The picture is just for reference. My system has different speakers, and the remote is a larger console powered by 1.5V D cells. I’ll share my picture if I manage to find it again.
I received this system from an inheritance. There were some problems with it: after switching on, the volume was set to maximum and the remote control responded slowly — panic in the room! The loudspeakers had to be refurbished with new rubber surrounds, as the originals had completely deteriorated.
The cartridge of the special linear turntable with servo system no longer followed the groove, seemed damaged, and was very expensive. Presumably this happened during earlier transport; when moving the unit, remove the valuable cartridge and place it separately in a protective box, and use the transport lock. The heavy platter also remains loose, so that should be removed as well. Unfortunately, too late — the damage had already been done.
When moving the pickup arm, the servo followed the movement, so the servo system seems to be working normally.
In my workshop I connected the system, but the power‑on problem persisted. Possibly the infrared LEDs or detectors are faulty? The receiver and amplifier did work, until suddenly a short hum was heard, a burning smell appeared, and the system shut down. I saw that the TC9148P had burned out and the two transistors that were supposed to regulate 12 V down to 8 V had melted.
After thoroughly testing all the transistors and confirming they were functional, I finally traced the Bang & Olufsen Beomaster 5500 power supply failure to diode D7. Removing D7 brought the ±12V back! It’s connected via resistor R48 (470 Ω) to the base of transistor TR3. Even a slight leakage in D7 causes TR3 to conduct, pulling the –28 V rail into the regulation circuit and throwing the ±12 V rails out of balance.
Catastrophic Consequences
Due to this leakage, the ±12 V rails surged to ±40 V, which destroyed the audio control ICs (TC91xx series). This failure mode is particularly dangerous because the leakage current was minimal — only 11 µA — yet sufficient to bias TR3 into conduction.
Component Testing
Faulty D7:
Forward voltage: 706 mV
Reverse leakage: 11 µA
Capacitance: 0 pF
New 1N4148 diode:
Reverse leakage: 4 nA
This comparison highlights the critical importance of low-leakage diodes in sensitive biasing circuits.
Functional Role of D7
While the exact function of D7 remains unclear, it appears to be part of a feedback or soft-start mechanism during power-on. Its placement suggests it influences the biasing of TR3, possibly to prevent overshoot or to coordinate startup behavior between the positive and negative rails.
Preventive Measures
To avoid future damage:
Replace D7 with a high-quality, low-leakage diode (e.g., Vishay or Nexperia 1N4148).
Consider increasing R48 to reduce sensitivity to leakage.
Add local clamps (TVS or zener diodes) on the ±12 V rails near the ICs.
Introduce series resistors or polyfuses to limit current in case of overshoot.
Final Notes
This repair highlights how a seemingly minor component — a single diode 100 for 50 cents..— can trigger catastrophic failure in a precision audio system. Careful measurement, hypothesis testing, and component isolation were key to identifying and resolving the issue.
Documenting this fix ensures others can recognize and prevent similar failures in their Beomaster 5500 units.
The volume and tone control unit uses Toshiba ICs TC9164P, TC9184, TC9177, along with some op-amps powered by a +/- 12V supply. Although the Toshiba TC91xx series is no longer in production, there are still parts available on eBay, so repairing this unit should be possible, even though I have no intention of using it.
 |
Comments