Sansui AU-505: Introduction

This is was our bedroom amplifier. It has a divine warm and beefy sound. That of a good output capacitor coupled amplifier. The nice thing about the AU-505 is that it sounds very good even at ultra-low listening levels. At night, it sings music with a power of under 0.5 W while still delivering its signature sound. During weekend mornings, I occasionally crank it up a little. It helps with the wake-up. But then I get lost in the sound forgetting to get up.

At the moment I am writing these lines, this amplifier has no problems. It is in full working condition. But I decided to take it through a restoration process in order to double its life.

Disclaimer

The following articles are not to be treated as do-it-yourself tutorials on how to fix, restore, rebuild, or improve the unit in cause. This was not my initial intention. But you can consider this whole content as a general guideline, should you decide to launch into such an adventure.

The entire documentation is just a reflection of my work and I cannot be held responsible if you damage your unit, or even harm yourself in the process.

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Sansui AU-505: Technical Data

This amplifier has the following technical characteristics.

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Sansui AU-505: Parts List

I have assembled a parts list for this amplifier. My restoration also touches some of original transistors. Even if I consider them irreplaceable for an authentic Sansui sound, I want to increase the lifetime of this amplifier. I don't believe that transistors actually contribute to any specific sound signature. But mostly I am interested in exchanging the electrolytic capacitors. So this is what you will find listed below.

The schematic value corresponds to what normally can be found in the electrical schematics. The recommended value is what I replaced the former part with. The BP inscription signifies a bipolar capacitor. Where I found appropriate, I have chosen a film capacitor replacement instead of an electrolytic. My reasons are reliability in time. Less electrolytic capacitors, less time-ticking bombs.

Descriptions and Parts Listings

The Power Supply filtering capacitor is mounted on the Chassis. I have not decided to exchange this one as it tests incredibly good for its age. In order to maintain the original aspect, you need to source a fat capacitor that will fit the original metal clamp fixture.

The Equalizer and Tone Control Block is coded F-1303A and implements the Phono preamplifier and the RIAA curve corrector along the tone control circuit. This board is held in place by the axles of the potentiometers bolted to the front face. To release it for an easy access, you need to remove the black front face then release the potentiometer axle nuts. Or if you are really skilled, you can do everything with the board still in place. I have chosen not to release the board.

The Filter Block is coded F-2001 and is a small square board bolted directly in the steel chassis, upside down. Beware that in order to release this board, you need to undo a big solder point that doubles as a ground point. You need a really powerful soldering gun or a high temperature capable soldering station. With a 40 W soldering iron, you stand no chance. Unfortunately this board really needs to be released from its fixture or you cannot even see the electrolytic capacitors. Note that TC signifies a tantalum capacitor.

The Power Amplifier Block is coded F-1266A. This is easy to spot as it occupies most of the right of the steel chassis. It has the two black large audio-class electrolytic output capacitors. I suggest you don't change these ones, should they test OK. These days, power output coupling capacitors are not made to last. If you find a good source of coupling capacitors capable of high current then go ahead and replace them. I have not touched them. Be careful as these define the capacitor coupled Sansui sound.

From an electronics point of view, this amplifier has a simple construction. It rises absolutely no problem whatsoever during the restoration process. You can observe the large number of 1 uF electrolytic capacitors that can easily be replaced by 1 uF film counterparts for a solid reliability in time.

As always, let me throw a piece of advice. Do not hurry, take your time and do the job once. And do it well.

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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.

General Considerations

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.

Aftermath

This concludes the restoration of this amplifier.

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Sansui AU-505: Impressions

This was my first Sansui amplifier. It started my appreciation for this brand and formatted my ears to this kind of sound. It was our bedroom amplifier for some time and we really enjoyed it. After the restoration it sounds virtually the same both in the low-end and mid-end registers. The high frequencies are now rendered more clearly. That is a great improvement over how it sounded before.

It is connected to a pair of two way 10 W bookshelf Russian speakers. They make for a great pair in the evenings and in the mornings. Occasionally during the weekends. I really like this vintage small amplifier with a round sound.

Overall I declare the restoration a success. Until the next maintenance, this topic is closed.

Later Edit: I ended up trading this amplifier to a friend.

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Please note that all the work presented herein this site is non-commercial. This is my hobby and I am doing this in my spare time. Through this page I freely share my knowledge with you. But if you like my work, please consider helping me buy a transistor or a capacitor for my projects.

Thank you!

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