Vintage Audio
this section covers some restoration work that I did on my vintage HI-FI gear
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Article #1 | 12:00 PM Sunday 02/10/2016

Sansui AU-7500: Introduction

I found this little gem in the local flea market. This was a lucky day. However the price was premium.

When I plugged the AU-7500 amplifier and played a few audio tracks, I instantly recognized the authoritative sound of the AU-x500 series filling the room. Naturally this amplifier replaced the dual amplification rig powered by AKAI AA-5210 units in my study.

This will be a long process but I document it well. Maybe you find my work inspiring or helpful. Or my descendants, during a distant future maintenance, should this amplifier survive the years to come...

Article #2 | 12:46 PM Monday 02/10/2016

Sansui AU-7500: Technical Data

This amplifier has the following technical characteristics.

TECHNICAL PARAMETERS
ParameterValue
Continuous RMS power (both channels)40 W + 40 W (8 Ω at 1,000 Hz)
Total Harmonic Distortion<0.1% (at rated output)
Intermodulation Distortion (70 Hz : 7,000 Hz = 4:1 SMPTE method)<0.1% (at rated output)
IHF power bandwidth (each channel driven at 8 Ω)5 Hz - 40,000 Hz
Frequency Response (at normal listening level)10 Hz - 30,000 Hz +0 dB / -1 dB
Load Impedance4 Ω - 16 Ω
Damping Factorapproximately 40 at 8 Ω load
SemiconductorsTransistors: 38 / Diodes: 15
Power Voltage100 V, 110 V, 117 V, 127 V, 220 V, 230 V, 240 V, 250 V at 50/60 Hz
Power Consumption100 W (max)
Dimensions / Weight140 mm (H), 440 mm (W), 322 mm (D) / 12.7 kg

Article #3 | 09:32 PM Monday 23/10/2017

Sansui AU-7500: Parts List

I have assembled a parts list for this amplifier. My restoration targets mainly the electrolytic capacitors. But some transistors are subject to change as well. Even though I consider them irreplaceable for an authentic Sansui sound, I am aiming for maximum reliability and low maintenance. Below you will find lists consisting of the various capacitors in this amplifier.

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 capacitors mounted directly on the chassis are for decoupling purposes. They are located directly on the transistor sockets beneath the unit. These capacitors are directly soldered on the rightmost and leftmost edges of the power transistor socket array. For some reason, these capacitors don't appear on the schematic.

Chassis
IdentifierSchematicRecommended
N/A3.3 uF / 50 V
N/A3.3 uF / 50 V

The Power Supply Block is coded F-2013-1 and is located below the chassis, mounted in a centered position on a tilted printed circuit. You can quickly recognize it due to the stabilizer transistor cooled by a small aluminum radiator and the four small current diodes that form a rectifying bridge.

Power Supply Block
IdentifierSchematicRecommended
C0094.7 uF / 63 V10 uF / 100 V
C01010 uF / 50 V10 uF / 100 V
C01210 uF / 50 V10 uF / 100 V
C014220 uF / 50 V

The Protector Block is coded F-2041 for stock no. 7591230 and F-1215A for stock no. 7591300. That's right, two boards for one schematic. F-2041 is located on the left side of the chassis mounted directly on it. You can quickly recognize it due to the big speaker coupling relay. F-1215A is located beneath the steel chassis directly under F-2041. There are three wires connecting the two boards. Both boards are easy to service.

Protector Block F-2041
IdentifierSchematicRecommended
C90147 uF / 10 V BP47 uF / 16 V BP
C902220 uF / 6.3 V220 uF / 16 V
C903100 uF / 50 V
C9061 uF / 50 V1 uF / 50 V FILM
C9071 uF / 50 V BP

F-1215A is a little bit weird but beautiful at the same time. It does not have a green solder mask and the copper tracks are connecting the part terminals via rivets. Interesting.

Protector Block F-1215A
IdentifierSchematicRecommended
C908220 uF / 35 V220 uF / 50 V

There is one combined audio driver board in this amplifier responsible for both left and right channels. It is coded F-2034 and can be found mounted on the chassis just between the power transistors array and the tone control board. In my unit there were two capacitors directly soldered on underneath this board between the emitter and collector of TR807 and TR808. For some reason, these don't appear on the schematic but are easy to change. I have marked them as N/A below.

