Leestereo
Super Member
A nice Sansui AU-7500 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).
2. Replace all of the signal path polarized electrolytic capacitors with film types (whenever possible) or bi-polar electrolytic types.
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 noisy failure-prone 2SA726 and 2SC1313 transistors.
6. Replace non-original mis-matched output transistors.
Part 1: Tone Control Block (F-2014)
The tone control circuit in the AU-7500 is always in the signal path as there is no "tone-defeat" switch
on this model.
Signal Path:
The signal first passes through the input high-pass filter capacitors (C701/C702) which were 0.22µF polyester film types; these were upgraded to 0.47µF Panasonic polypropylene film types. The low leakage electrolytic capacitors at C709/C710; C719/C720; C729/C730; C739/C740 and C741/C742 were replaced with WIMA polyester film types (at the same capacity as the originals). The polarized electrolytic capacitors (C721/C722) in the local feedback loop were replaced with Nichicon ES bipolar types (at the same capacity as the originals).
The stock polyester capacitors at C711-C718 (tone controls) and at C731-734 (low filter) were replaced with polypropylene types. Although the polyester capacitors for the low filter can be switched out of the signal path, they were also upgraded to polypropylene types to match the aesthetic of the rest of the board.
The transistors at TR701/TR702, TR705/TR706, TR707/TR708 and TR709/TR710 were the failure prone 2SA726 types (rather than the 2SA493 shown in the schematic) and these were replaced with Fairchild KSA992 types. The transistors at TR703/TR704 were 2SC1313 types (rather than the 2SC1000 shown in the schematic) and they were replaced with Fairchild KSC1845 types.
Non-signal Path Capacitors and other Components
The emitter by-pass capacitors at C703/C704, C707C708 and at C723/C724 were replaced with low ESR types (at the same capacity as the originals). The local decoupling capacitor C743, originally rated at 220µF/50V, was replaced with a low ESR type rated at 330µF/50V.
The restoration/upgrade plan included the following:
1. Replace all power supply electrolytic capacitors with low ESR types (increasing capacity where appropriate).
2. Replace all of the signal path polarized electrolytic capacitors with film types (whenever possible) or bi-polar electrolytic types.
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 noisy failure-prone 2SA726 and 2SC1313 transistors.
6. Replace non-original mis-matched output transistors.
Part 1: Tone Control Block (F-2014)
The tone control circuit in the AU-7500 is always in the signal path as there is no "tone-defeat" switch
on this model.
Signal Path:
The signal first passes through the input high-pass filter capacitors (C701/C702) which were 0.22µF polyester film types; these were upgraded to 0.47µF Panasonic polypropylene film types. The low leakage electrolytic capacitors at C709/C710; C719/C720; C729/C730; C739/C740 and C741/C742 were replaced with WIMA polyester film types (at the same capacity as the originals). The polarized electrolytic capacitors (C721/C722) in the local feedback loop were replaced with Nichicon ES bipolar types (at the same capacity as the originals).
The stock polyester capacitors at C711-C718 (tone controls) and at C731-734 (low filter) were replaced with polypropylene types. Although the polyester capacitors for the low filter can be switched out of the signal path, they were also upgraded to polypropylene types to match the aesthetic of the rest of the board.
The transistors at TR701/TR702, TR705/TR706, TR707/TR708 and TR709/TR710 were the failure prone 2SA726 types (rather than the 2SA493 shown in the schematic) and these were replaced with Fairchild KSA992 types. The transistors at TR703/TR704 were 2SC1313 types (rather than the 2SC1000 shown in the schematic) and they were replaced with Fairchild KSC1845 types.
Non-signal Path Capacitors and other Components
The emitter by-pass capacitors at C703/C704, C707C708 and at C723/C724 were replaced with low ESR types (at the same capacity as the originals). The local decoupling capacitor C743, originally rated at 220µF/50V, was replaced with a low ESR type rated at 330µF/50V.