Sansui AU-666: Introduction

This is was my wife's amplifier. People speak that this amplifier has a tube sound but I strongly argue. My unit has a controlled, fast and precise sound with emphasis on medium-high end of the spectrum. I can't say I like it but I don't dislike it either. It is different than the others that I have in a sense that it lacks that sweet and slow sound. It sounds best with speakers that are not that rich in high frequencies reproduction. I have such pair and connected it to this amplifier. They make for a very nice pair. Power is sufficient for a 10-14 square meter room. Bass reproduction is good also. The construction, on the other hand, is like solid reinforced concrete. Built to last.

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-666: Technical Data

This amplifier has the following technical characteristics.

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Sansui AU-666: Parts List

I have assembled a parts list for this amplifier. My restoration does not touch original transistors as I consider them irreplaceable for an authentic Sansui sound. I don't believe that transistors actually contribute to any specific sound signature. Thus, I am only 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

There are some Power Supply electrolytic capacitors mounted directly on the steel Chassis. These are either filter (in the case of this amplifier) or decoupling capacitors. Normally I don't recommend replacing the big filter capacitors. Should they test OK, leave them alone. Otherwise, get replacements. But the smaller ones need to go.

There are two Driver Block boards in this amplifier. They are coded as F-1159A. These boards are positioned in between the big black power transistor aluminum radiators. They are socketed in blue sockets mounted on the chassis. In the table below, I have listed the electrolytic capacitors for only one board. So be careful to double order each part. Thankfully, on this board, the engineers have spared electrolytic capacitors in favor of Mylar film ones. I cannot but greet them! Thank you, Sansui!

The Minus Ripple Filter Block board is mounted under the steel chassis, near the big filter capacitors. It is coded as F-1277A. Easy to spot, even easier to operate changes.

The Plus Ripple Filter Block board is mounted under the steel chassis, left of the Minus Ripple Filter Block board. It is coded as F-1231A. Easy to spot, even easier to operate changes.

The Tone Control Block board is coded F-1279 and is mounted upside down in front-left of the amplifier chassis. It is held in place by the potentiometer axles. In order to remove it, you need to undo the potentiometer axle nuts. I recommend to get it out in order to gain better access to it.

The Speaker Protector Block board is coded as F-1234 and is mounted under the steel chassis, just above the two blue sockets for the Driver Block boards. If we take the blue sockets as a reference point, then the Speaker Protector Block is diametrically opposing the Minus and Plus Ripple Filter Block boards. Easy to spot, even easier to operate changes.

The Ripple Filter Block board is coded as F-1268A and is mounted next to the the mains transformer, in the left region on the steel chassis. This board does not require removal in order to exchange parts. The solder side is accessed from below.

The Head Amplifier Block board is coded as F-1284-1 and is socketed in the far front-right region of the steel chassis. You cannot miss it.

A lot of capacitors, indeed. But no need to worry. Take one board at a time. Success is guaranteed.

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-666: Woodwork

I started by restoring the wooden case. As you would have expected, it is crafted from real wood with a walnut (?) wooden veneer applied on top. I find it beautiful but it has a few scratches and usage marks here and there. Here are a few pictures of what the case looked before the restoration.

This is the underside of the wooden case. You can see the real wood grain. In my opinion, it is beautiful on the inside too.

To restore this part, I first opted for a thorough rubbing alcohol cleanup on the entire veneer. It came up pretty good. Then I used some felt to even up the surface and remove minor imperfections. Deep scratches, however are here to stay. I don't have means nor skills to restore them. Then I took some walnut wood stain that I applied with a soft sponge brush. After the first coat, it looks like this.

Or, in daylight it appears like this.

A second and final coat of walnut stain produced this end result. Before applying this final coat, I have slightly sanded the case with my trusty felt. Light scrubbing with alcohol helped remove the dust. Don't insist as stain is easily removed by sanding or alcohol rubbing.

This is all I am going to do to this case. I think it's current condition nuances the fact that this amplifier is over four decades old and adds to the appealing aging wood character. A little secret I wanted to share: in time, I have learned to like imperfection. Anyway, should I ever put my hands on a nice piece of oak or walnut wood, I would very much like to find a workshop to build a replica of the original case. I would not use veneer but wood stain and lacquer instead. It might be a nice future project.

But for now, let's continue with the next stage.