Driver Block
IdentifierSchematicRecommended
N/A3.3 uF / 50 V
N/A3.3 uF / 50 V
C8014.7 uF / 50 V
C8024.7 uF / 50 V
C80547 uF / 50 V
C80647 uF / 50 V
C80747 uF / 10 V BP47 uF / 16 V BP
C80847 uF / 10 V BP47 uF / 16 V BP
C81147 uF / 50 V
C81247 uF / 50 V
C815100 uF / 50 V
C816100 uF / 50 V
C819220 uF / 6.3 V220 uF / 25 V
C820220 uF / 6.3 V220 uF / 25 V
C821100 uF / 50 V
C822100 uF / 50 V

The Equalizer Block is coded F-2028 and implements the Phono preamplifier and the RIAA curve corrector. This board is right positioned, under the large Faraday cage metallic shield. This board is socketed. Note that the TC notation signifies a tantalum capacitor.

Equalizer Block
IdentifierSchematicRecommended
C6012.2 uF / 25 V TC2.2 uF / 50 V
C6022.2 uF / 25 V TC2.2 uF / 50 V
C60310 uF / 25 V10 uF / 100 V
C60410 uF / 25 V10 uF / 100 V
C60947 uF / 16 V47 uF / 25 V
C61047 uF / 16 V47 uF / 25 V
C61133 uF / 10 V33 uF / 25 V
C61233 uF / 10 V33 uF / 25 V
C62310 uF / 25 V1 uF / 50 V FILM
C62410 uF / 25 V1 uF / 50 V FILM
C6251 uF / 50 V1 uF / 50 V FILM
C6261 uF / 50 V1 uF / 50 V FILM

The Tone Control Block is coded F-2014 and is located on the steel chassis in the front part of this amplifier. Of them all, this board is the most complicated to work on because there are a lot of electrolytic capacitors and small signal transistors. Also a lot of wires on the solder side. I list the capacitors below.

Tone Control Block (F-2014)
IdentifierSchematicRecommended
C70347 uF / 16 V
C70447 uF / 16 V
C70733 uF / 16 V33 uF / 25 V
C70833 uF / 16 V33 uF / 25 V
C7094.7 uF / 50 V
C7104.7 uF / 50 V
C7191 uF / 50 V1 uF / 50 V FILM
C7201 uF / 50 V1 uF / 50 V FILM
C72110 uF / 16 V10 uF / 100 V
C72210 uF / 16 V10 uF / 100 V
C7234.7 uF / 16 V4.7 uF / 50 V
C7244.7 uF / 16 V4.7 uF / 50 V
C7291 uF / 50 V1 uF / 50 V FILM
C7301 uF / 50 V1 uF / 50 V FILM
C7391 uF / 50 V1 uF / 50 V FILM
C7401 uF / 50 V1 uF / 50 V FILM
C7414.7 uF / 50 V
C7424.7 uF / 50 V
C743220 uF / 50 V

As you can see, there are a lot of electrolytic capacitors to be replaced. While this amplifier desires a lot of respect, I cannot but advice you to take your time and observe the electrical schematics and my capacitor lists. You can stick to the original values and types or you can follow my list. It is up to you. But the results will be similar. Your amplifier will shine again and will produce that great Sansui sound. After all, this is a top performer and will surely reward your ears.

Do not hurry, take your time and do the job once. And do it well.

Article #4 | 10:00 AM Tuesday 24/10/2017

Sansui AU-7500: Restoration

As always before I start working on a vintage unit restoration, I organize all the parts that will be changed. Further more I am sorting transistors based on the hFE factor. This is a boring job but it pays for itself. Here I am sorting Fairchild KSA992 transistors. Careful with these transistors as their terminals are designated as ECB. The old 2SA726 are BCE. So the replacements must be mounted in reverse.

An array of Panasonic stacked film capacitors. Good quality.

This is one of the most easy to repair of them all amplifiers. I have seen quite a few units during my restorations but none like this. Nearly all boards can be serviced directly in place. Not a screw to remove. My way of working on this is to put it upside-down on the workbench. I can reach with my hand under the chassis all the components that I need to service. On the soldering side there is enough clearance to work in a clean manner. If you are careful enough (!) then you don't need to even desolder any wire. With a lot of patience, I managed to do a clean job without heat-damaging the various soldered wires. Overall it is a joy to work on.

General Considerations

I should begin by saying that working on this amplifier presents a very big health hazard. There are lethal voltages inside. Not knowing what you are doing might result in severe accidents and possibly death by electrocution. I am very skilled in electronics repair and I have been doing such repairs for almost 20 years. This qualifies me to work in this field. But if you do not have experience, please take this information just as a knowledge base. Do not attempt to repair something that you cannot handle as there is a big probability to severe it further while also suffering accidents.

