Leestereo
Super Member
A little while ago (last summer), a very nice AU-777A was on the workbench for a complete restoration and upgrade.
Part 1: HEAD PRE-AMP BLOCK (F-1194)
The F-1194 board includes the phono stage (600-series components) and the line level stage (700-series components).
A. Phono Stage
The stock capacitors (C601/C602) in the high-pass filter for the phono input were 1.5µF polarized electrolytic types; these were upgraded to 1.0µF polyester film types. The emitter bypass capacitors at C605/C606 were originally 33µF/10V and were replaced with 47µF Nichicon KZ types; the increased capacity of the replacements extends their effectiveness to 20Hz. Similarly, the 10µF emitter bypass capacitors at C613/C614 were replaced with 100µF Nichicon FG types. The C607/C608 capacitors in the feedback high-pass filter were 33µF polarized types and these were replaced with 100µF Nichicon ES bipolar types. The stock phono stage output capacitors (C611/C612) were originally 10µF polarized electrolytic types and were replaced with 22µF Nichicon ES bipolar types. The failure-prone 2SC458 transistors (TR601-TR604) were replaced with KSC945C(G) transistors (hFE matched pairs).
The capacitors for the RIAA correction, C615-C618, were originally 10% polyester film types and these were upgraded to 5% polypropylene types. The RIAA resistors at R625/626 were originally 22kohm 10% carbon film types and their replacements were 22.6kohm 1% metal film types. Similarly, the RIAA resistors at R627/628 were originally 270kohm 10% carbon film types and the replacements were 1% 267kohm metal film types. Note that changes in the resistor values are deliberate and improve the RIAA accuracy to -0.1dB/+0.2dB. Furthermore, although the stock component values can theoretically yield excellent RIAA accuracy, given the 10% tolerance of the stock components, that is not very likely.
B. Line Stage
The input high-pass filter capacitors (C721/C722), originally 0.22µF polyester film types, were upgraded to 0.47µF Panasonic polypropylene film types. All other stock polyester signal path capacitors (C731-C738) were upgraded to polypropylene film types of the same capacity. The coupling capacitors at C727/C728 were originally 10µF polarized electrolytic types and these were replaced with 22µF Nichicon ES bipolar types. The 1µF signal path capacitors at C729/C730 and C739/C740 were upgraded to stacked film types. The 47µF emitter bypass capacitors at C725/C726 were replaced with 100µF Nichicon FG polarized types; the increased capacity extends their effectiveness to below 20Hz. The failure-prone 2SC458 transistors (TR705-TR710) were replaced with KSC945C(G) transistors (hFE matched pairs). The local decoupling/filtering capacitor at C743 was originally 470µF/25V and was replaced with a 1200µF/35V low ESR type.
Part 1: HEAD PRE-AMP BLOCK (F-1194)
The F-1194 board includes the phono stage (600-series components) and the line level stage (700-series components).
A. Phono Stage
The stock capacitors (C601/C602) in the high-pass filter for the phono input were 1.5µF polarized electrolytic types; these were upgraded to 1.0µF polyester film types. The emitter bypass capacitors at C605/C606 were originally 33µF/10V and were replaced with 47µF Nichicon KZ types; the increased capacity of the replacements extends their effectiveness to 20Hz. Similarly, the 10µF emitter bypass capacitors at C613/C614 were replaced with 100µF Nichicon FG types. The C607/C608 capacitors in the feedback high-pass filter were 33µF polarized types and these were replaced with 100µF Nichicon ES bipolar types. The stock phono stage output capacitors (C611/C612) were originally 10µF polarized electrolytic types and were replaced with 22µF Nichicon ES bipolar types. The failure-prone 2SC458 transistors (TR601-TR604) were replaced with KSC945C(G) transistors (hFE matched pairs).
The capacitors for the RIAA correction, C615-C618, were originally 10% polyester film types and these were upgraded to 5% polypropylene types. The RIAA resistors at R625/626 were originally 22kohm 10% carbon film types and their replacements were 22.6kohm 1% metal film types. Similarly, the RIAA resistors at R627/628 were originally 270kohm 10% carbon film types and the replacements were 1% 267kohm metal film types. Note that changes in the resistor values are deliberate and improve the RIAA accuracy to -0.1dB/+0.2dB. Furthermore, although the stock component values can theoretically yield excellent RIAA accuracy, given the 10% tolerance of the stock components, that is not very likely.
B. Line Stage
The input high-pass filter capacitors (C721/C722), originally 0.22µF polyester film types, were upgraded to 0.47µF Panasonic polypropylene film types. All other stock polyester signal path capacitors (C731-C738) were upgraded to polypropylene film types of the same capacity. The coupling capacitors at C727/C728 were originally 10µF polarized electrolytic types and these were replaced with 22µF Nichicon ES bipolar types. The 1µF signal path capacitors at C729/C730 and C739/C740 were upgraded to stacked film types. The 47µF emitter bypass capacitors at C725/C726 were replaced with 100µF Nichicon FG polarized types; the increased capacity extends their effectiveness to below 20Hz. The failure-prone 2SC458 transistors (TR705-TR710) were replaced with KSC945C(G) transistors (hFE matched pairs). The local decoupling/filtering capacitor at C743 was originally 470µF/25V and was replaced with a 1200µF/35V low ESR type.