Beocord 1700/2200
12th March 2009
Although these models are generally well designed the input stage attracted some criticism when new. In particular if you want to record from a line level source (e.g. a CD player) through the AUX socket there is a noticeable loss of treble that spoils the recordings.
The cure is to change 0R1 and 0R2 (mounted on the back of the DIN sockets) from 2M2 to 470K and 1C4 and 1C24 from 220pF to 22pF.
When new these models were set up for use with rather basic tapes, as a result they do not suit modern ones well. Readjustment is therefore strongly recommended, to do this properly with the Beocord 2200 you will need the filter unit that the Beocord 1700 has in place of the Dolby processor. This is a difficult thing to find but it is easy to make your own on a piece of 0.1” header strip and a small piece of veroboard. The circuit is given on the service sheet. [Beocord 1700, Beocord 2200]
Beogram 4002
12th February 2009
A Beogram 4002 (the later type with a DC motor for the turntable) would change speed from 33 to 45 RPM within the first minute of play of an LP record. The 33 RPM key could be used to set the speed back but it would not remain set for long before the problem recurred. These models use a relay to switch different resistors into the speed regulator circuit, the resistors include the panel controls and some extra trimmers inside. Measurements showed that the relay control circuit was unstable, before the speed change occurred rattling and screeching could be heard from the relay itself.
As well as changing the speed control resistors the relay is also used to control the lamps that illuminate the variable speed controls. The lamps also form part of the latch circuit that holds the relay in the correct position for the selected speed. In the case of this Beogram 4002 the indicator lamp for 33 RPM was open circuit, so as well as not lighting up the display this upset the operation of the latch by leaving the base of the relay control transistor almost “floating” and therefore susceptible to picking up noise. A replacement lamp cured the fault, the symptoms were considered to be unusual as whilst it is not uncommon for the lamps to fail normally this does not cause other problems. [Beogram 4002]
Digital TV on old Beovisions
11th February 2009
For viewing digital TV services on old Beovision sets that do not have DIN AV or SCART connections the Tvonics MFR-200 set-top box is an excellent design. It is fully featured yet very small (smaller than a cigarette packet) and features a built-in fully tuneable UHF modulator, so it can be connected directly to the aerial socket of any set. There is also a CVBS/stereo audio output for which a special cable is needed.
Clear pictures are however very dependant on setting the modulator to a genuinely clear channel, it’s worth taking some time to get this right.
Beovox MS 150
11th February 2009
The very distorted sound that these loudspeakers produced gave the impression that the mid bass drivers were defective. Moving the cones carefully by hand did not reveal any rubbing or loose parts so the crossover circuit was investigated. It was found that the contact resistance of the protection relays was quite high (a couple of ohms) and varying. It would of course be possible to short out the relay contacts but as the tweeter and midrange units are both delicate and no longer obtainable this is not a good idea. The relays are also of an unusual type but they can be dismantled and the contacts cleaned. In the case of these particular loudspeakers this provided a complete cure. [Beovox MS 150]
Beomaster 6000 4channel
6th January 2009
These models must be amongst the least reliable of all Beomasters and the densely packed electronics and poorly planned wiring make the necessary repairs very difficult.
One common problem is the failure of the output transistors and their accompanying 0.12R emitter resistors. As well as the usual reasons (excessive bias current, shorted loudspeaker leads) this can be caused by the long thin amplifier panel warping and shorting out on the sides of the metal trough where it is mounted. It is wise to cut some stout cardboard to size and fit it between the print side of the panel and the metal whenever one of these sets is serviced, this can prevent nasty (and potentially expensive) problems later. [Beomaster 6000 4channel]
Beomaster 900
6th January 2009
The fault with this receiver was no results with severe overheating of the rectifier and regulator. A short circuit was found across the 32V supply so the output transistors were immediately suspected. On disconnecting the collector connections the short remained however, a meter check showed only a few ohms between the supply and the chassis.
