Bang & Olufsen Beomaster 1000 (1968)

The amplifier had spent a few years in the screed. The condition generally looks good and it has no signs of moisture. Only the holders of the transformer fuses had verdigris deposits, which were easy to remove. Incidentally, one of the 800 mA fuses on the secondary transformer voltage was also defective. The device can now be switched on perfectly. The amplifier stage also seemed to work at first. However, after a few minutes there is a bang in the sound, it starts to boom loudly at a higher frequency and the 2 amp fuse in the left power amplifier blows.

Some capacitors in the LF stage need to be replaced because the ESR values are out of tolerance. Elcos in the preamplifier also no longer look promising in terms of their condition; some of them were replaced as a preventative measure, even if they were still intact according to the measured values. The large gray capacitor in the secondary supply next to the transformer had a higher capacity than originally stated, which could also be an indication of aging and recommends replacing this component.

The surfaces of the switches have a clearly yellowed patina, which is rarely addressed because otherwise the old plastic and the labels could be damaged. It is therefore left as is. Mechanically, the buttons still work perfectly, the mechanics just need to be activated using a special contact cleaner and care product.

The layer of dust that has accumulated over the decades is everywhere inside the device, which easily adheres to the surfaces and has to be removed with a fine brush and a dust removal spray and vacuum cleaner.

Some capacitors were replaced because their ESR value was out of tolerance. Some 22 ohm resistors also had to be replaced as a precaution because they no longer made a good visual impression. The driver transistors AC187/188 are not original like the original transistor pair AC127/AC132. The change to the transistors was probably made in the past due to a repair and possibly the problem related to the loose crimp connections of the AD149 power stage transistors, which were probably not recognized at the time. This soon became a problem because rather well-sized speaker drivers were connected and a collector current of 500 mA compared to 1 A (AC187/AC188) may be a bit too tight in terms of tolerances. Due to a more practical case type, the AC187/AC188 can be easily attached to a heatsink/case using a screw. The transistor in position AC132II had a short circuit due to the following defects and had to be replaced. It was also found that the NTC resistor of the bias setting in the left LF stage had a 2-3 times steeper characteristic curve compared to the NTC in the right channel. This was determined using heat and cold spray. In addition to the defective output stage transistors, the NTC resistor is also partly responsible for the suddenly excessive currents on the driver transistors AC127II and AC132II. The NTC transistor is therefore to be assessed as defective. After removing the NTC and readjusting the bias using suitable resistors, the problem on the left audio channel finally disappeared and the repair was therefore successful. As a consequence, the bias setting was also modified in the right amplifier stage. In addition, as a preventive measure, the sole trimming potentiometers were replaced with new ones (100 ohms) and the bias was readjusted to the left and right.

The transistors AD149 in the LF stages and in the supply were replaced with new ones. Because all of them were either defective or the inner layers had already short-circuited after a while, which led to the rumbling. The cause was probably the original crimp connections, which were oxidized and even loose. This in turn caused loose contact, which must have led to higher current consumption and destroyed the transistors over time. Fortunately, these guys could still be found. The sound is now really impressive, and that's only on the test speakers. A beautifully colorful, warm sound with a good foundation now comes from the speakers.