Just so you know the old caps may be twist-lock-lytics. If that is the case I usually replace them. You can usually read a date code off of them to see when they were manufactured. Twist-lock caps have a high degree of ESR or equivalent series resistance. Even though the cap might say 300mfd on the can, because of high internal resistance it will act in circuit more like a 150mfd cap. They are unwilling to give up their charge easily. The average life of a filter cap depending of course on usage, age, and temperature exposure is approx. 15-20 yrs. If the amp sat unused for years that will shorten the life of a capacitor as well. After 15-20 yrs. of use they tend to get muddy in the bass and I replace them as the slurry they use in the process of manufacturing dries out. They can internally heat up and begin to leak. The material that leaks out is caustic, conductive, and can damage the circuit board if not removed in a timely fashion. The one place you want a low source impedance is in your power supply. The caps that now come from ARC as described by Stonedeaf are low ESR types and a clever little circuit board on them that adapts the new style computer grade electrolytics to the old circuit board footprint and they drop right in. This makes a nice very nice improvement in sound and especially the bottom end and transient performance. The amp is capable of sounding better then when it was new because when this amp was built these low ESR caps did not exhist! If I get a chance I will look at a D90 circuit diagram and see if there are other areas you need to check i.e. zener diode strings in the voltage regulator to make sure the regulator is running at proper voltage etc. Off hand I don't remember the circuit. G
