A very nice Sansui AU-717 was on the workbench for a complete restoration and upgrade. The restoration/upgrade plan included the following: 1. Replace all power supply electrolytic capacitors with low ESR types (increasing capacity where appropriate and physically possible). 2. Replace all of the signal path electrolytic capacitors with either "audio grade"/bi-polar electrolytic types or film types (whenever possible). 3. Upgrade original polyester film capacitors to polypropylene film types (as appropriate). 4. Replace signal path ceramic capacitors with film or C0G types. 5. Replace fuse resistors with metal film types. 6. Replace VD1212 (dual diodes) with pair of 1N4148 diodes (in series). 7. Replace any components damaged by the infamous corrosive board glue. Although cosmetically near mint, there was a noticeable "hum/buzzing" noise on both channels (independent of the input source and the volume control setting). Further investigation revealed that the noise originated from the preamplifer stage; specifically, failing capacitors in the regulated 35V power supply for the preamplifier. Accordingly, the first board to be restored/upgraded was the power supply board. Part 1: Regulated Power Supply & Protector Circuit Board (F-2663) The F-2663 board contains 2 independent regulated 35V power supplies (one for each channel). Unlike the main amplifier power supply, the regulated power supply uses rather modestly sized capacitors; this is one of the few weak spots in this otherwise excellent design. Fortunately, since modern electrolytic capacitors are physically smaller than their vintage counterparts, the capacitance reserve can be significantly increased with the replacement capacitors. Capacitors C05, C06, C07 and C08 filter the raw DC from a diode bridge, and were originally rated at 220µF/63V; the replacements were low ESR types rated at 820µF/63V. Similarly, capacitors C09, C10, C11 and C12 were originally rated at 100µF/63V and the replacements were low ESR types rated at 330µF/63V. Capacitors C17 and C18 which shunt the 13V zeners were originally rated at 470µF/16V and were replaced with low ESR types rated at 470µF/25V. The small value (0.47µF) low leakage capacitors, C19, C20, C21 and C22, were replaced with 0.47µF stacked film types. The final filtering capacitors in the regulated supply, C23, C24, C25 and C26 were originally rated at 100µF/35V and the replacements were low ESR types rated at 220µF/50V. Note that the voltage rating of these capacitors must be increased to 50V from the original 35V since these capacitors are subject to ~35V. The VD1212 double diode was replaced with a pair of 1N4148 diodes in series. Also, since the integrity of a number of solder joints appeared suspect, in particular the ones for the regulator transistors (TR01-TR04), these were re-flowed/renewed. Due to the cramped installation of the components on this board and the location of the board within the amplifier, this circuit runs rather warm. In order to improve the air cooling of this circuit, most of the replacement capacitors were selected to be physically smaller than the originals, except for C23-C26 which were the same size as the originals. A number of components had to be replaced due to damage from the corrosive glue used to secure the larger capacitors on this board. The damaged diodes, ZD01 and ZD02, were replaced with precision (2%) 13V BZX55B13 zeners. Similarly, the glue damaged 10kohm resistors (R7-R14) and the 22kohm resistors (R33, R34) were replaced with same value metal film types. Note that all remnants of the corrosive glue was removed from the board prior to the installation of the new components. The measurements of the restored/upgraded regulated circuit confirmed very stable ±35V rails, with no measurable AC ripple. Also, a quick listening test confirmed that the "hum/buzzing" noise was completely eradicated.