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Sansui AU-666: Restoration

There is a lot of work to be done on this amplifier due to the unusually high number of printed circuit boards. In my opinion, at least four boards could have been coupled together on a single larger PCB. But I have to deal with what I have. Fortunately the removal of boards is easy and servicing is at hand. You have clearance on almost any board, bar the tone control one. Overall it is a fun project and with a little patience, you can do it in a single cloudy Saturday afternoon. If you have all the parts, of course.

And speaking about parts, I have sorted and matched each capacitor pair that I was about to replace in this unit. Once you have everything sorted out, it makes for a very quick servicing procedure. So let's get started.

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.

Speaking of fumes, this is the worst amplifier of them all. The boards were treated with an unknown rosin that when heated smells very pungent. Wide open windows are mandatory while working on the AU-666 boards.

Chassis Restoration

A number of six capacitors are directly soldered to some kind of multi-bracket connectors bolted on the underside of the chassis. These capacitors are forming a filter that should block frequencies below 50 Hz. From what I read on various sources, people recommend removing this filter altogether. I have decided to keep it in place however. Thus, follows the restoration. Here are the signal filtering capacitors before reconstruction.

I have replaced electrolytic capacitors designated C905, C906, C909, and C910 with the same 1 uF / 50 V rating Panasonic stacked film non polarized capacitors. C907 and C908 were rated 2.2 uF / 50 V and were replaced with the same rating Nichicon FG series modern parts.

And after the job.

F-1159A Driver Board Restoration

Each driver board has a thick steel shield that also acts as a fixture. There are two screws per shield. Remove these in order to liberate the steel plate. There is a bias transistor per board. This is fixed directly on each of the black radiators by a single screw. Undo these screws first. Then the boards are free to be pulled from their blue sockets. Gently get them out, one by one.

This is the left driver board prior to restoration.

C807 was a bipolar capacitor rated 47 uF / 10 V. I have replaced it with a Nichicon ES series bipolar capacitor rated 47 uF / 16 V. C809 provides local supply filtering and was rated 100 uF / 50 V. I have replaced it with the same rating Nichicon KZ MUSE counterpart.

And this is after the job.

Check out the weird medium power 2SA537 / 2SC708 transistor pair. Nice radiators by the way. Even nicer are the gold legs UFO-class transistors. People say on various forums that these are prone to failures. For some reason mine are behaving correctly.

I have reapplied solder to all the joints as you can see in the following picture.

Moving on to the right channel, prior to servicing.

C807 was a bipolar capacitor rated 47 uF / 10 V. I have replaced it with a Nichicon ES series bipolar capacitor rated 47 uF / 16 V. C809 provides local supply filtering and was rated 100 uF / 50 V. I have replaced it with the same rating Nichicon KZ MUSE counterpart.

And after the restoration.

These transistors sure do look nice. I have said it before already.

The same re-soldering treatment was applied to the joints.

F-1277A Minus Ripple Filter Board Restoration

There are two capacitors to replace on this board. But in order to do so, you need to remove the two screws securing the board to the chassis.

This is the board before restoration.

C814 was rated 100 uF / 50 V and was replaced with a Nichicon KZ MUSE series modern capacitor of the same rating. On the other hand, C816 was 4.7 uF / 50 V and was replaced with the same rating Nichicon FG series capacitor.

And after the job.

On the solder side I have done the re-soldering treatment. Here it is.

F-1231A Plus Ripple Filter Board Restoration

Like the Minus Ripple Filter circuit, this board has only two capacitors to be replaced. It is very similar to the one above thus you need to unfasten the two securing screws before having clearance for the replacements.

This is the board prior to restoration.

C813 was rated 100 uF / 50 V and was replaced with a Nichicon KZ MUSE series modern capacitor of the same rating. On the other hand, C815 was 4.7 uF / 50 V and was replaced with the same rating Nichicon FG series capacitor.

And this is after the restoration.

On the solder side I have done the re-soldering treatment. Here it is.

F-1279 Tone Control Board Restoration

It is very hard to work on this board with it in place because you have no clearance for your hand to position the parts. However the soldering side is exposed and easy to work on.

Here is the board before the restoration. There is an assorted array of colored capacitors. I believe originally these were sorted for low leaking current and stable characteristics. I have measured one 1 uF / 50 V of them and it wasn't doing very well. Capacity reported by tester was 1.5 uF while the ESR was about 20 R. Voltage loss at about 6.8 %.

