PA0O ham radio

  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...