Sonab Amplifier Repairs

Sonab R3000 FM Stereo Receiver

The unit initially failed to power up. On investigation, it was found that the two main reservoir capacitors were effectively absorbing the supply voltage, rather than charging correctly. This was placing considerable stress on the power supply and left the capacitors at imminent risk of failure. Both capacitors were therefore replaced.

Following this repair, the tuner section became operational but would not tune and lock onto a signal reliably. The tuner employs varicap (voltage-variable capacitance) tuning, and measurements indicated that the tuning voltage was approaching the limit of the available tuning range. In effect, the tuner had run out of varicap tuning capability before the required tuning point was reached.

This suggested that the fault was not simply a matter of alignment, but was associated with the tuning voltage/range within the varicap circuit. Further investigation was therefore required to establish why the tuning voltage was outside the range necessary for stable reception.