The cause of the fault was the bulb holder for the stereo indicator, the +32V supply is connected directly to the outer (screw) part and the terminal to which the wire is soldered was touching the chassis. Careful bending cleared the short, luckily no other damage had been done. [Beomaster 900K]
Beomaster 5500, 6500 and 7000
6th January 2009
If you have owned a lot of B&O equipment then you will have probably noticed that thesse models give a slightly disappointing sound when used with passive loudspeakers when compared to some of the earlier designs. This is surprising as the power amplifier circuit is very similar to that of the Beomaster 2200, Beomaster 5000 and the Beocenter 7700 (amongst many others), all of which give very satisfactory performance.
The Beomaster 5500, 6500 and 7000 have a rather basic pre-amplifier circuit compared to that used in the original 5000 model but this is not the cause of the problems, the same arrangement is used in the Beocenter 9000 where it seems to work reasonably well. Therefore the power amplifier circuit was investigated to see what could be causing the trouble.
The only substantial difference between the Beomaster 5000 circuit and that used in the later models is the addition of four 220nF capacitors C215, C216, C415 and C416. They are not shown on all the Beomaster 5500 circuit diagrams but many sets have them. As these do not appear in any other versions of the circuit it seemed safe to remove them and see if the performance improved. With the capacitors in circuit the sound from one Beomaster 5500 was lifeless, bland and uninteresting, without them it was more similar to a 5000, engaging and lively with a full, rich tone.
It is not clear what the purpose of these capacitors is but performance definitely improves if they are disconnected. Before doing this make sure that the muting relay 3RL1 is in good condition (contact resistance less than 0.5R at rest) and that the output bias current for each channel is correctly set. [Beomaster 5500, Beomaster 6500, Beomaster 7000]
Beovision LX 2802
18th December 2008
This set displayed corrugated verticals when first switched on from cold. The line output and driver stages had already been overhauled in line with B&O’s recommended reliability modifications so the fault clearly lay elsewhere.
The cause turned out to be C40 (100uF 16V) in the timebase generator circuit (near the line hold and line phase controls on the left hand side signals panel). This capacitor is located near some hot running transistors and had clearly dried out with age. A high-temperature replacment ensured that the fault could not recur. [Beovision LX 2802]
Beocord 9000
9th December 2008
This recorder (an early model with the complex Dolby PCB) played back OK but its recordings were spoiled by a faint but regular clicking sound on one channel, between one and two clicks per second was typical. An earthing fault was suspected and upon investigation the earthing arrangements for the deck were found to be odd indeed. The motor can was earthed through the supply cables (returning via P1/12) but the earthed side of the motor supply filter was returned to the deck itself. The only deck earth was via a tag on the combination record / replay head, this was returned to P19/6. P19 is also the connector for the replay head, so the deck earth effectively channeled any surplus charge into the sensitive amplifier circuitry.
The lead between P19/6 and the head was removed, the deck was then earthed by fitting an eyelet tag under one of the three mounting screws and soldering a wire between this and the screened cover of the microphone amplifer. This arrangement removed the clicks completely. It was thought that the capstan belt was becomming statically charged and that the poorly designed earthing arrangement was allowing the resulting dischages to be recorded onto the tape. [Beocord 9000]
NICAM decoders
11th November 2008
B&O nicam decoders have proved fairly reliable but when faults do occur they tend to worry engineers as the circuitry is not generally well understood.
The earliest design, as used in the Unity I TV chassis and the VX 5000/VX 5500 VTRs, has proved reliable but one common fault is uneven channel balance of the audio output, either from NICAM or from the FM intercarrier sound that is also routed through the NICAM decoder. The faulty components are usually the 10uF coupling capacitors around the switching stage, replace them all if one is found to be faulty.
The later type of decoder, as used in the Unity II chassis and early variants of the Beovision Avant, is of a simpler design. Its main fault symptom is spluttering or intemittent NICAM sound, the defective component being the VCO trimmer. Cleaning it is a waste of time, fit a new one and set it up carefully as per the manual. Another weak component in some variants of this design is the 220uf 16V capacitor that smoothes the 5V output of the switching regulator. Despite being a “long life” type after a few years use its contents can leak out and make a mess of the board. Fit a replacement and clean away any contamination, it is corrosive to the copper trackwork in the long term.