Introduction

This is was our bedroom amplifier. It has a warm 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, even at a power level 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.

As I write these lines, this amplifier has no real issues. Even though it is in full working condition, I decided to take it through a restoration process in order to make it future-proof.

Technical Data

This amplifier has the following technical characteristics.

Parts List

I have assembled a parts list for this amplifier. Since I want to increase the lifetime of this amplifier, and I don't believe that transistors actually contribute to any specific sound signature, I will replace the failure-prone ones. Otherwise, I will focus on replacing electrolytic capacitors.

The schematic value corresponds to what can normally be found in the electrical schematic diagrams. The recommended value is what I replaced the 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, fewer time-ticking bombs.

Descriptions and Parts Listings

The Power Supply filtering capacitor is mounted on the chassis. I have not decided to replace this one as it tests OK 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 faceplate. To release it, you need to remove the black faceplate, and 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 into the steel chassis, upside down. Beware that in order to release this board, you need to undo a big solder joint 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 could probably not succeed. Unfortunately, this board really needs to be released from its mounting point, otherwise 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 side of the steel chassis. It has two black large output electrolytic capacitors. I suggest you don't replace these, should they test OK. If you find a good source of capacitors capable of high current, then go ahead and replace them. I have not touched them. Be careful as these define theat capacitor-coupled Sansui sound.

From an electronics point of view, this amplifier has a simple construction. There are no real issues to be encountered during the restoration process. You can observe the large number of 1 uF electrolytic capacitors that can easily be replaced by film counterparts for a solid reliability in time.

Restoration

At some point in time, I attempted a failed restoration of this amplifier. Why failed? Because I mostly used capacitors that I recovered from various electronic devices. It was OK, but this cannot be called a proper restoration. In fact, replacing old parts with ... old parts is just unacceptable. So I have decided it is time to revisit this amplifier. Now, 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 at 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 have both standard and precision desoldering pumps and desoldering wicks of various widths. To clean the flux, I use isopropyl alcohol and high-purity acetone.

Empirically, I found that soldering at 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 for 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.

Sometimes, I like to take unnecessary actions, such as replacing perfectly working components for futureproofing, or completely stripping down PCBs of parts for a better cleaning. While tedious, I have fun while doing so, and I also learn a thing or two in the process.

F-1303A Equalizer and Tone Control Board Restoration

Removal of the PCB is not mandatory. You can work directly on the board with some difficulties caused by space constraints. Which I did. On this board​ 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 emitters of TR601, TR602, TR603, and TR604. They were originally rated at 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 into the raster holes. C709 and C710 are decoupling the emitters of TR701 and TR702 for improved working stability. I used Nichicon KZ MUSE series parts rated 100 uF / 25 V.

I did not take pictures before the restoration as I simply forgot. 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 good pictures. But you get the point.

A trained eye will quickly spot that those transistors have corroded legs.

Film capacitors used as replacements.

F-2001 Filter Board Restoration

To remove the filter board you need to remove two screws. Furthermore, you need to desolder a steel pin, part of the chassis, protruding through the board directly onto the ground plane. You need a powerful iron and a wide tip for 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 latter. 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 with 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 removed the bottom steel shielding plate. Then I grabbed the capacitors with my hand from underneath the amplifier while operating the soldering iron with the other 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 previous restoration with vintage assorted capacitors; between them, even Rubycon parts.

I still don't know what was I thinking back then.

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 form 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.

With all brand-new proper capacitors.

Aftermath

These are the old parts that I replaced.

This concludes the restoration of this amplifier.

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 great, with a deep low-end and mellow mid-frequency reproduction. The high frequencies are now rendered more clearly. There's a bit of improvement over how it sounded before.

It is connected to a pair of old two-way 10 W bookshelf speakers made by Tento. 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, the restoration was a success. Until the next maintenance session, this topic is closed.

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

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

Thank you!