Interesting socket for transmitting power and audio signals to and from the tone control board. Reminds me of the old days when tube amplifiers used to be common.

C705 and C706 were rated 10 uF / 16 V while C709, C710, C723, and C724 were rated 10 uF / 50 V. I have replaced all these with Nichicon FG series parts rated 10 uF / 50 V. C707, C708, C721, and C722 were rated 47 uF / 6.3 V and were replaced with Nichicon FG series rated 47 uF / 16 V. C725 and C726 were originally rated 1 uF / 50 V and were replaced with the same rating Panasonic stacked film parts.

I have replaced all six 2SC871 transistors with hFE matched KSC1845 modern transistors made by Fairchild.

And here it is after.

Re-soldering treatment is carried as well.

F-1334 Speaker Protector Board Restoration

This small circuit implements a spartan speaker protector that works by shorting the triacs when DC is found on the speaker terminals. A weird circuit, if you ask me. For the uniqueness, I have decided to restore the board. There is only one bipolar capacitor present.

This is the board prior to service.

The only capacitor on this board is designated C903 and is a 220 uF / 10 V bipolar part. As a replacement I have used a Nichicon ES series bipolar capacitor rated 220 uF / 16 V.

And here is the board after servicing.

On the solder side I have done the re-soldering treatment. Here it is.

F-1268A Ripple Filter Board Restoration

This board is easy to service as it does not require removal. Simply turning the unit upside-down allows you to use your other hand to manipulate parts while soldering or desoldering them. A number of three wires are soldered directly on the plus terminals of their respective capacitors. These need to be de-soldered before you can take out the capacitors. Another weird particularity is that the minus terminal of one of the capacitors is connected directly to the ground pad. There is no printed track connecting that capacitor with the main ground track.

This is the board before the replacement procedure.

C011, C012, and C014 are supply rail filtering capacitors and are of type 100 uF / 50 V. I have replaced these with Nichicon KZ MUSE series parts of the same rating. I believe C011 is marked on the silkscreen while it is actually C029 on the schematics. I cannot confirm this as I have not studied the circuit. I just shotgunned all these capacitors. TR001 and C013 along with the other parts are forming a power stabilizer. C013 is rated 220 uF / 50 V. I have replaced it with the same rating Nichicon KZ MUSE series capacitor.

And here is the board after servicing.

On the solder side I have done the re-soldering treatment. You can also see the minus terminal of C011 (C029) going directly to the ground point. Here it is.

F-1284-1 Head Amplifier Board Restoration

To remove this board you need to undo the one screw on top. It comes out with the small metallic piece that doubles as a fixture. After that you can gently pull the board out of its blue socket.

This is the board prior to the parts exchange.

C605 and C606 were 47 uF / 6.3 V and were replaced with Nichicon FG series rated 47 uF / 16 V. C607 and C608 were rated 100 uF / 6.3 V. I have replaced these with Nichicon KZ series rated 100 uF / 25 V. C611 and C612, originally rated 10 uF / 25 V were replaced with modern Nichicon FG series capacitors rated 10 uF / 50 V. C621 was originally rated 33 uF / 16 V and was replaced with a Nichicon KZ MUSE series capacitor rated 33 uF / 25 V. C623 and C624, rated 33 uF / 6.3 V were replaced with Nichicon FG series parts rated 33 uF / 16 V.

Here is the board after servicing.

On the solder side I have done the re-soldering treatment.

I have secured the head amplifier board back into its socket.

Aftermath

Old parts.

This board concludes the restoration of this unit.

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Sansui AU-666: Impressions

I have tested this amplifier with a smile on my face. Sound clarity and brute power is what characterizes this unit. Turning the volume potentiometer has a rewarding effect caused by the solid wall of sound emitted by the speakers.

Good job. I like this unit. The mid frequencies are very clearly rendered. Highs as well. Bottom-end of the spectrum is rendered as well as before. Punchy and powerful. I will continue to listen to this amplifier in the days to come, in our bedroom. I really like the fact that it doesn't warm up even after prolonged use. But anyway, this is a Sansui as-designed feature. Overengineering plays its role very well on this unit.

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

Later Edit: I ended up trading this amplifier to the same guy that now owns the AU-505.

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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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