Grand Prix Moderne 610

4th September 2008

This set showed some signs of life, the valve heaters were lit and the tuning indicator showed that stations could be tuned in on all wavebands. There was no sound however on either channel. Looking for a common cause, the HT supplies to the amplifier panel were checked and found to be in order and the cathode voltages of the output valves were around the expected figure, indicating normal functioning. The unexpected cause of the fault turned out to be the 220K anode load resistors for the triode sections of the output valves, all four were completely open circuit. Replacements restored the sound but it was then found that the AFC did not work. The AFC system unusually uses a small double triode valve as a DC amplifier and the anode load resistor to one side of this was also found to be open circuit, replacement provided a complete cure. The reason why these five resistors had all failed was not clear, although small high-value resistors with large voltages across them are vulnerable components. [Grand Prix Moderne 610]

Beolit De Luxe 611-FMT

29th August 2008

This set had worse than usual background noise on all wavebands. As the noise continued even with the volume reduced to the minimum setting it was clear that the fault must be in the audio amplifier stages. Shorting the signal path to ground using a 100nF capacitor confirmed that the fault was in the region of the second transistor AC151-2 but a replacement transistor made no difference. The noise even continued with the transistor removed, pinpointing resistor 145 (4K7) as the defective component.

The amplifier of these sets is not particularly well designed and has rather too much gain. This tends to lead to excessive background hiss and a poor usable range on the volume control. These problems can be mitigated by disconnecting resistor 145 from the supply rail and putting an extra 1K resistor in series with it. If capacitor 150 (10uF) is then connected to the junction of the two resistors instead of the collector of the transistor the gain (and noise) will be reduced to a more usable level. Along with this change, adding a 10K resistor in parallel with resistor 149 (22K) improves the effectiveness of the negative feedback and results in better sound quality. [Beolit De Luxe 611-FMT]

Beovision 3802 (20AX chassis)

29th August 2008

This set suffered from annoying intermittent width variations, mainly in the corners of the picture. The first suspect was of course the EW transformer 6T3 but as the new type was fitted this clearly was not the case on this occasion. The EW modulator diodes 6D4 and 6D5 were tried, along with the EW driver transistor 6TR6 but the symptoms returned after a few hours use. The line scan coil plug P13 was found to be in poor condition so the wires were removed and soldered directly to the underside of the pins, this helped but the fault was still not completely cleared. Eventually it was discovered that the plug on the frame timebase module (P19) was making poor contact at pin 8, the field scan current return path. The plastic body of the socket was melted, showing that a high resistance connection had been getting hot. As the EW circuit takes its frame drive waveform from this point this explains why the geometry was affected. Removing the plug and wiring the module in directly finally resulted in stable operation. [Beovision 3802]

Beomaster 5000 (tuner)

29th August 2008

Reception weakened and the stereo decoder stopped working after about half an hour’s use. Meter checks showed that the output of the power supply was falling over this period because the selenium rectifier (mounted under the chassis) was defective. A modern silicon replacement restored correct operation, the extra output was not a problem as it was still well within the capability of the AD149 voltage regulator transistor. [Beomaster 5000]

Beolab Penta 2

29th August 2008

Crackling and rustling noises remained after the usual potentiometer and capacitor replacements. As the fault got worse as the amplifier warmed up something had clearly become thermally sensitive, eventually the careful use of freezer spray identified 1D8 on the main panel as the culprit. [Beolab Penta 2]

Beogram CD 50

29th August 2008

The fault was no sound, although the disc was clearly being played as it turned normally and the time counter was reading correctly. The muting transistors were turned on all the time however, the cause was C351 in the muting circuit, it had dried up and gone open circuit. [Beogram CD 50]

Beocord VX 5000

27th August 2008

This machine gave the correct random light show in the display followed by going into standby mode when the mains power was connected, indicating that the power-on reset circuit was working correctly. However, every attempt to turn the machine on after that resulted in an automatic shutdown a few seconds later. Disconnecting the collector of the fan motor reset transistor QR1654 proved that the fan motor circuit was where the fault lay. The fan impeller was free to turn but the over-current trip was being operated as the voltage at pin 6 of IC1653 was 2.8V instead of the more usual 1.2V. R1655 (47R) was checked and found to be the correct value so attention was instead turned to the UNREG 16V line which was rather high at over 21V. The machine had been imported from Germany and was fitted with a 220V mains transformer, fitting a 240V replacement restored normal functioning. [Beocord VX 5000]

Beovision 2600

27th August 2008

This vintage colour set was in good condition but the picture was marred by intermittent height variations. As the frame hold remained solid it was decided that the fault was more likely to be in the output stage than in the oscillator. A new PL508 frame output valve made no difference, leaving the sometimes troublesome output transformer as a possible cause of the trouble. As a replacement transformer was not immediately available further tests were made and it was later found that resistor 505 (68R 1W) in the cathode circuit of the PL508 had gone high in value. Such a breakdown does not normally result in intermittent symptoms but on this occasion a new resistor removed all the faults. [Beovision 2600]

Beomaster 1900

27th August 2008

A very noticeable background hum spoiled the otherwise quite reasonable performance of this stereo receiver. The 15V rectifier D50 was initially suspected but proved blameless, eventually it was discovered that there was excessive ripple on the 28V supply. The cause was C66, it was open circuit and in very poor condition physically. This should have been obvious but the capacitor is hidden by the resin moulding and not visible until the main PCB is removed. [Beomaster 1900]

Beocenter 9500

27th August 2008

All the functions of this complex machine worked correctly except for the FM radio which gave noise but no stations, even when the manual tuning mode was tried. As the AM section was working this cleared the tuning voltage supply and much of the microcomputer from suspicion, so what next? Pin 8 of the tuner module (the tuning voltage input) was stuck at 29V regardless of what the frequency readout was set to so it was disconnected and a 0 > 30V variable bench supply connected in its place. Altering the voltage allowed stations to be tuned so the FM front end, IF amplifier and stereo decoder were deemed to be in order and attention turned instead to the pre-scaler circuit. Sure enough there was no divided-down output at pin 4 of 10IC6 and so 10IC4 (SP8529), which has not proved to be especially reliable, was suspected. Before replacing it however more oscilloscope checks showed that there was no input signal to 10IC4 either and 10TR3 (BF240) was found to have 5V on both its collector and base with nothing at its emitter. The transistor was open circuit of course and a replacement brought all the FM stations back with no further complications. The same fault could also occur in the Beomaster 5500 and its derivatives as they use the same circuit for the radio section. [Beocenter 9500]