Good working condition tools are also necessary for this restoration. I am using a quality temperature regulated soldering station with multiple tips for every situation that I could possibly encounter. Also I am using a good solder pump and quality desoldering wick in two sizes. Isopropyl alcohol is handy to decontaminate boards of old flux and other residues. I also use eutectic formulation quality solder. Every replacement part is brand new, from a reputable manufacturer, ordered from U.S.A., Japan, or Germany. Also I only use parts that are suitable as replacements in various sections of the amplifier, after inspecting and comprehending the original schematic. Last but not least I have the years of experience backing up every action that I take while working on this unit.

I have found out that working with a temperature of exactly 323 degrees Celsius is sane for these vintage printed circuit boards. I have never lifted a pad with this temperature. But it is also true that I never wait more than three seconds with the soldering tip on a pad. While working on the chassis, I am pumping up between 360 and 440 degrees Celsius in the soldering iron.

Flux fumes are extremely toxic and should be avoided at all costs. Especially toxic are the fumes released while working on these old Japanese electronic boards.

Chassis Restoration

The two decoupling capacitors, subject to change on the chassis are not figured in the original schematics. Or I could not find them. They are decoupling the power rails to the power transistors array. There is one on the leftmost side of the unit and another one on the rightmost side, directly under the fuse board. You cannot replace the one under the fuse board without removing this first. Disassembly is simple because only two screws are holding the board in its place. Changing these capacitors is very difficult due to the direct soldering to the steel chassis. This acts like a very large heatsink which gave a hard time to my 80 W soldering station. Even though the temperature was set to 450 degrees and I was using the widest tip I had available. But in the end I managed to solder the terminals. Here I show the original parts.

I have used Nichicon FG series replacements rated 3.3 uF / 50 V. Unfortunately I have not took before pictures. But this is after the replacement.

Very hard to spot the small electrolytic capacitor under the fuse board.

F-2013-1 Power Supply Board Restoration

Easy to work on; no need for disassembly. Here it is in a before state.

A particularity is that C009 is rated 4.7 uF / 63 V on the schematic but in my unit it was 10 uF / 50 V. For C009, C010, C012 I have used Nichicon KZ MUSE series rated 10 uF / 100 V. C014 was replaced with a Nichicon KZ MUSE modern counterpart rated the same as the original.

This is after the restoration.

And on the track side I have re-soldered the terminals.

F-2041 / F-1215A Protector Board Restoration

F-2041 is a joy to restore. A few capacitors and the coupling relay are targeted here. This is before.

C901 was a 47 uF / 10 V bipolar capacitor and it was replaced with a Nichicon ES bipolar series modern part rated 47 uF / 16 V. C902 acts as the relay turn-on delay and it was rated 220 uF / 6.3 V. I replaced it with a modern part rated 220 uF / 16 V. C903 is a supply rail filter capacitor for this circuit. I replaced it with the same 100 uF / 50 V rating capacitor from the Nichicon KZ MUSE series. For C906 rated 1 uF / 50 V I have used a Panasonic stacked film capacitor of the same rating. C907 is a 1 uF / 50 V bipolar capacitor that was replaced with a Nichicon ES series counterpart of the same rating.

The relay can be ordered by the OMRON MY4-02-DC24 part number. This fits perfectly in place of the old relay. In fact it is identical despite the 40+ years that have passed.

Relays. Old and new OMRON parts.

And this is after.

F-1215A is very easy to work on because there is exactly one capacitor to be replaced. Removal is easy as well. Only one screw to remove. Here is a picture before the job.

I have replaced the old 220 uF / 35 V capacitor with a modern Nichicon KZ MUSE series counterpart rated 220 uF / 50 V. And here is after the job.

While putting this board back, pay attention to the metal fixture. It has a tendency to run off and misalign with the screw hole. Nothing to worry about. With minimal care I have secured it back in place in no time.

F-2034 Driver Board Restoration

The driver board is just near the power transistors array. Very easy to service if you turn the unit upside down to have access to the soldering side while holding parts with your left hand from beneath the chassis. This is the board prior to restoration.

Input coupling capacitors C801 and C802 were originally rated 4.7 uF / 50 V. I have used same rating Nichicon FG series replacements. C805, C806, C811, and C812 were rated 47 uF / 50 V and were replaced with same rating Nichicon KZ MUSE series modern capacitors. The bipolar capacitors in positions C807 and C807 were rated 47 uF / 10 V while in my unit were 68 uF / 10 V. I have replaced these with Nichicon ES bipolar series capacitors rated 47 uF / 16 V. Power supply rail decoupling capacitors C815, C816, C821, and C822 were rated 100 uF / 50 V and were replaced with the same type modern parts from the Nichicon KZ MUSE series. C819 and C820 are used for emitter decoupling of transistors TR805 and TR806 for static functioning point stabilization. These were 220 uF / 6.3 V and were replaced with Nichicon KZ MUSE series parts rated 220 uF / 25 V.

