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Akai GX-635D has black leads to many transistors

Bought new in 1983, my Akai GX-635D has developed a fault in the Right recording channel. I believe there is a known issue with transistors in this Akai model. Now on the bench and stripped down with the Pre-amp PCB available to change components, my observation of the transistors showed many had a black, furry deposit on their leads and I wondered if this was consistent with this 'bad transistor' issue. One of the worst affected is tr11 - 2SK68M19Y. There are several 2SC458 transistors likewise affected. It has been suggested that the leads were silver-coated and the black is therefore Ag20 - or silver oxide. But were this the case then all the transistors would exhibit this coating. Any suggestions would be appreciated. Michael (UK)