the loading motor and powering it with a battery proved that the mechanism was in good working order. It was noticed that the power supply was making the sounds of distress in time with the flashing clock so attention was turned to the switch mode regulator section. Every electrolytic capacitor inside the screened enclosure was virtually open circuit, replacing them with 105 degree rated ones restored surprisingly good results. [Beocord VHS 82]
Beolab 8000
24th October 2008
It is not unusual to find these loudspeakers to be completely dead with not even the red standby indicator showing. The cause is usually the standby transformer, the primary winding burns out. B&O now supply only the complete amplifier as a replacement part, this is a very expensive way to deal with such a simple fault. An alternative is to source your own transformer, RS Components 504-363 being one alternative. The pin arrangement is slightly different so some cutting and bridging of the trackwork of the PCB will be necessary. [Beolab 8000]
Beomaster 5500
20th October 2008
This receiver would switch itself off a second or so after being switched on. The usual cause of this is an excessive DC potential at the loudspeaker outputs so and further tests must be made with the loudspeakers disconnected. In this case -37V was found at the midpoint of the two right hand channel output transistors, normally the voltage should be zero, give or take a millivolt or two. In all B&O amplifiers that use a BF423 transistor this is the first thing to check and sure enough TR205 showed considerable leakage when tested with a meter. A replacement failed to cure the fault however. TR206 (BC547B) read strangely in circuit so this was replaced too but this also made no difference. The true culprit was C215 (220nF / 63V film type) which was leaky. This capacitor is not shown on all issues of the circuit diagram, making tracking down the fault more difficult than it could have been. [Beomaster 5500]
Beovision MX 6000
15th October 2008
Erratic line output stage operation and occaisional failure of the BU2508AF line output transistor was traced to poor soldering of the connections of coil L8. It was the connections of the windings themselves to the pins that was defective, the coil had to be removed from the PCB to see the damage. Scraping the wire clean and resoldering it to the pins provided a complete cure. While doing this it is also wise to add some insulating sleeving over the wires that lead to the top winding, they can short to the lower winding where they pass over it. Short circuits like this were a common fault with a similar coil fitted to the ICC5 chassis (used in the MX 3000/4500/5000) as well. [Beovision MX 6000]
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]
Unity 2 chassis LOPT confusion
7th October 2008
An unusual and non-standard line output transformer has been used in a few examples of the Unity 2 chassis (Beovision LX5000 etc). This has the code number 40308-17 70420461 and is used in conjunction with a early issue of PCB4 (power and deflection) which lacks the link to select the supply voltage to the RGB output transistors (157V or 190V). If this transformer is fitted to a conventional PCB4 the set will not start up as a short circuit appears across the line output transistor. The transformer can be used in these models however if pins 11 and 12 are cut off and the RGB supply voltage link is set to the 157V position (the screen control may have to be adjusted if the link was previously set to 190V). The newer type of transformer with a winding connected to pins 11 and 12 can be used in the earlier panel without modification.
Beovision 3000/3200/2600
2nd October 2008
If there are problems with line synchronisation or phasing (picture pulls to the side or breaks up into lines) and the PCF802 line oscillator valve proves blameless the two diodes in the flywheel sync discriminator are a likely suspect. They are often found to be housed in a common block next to the valve and slowly increase their forward voltage drop and become unbalanced as the years pass. Two ordinary silicon types (1N4148 etc) make a good replacement but be careful when fitting them as in some circuit diagrams they are shown the wrong way round. [Beovision 3000, Beovision 3200, Beovision 2600]