I have also replace all four 2SA726 transistors in positions TR801, TR802, TR803, and TR804. hFE matched Fairchild KSA992 transistors were used instead.

And here it is after.

I have reapplied solder on the component terminals. Here is a view on this.

On the underside of this board there are the two 3.3 uF / 50 V capacitors mounted directly on the emitter and collector terminals of each bias transistor. I have changed these with Nichicon FG series modern parts of the same rating.

This is a view before the replacement.

And after the restoration.

I really enjoyed working on this board as everything worked flawlessly and clean. If only there were more of these vintage units built like this. Normally with vintage amplifiers you face a deep forest of wires. Think about Pioneer for instance.

F-2028 Equalizer Board Restoration

The equalizer board is socketed and can be found in the rightmost side of this amplifier. It has a thick steel Faraday cage protecting the circuit of electromagnetic radiation. Removal of the shield is done by removing the two screws securing it in place. Gently pull the board out of its socket presents you with this.

Input capacitors C601 and C602 were of tantalum type, rated 2.2 uF / 25 V. I have replaced these with Nichicon FG series modern parts rated 2.2 uF / 50 V. C603 and C604 in the RIAA correction loop were polarized parts rated 10 uF / 10 V. They were replaced with Nichicon ES series bipolar parts rated 10 uF / 16 V. C609 and C610 are rated 47 uF / 16 V on the schematic while on the board were 47 uF / 50 V. I have measured the voltage drop on their terminals and it is about 7 V. So I have replaced these with Nichicon KZ MUSE series capacitors rated 47 uF / 25 V. C611 and C612 were rated 33 uF / 10 V and were replaced with Nichicon KZ MUSE series modern parts rated 33 uF / 25 V. C623 and C624 were marked as 10 uF / 25 V on the schematic while on the printed circuit board were 1 uF / 50 V. Output coupling is done through C625 and C626, rated 1 uF / 50 V. I replaced all these with Panasonic stacked film capacitors rated 1 uF / 50 V. All fault-prone transistors were replaced with modern Fairchild parts, hFE matched.

Here is the board after the restoration.

I have reapplied solder on the component joints.

F-2014 Tone Control Board Restoration

On this board things are more complicated than on the other ones. This is because of the wires soldered directly on the solder-side of the board. Nothing to worry about as I could work my way on avoiding damaging or desoldering them. However extra care should be taken. There are a lot of capacitors and a handful of transistors subject to replace. In particular there is the big local filtering capacitor that is very hard to remove due to the fact that it has some ground wires soldered directly onto its terminals. These are the wires that I had to desolder.

Here is a picture of this board prior to servicing.

C703 and C704 were rated 47 uF / 16 V and were replaced with Nichicon FG series parts rated 47 uF / 16 V. C707 and C708, rated 33 uF / 16 V were replaced with Nichicon KZ MUSE series capacitors rated the same as the originals. Coupling capacitors C709, C710, C723, C724, C741, and C742 were rated 4.7 uF / 50 V and were replaced with same rating Nichicon FG series parts. C719, C720, C729, C730, C739, and C740 were 1 uF / 50 V polarized capacitors. I have replaced these with Panasonic stacked film 1 uF / 50 V capacitors. C721 and C722 were 10 uF / 16 V and were replaced with 10 uF / 100 V Nichicon KZ MUSE series parts. Why the high voltage? Because this is what I had available at the moment. Finally C743 which is also the hardest to remove due to the fact that two wires are soldered directly on one of its terminals, was originally rated 220 uF / 50 V. It was replaced with the same rating Nichicon KZ MUSE series modern part. Faulty transistors were replaced with modern Fairchild parts, hFE matched.

Here is the board after the restoration.

This is the hardest part to work on. You can see various soldered wires that I didn't want to mess around with. So I carefully worked my way through them. Keep in mind that the notion of easy is relative however to your experience. What I could find easy to service, you could find hard. And recto-verso.

An overview of the soldering side shows that I have applied new solder over the component terminals.

Aftermath

Old parts.

Cleaning the Front Panel

...

Article #5 | 10:35 AM Sunday 29/10/2017

Sansui AU-7500: Impressions

So how does it sound like? Long story short, I cannot stop listening to music through it.

...

This restoration was a success. Until the next maintenance, this topic is closed.

Copyright © 1998- Alexandru Groza