Hi Neil. What can you tell me about the Armstrong 521 please?
This was a stereo hi-fi amplifier from the Armstrong Audio company and was marketed as 2 x 25W amplifier.
It employed germanium AL102 transistors in its output stages and these had a reputation for failure and are now unobtainable although it is possible, with modification to replace these with newer, silicon transistors. The amplifier was a single rail design and employed an electrolytic output capacitor in the output stage. The amplifier featured inputs for tape, tuner and MM gramophone and on the front panel had 4 rotary controls for volume, bass, treble and balance. Underneath these controls were a series of black push buttons which allowed control of the input sources as well as loudness, high pass filters, rumble filters and tape bypass. The amplifier was presented with an aluminium, silk-screened front panel with a teak case and a matte black rear panel. The amplifiers were marketed from the 1968–1972 when it was replaced by 600 series.