After extensive testing of all transistors — which turned out to be functional — I discovered the root cause of the failure in the power supply of the Bang & Olufsen Beomaster 5500. The issue stemmed from diode D7, which connects via resistor R48 (470 Ω) to the base of transistor TR3. Even the slightest leakage in D7 causes TR3 to conduct, pulling the –28 V rail into the regulation path and destabilizing the ±12 V rails.
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, TC9164, 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.
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