Sansui AU-505: Restoration
At some point in time I have attempted a failed restoration of this amplifier. Why failed? Because I used mostly used capacitors that I recovered from various electronic devices. It was OK but this cannot be called a proper restoration. So I have decided it is time to revisit this amplifier. This time using proper parts.
Working on this unit exposes you to electrical hazards. There are lethal voltages inside.
Severe accidents and possibly death by electrocution might occur. I am qualified and skilled with electronics and I have been doing audio gear repairs for over 20 years. If you lack experience, please take these articles as just a knowledge base. Do not attempt to repair something that you cannot handle as there is a high chance of doing further damage while also possibly suffering accidents.
Good tools are a must for a quality restoration. I use eutectic soldering alloy and a temperature-controlled soldering station equipped with various tip shapes. I a standard and a precision desoldering pumps and desoldering wick in various widths. To clean the flux, I use isopropyl alcohol and high purity acetone.
Empirically, I found that working with a temperature of exactly 300 °C is safe for these vintage printed circuit boards. I have never lifted any pads and I never wait more than a couple of seconds with the hot tip on any pad. While working on the chassis, I use between 360 and 440 °C. Flux fumes are extremely toxic and should be avoided at all costs.
Every replacement part is brand new, from a reputable manufacturer, ordered from the U.S.A., Japan, or Germany. In addition, I only use parts that are suitable in specific circuit sections, after inspecting and comprehending the original schematic diagrams. Last but not least, I have years of experience backing up my choices and actions.
F-1303A Equalizer and Tone Control Board Restoration
Removal of the board is not really mandatory. You can work directly on the board with some difficulties caused by space constraints. Which I did. On this boart I have decided not to replace the small signal transistors. I am still looking for some that might be a better replacement for 2SC871.
Local filtering capacitors are designated C003 and C004 and were originally rated 470 uF / 35 V. I have used Nichicon KZ MUSE series counterparts rated 470 uF / 50 V. Stage coupling capacitors designated C601, C602, C611, C612, C705, C706, C711, and C712 were originally electrolytic capacitors rated 1 uF / 50 V. I have changed them all with Panasonic stacked film non polarized capacitors rated the same as the originals. C605, C606, C609, and C610 are decoupling the emitter of TR601, TR602, TR603, and TR604. They were originally rated 10 uF / 10 V. I have replaced them with modern Nichicon KZ MUSE capacitors rated 10 uF / 100 V. There is absolutely no need for such a high voltage rating but this is what I had available at the moment. These capacitors fit perfectly the raster holes. C709 and C710 are decoupling the emitter of TR701 and TR702 for improved working stability. I used Nichicon KZ MUSE series parts rated 100 uF / 25 V.
I have not take pictures before the restoration. Don't ask why. I myself don't know the answer to this question. But here are some post-restoration pictures. Given that I decided not to remove the board from its place, it was very difficult to take quality pictures. But you get the point.
F-2001 Filter Board Restoration
To remove the filter board you need to remove two screws. Further more you need to desolder a steel pin, part of the chassis, protruding through the board directly on the ground plane. You need a powerful iron and a wide tip for a good heat transfer. I managed to do it with the soldering station set to 450 degrees Celsius.
Stage coupling capacitors C751 and C752 were originally of electrolytic type, rated 1 uF / 50 V while C755 and C756 were of tantalum type, rated 1 uF / 50 V. I have replaced all with Panasonic stacked film parts of the same rating. Filtering capacitors C006 and C007 are also on this board. They were rated 100 uF / 50 V and 220 uF / 25 V. I have replaced both with Nichicon KZ MUSE series capacitors of the same rating as the originals. C761 and C762 were rated 47 uF / 6.3 V and were replaced with Nichicon FG series parts, rated 47 uF / 16 V.
I have replaced the 2SC871 transistors with hFE matched KSC1845. However, the former type of transistor has a much higher amplification factor than the later. So if you decide to do this swap you will encounter a 10 to 15% reduction in total gain of this stage alone. I have absolutely no problem against this.
Renewal of all solder joints on this board.
F-1266A Power Amplifier Board Restoration
This board does not need to be removed in order to be serviced. I have turned this amplifier upside down and have removed the bottom steel shielding steel plate. Now I grabbed the various capacitors with my left hand from underneath the amplifier while operating the soldering iron with my right hand. Overall the board is easy to service and the job is straightforward.
The board before restoration. You can also spot the thick layer of dust.
And my previously attempted restoration with vintage assorted capacitors; between them, even Rubycon parts.
Decoupling capacitors C805 and C806 were rated 1 uF / 50 V and were replaced with Panasonic stacked film capacitors of the same rating. C807 and C808 were 470 uF / 16 V. I replaced these with Nichicon FG series parts rated 470 uF / 16 V. C809 and C810 are part of some sort of negative feedback loop. In the service manual they are rated 33 uF / 16 V. However in the schematic and in my unit they were 47 uF / 16 V. I have replaced them with modern Nichicon KZ MUSE series capacitors rated 33 uF / 25 V. I believe bipolar capacitors would do in this position but I have not yet investigated this thought. Local filtering capacitors C813 and C814 were rated 47 uF / 50 V. I have used Nichicon KZ MUSE series replacements of the same rating. C815 and C816 are emitter decoupling capacitors for TR803 and TR804. They were rated 47 uF / 6.3 V. I have replaced these with Nichicon KZ MUSE series modern counterparts rated 47 uF / 25 V. C822 and TR815 forms a type of stabilizer circuit. C822 was rated 100 uF / 75 V. I have replaced it with a Nichicon KZ MUSE series part rated 100 uF / 100 V.
Here is the board after the job.
This concludes the restoration of this amplifier.