Problem Yamaha YST C11

I have a YST C11 about 16 years old. It sounds great until now. The tape drives pulse on intermittantly on power up and the sound level waves in and out for quite a while until it steadies out. Tape replay is always low volume. Feels like a power problem but a couple of looks have revealed no obvious component burn outs. The three d2061's on the heatsink do get warm quickly though not burn out hot. Has anyone experience of this? Does anyone know the dodgy component? Thanks.