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Old 1st December 2009, 12:40 PM   #1
diyAudio Member
Join Date: Apr 2008
Default Armstrong 523 tuner problem

I have been trying to repair an Armstrong 523 FM/AM tuner for a friend. The tuner is at least 40 years old. Mono reception is fine, but in Stereo mode the pilot light illuminates, implying that a stereo signal is present, but the audio remains in mono, with a faint 'chuffing' noise in the background. This noise stops when mono mode is selected.

Compared with modern tuners using IC's, the decoder circuit, which lives on a separate PCB, seems very simple, with only 4 or 5 transistors. I have tested every component and nothing appears to have failed. The owner of the tuner has suggested that the decoder is based on a Mullard standard circuit, but I have been unable to unearth either this or a diagram for the tuner on the web. Can anyone offer any suggestions?
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