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 unit #1 and #2 in my study.

This restoration will be a long process, but I will be documenting it thoroughly. Maybe you find my work inspiring or helpful. Or my descendants, during a distant future maintenance, should this amplifier survive the years to come, and they'd be interested.

Technical Data

This amplifier has the following technical characteristics.

Parts List

I have assembled a parts list for this amplifier. My main goal is to replace the electrolytic capacitors. But some transistors are also due for replacement. I don't really believe that transistors actually contribute to any specific sound signature, thus, if needed, I replace them without reserves. I am aiming for maximum reliability and low maintenance. Below you will find the individual parts lists for each sub-assembly of this amplifier.

The schematic value corresponds to what can normally 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, fewer 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. But they appear at page 26 of the service manual designated as C823 and C824. Also, there are some capacitor bleeding resistors underneath the steel chassis. These are soldered directly on the big capacitor terminals. These don't normally require replacement but in my case, one was of carbon composition type. These are known to drift in time.

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.

The Protector Block is coded F-2041 for stock no. 7591230 and F-1215-A 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-1215-A is located beneath the steel chassis directly under F-2041. There are three wires connecting the two boards. Both boards are easy to service.

F-1215-A 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.

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.

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.

Later edit: The capacitor marked with the * symbol is not on the original schematic nor on the circuit. I have explained in Article #6 below why I decided to mount it.

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 and transistors below.

As you can see, there are a lot of electrolytic capacitors to be replaced. While this amplifier deserves a lot of respect, I cannot but advise 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.

Restoration

As always, before I start working on a vintage unit, I organize all the parts that will be changed. Furthermore, I sort transistors based on the hFE factor. This is a boring job, but it pays for itself. The picture below depicts how 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.

It's very easy to work on this amplifier. I have seen quite a few units during my restorations, but a few of them were like this one. Nearly all boards can be serviced directly in place. Not a screw to remove. My way of working on the AU-7500 is to put it upside-down on the workbench. Then 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 even need to 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

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.

Chassis Restoration

The two decoupling capacitors, subject to change on the chassis, are not listed in the original schematic diagram. Or at least, I could not find them. They are decoupling the power rails to the power transistor 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 it first. Disassembly is simple because only two screws hold the board in its place. Changing these capacitors is difficult due to the direct soldering to the steel chassis. This acts like a very large heat sink, which gave my 80 W soldering station a hard time, 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 are the original parts.

I have used Nichicon FG series replacements rated 3.3 uF / 50 V. Unfortunately, I did not take a picture before the replacement. But this is after the replacement.

It is 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, and 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 tracks side, I have re-soldered the terminals.

F-2041 / F-1215-A Protector Board Restoration

F-2041 is a joy to restore. A few capacitors and the speakers 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 / 25 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.

C907 could've been replaced with a stacked film capacitor.

F-1215-A 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.

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 transistor 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 the same rating Nichicon FG series replacements. C805, C806, C811, and C812 were rated 47 uF / 50 V and were replaced with the 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 at 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.

I think that it would've been nice if I'd replaced the variable resistors as well.

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

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.

Details.

And after the replacement.

I reused the old isolation tubing on the leads of the new capacitors.

I really enjoyed working on this board as everything worked flawlessly and cleanly. If only there were more of these vintage units built like this. Normally, with old 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 from 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 / 50 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.

The symmetric construction is clearly visible.

This circuit board, with small variations, has been used throughout the entire AU-x500 series.

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 to avoiding damaging or desoldering them. However, extra care should be taken. There are a lot of capacitors and a handful of transistors subject to replacement. 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, C741, and C742 were rated 4.7 uF / 50 V and were replaced with the same rating Nichicon FG series parts. C723 and C724, which are emitter decouplers, were rated 47 uF / 16 V in the schematic, but 4.7 / 16 V in the service manual parts list. In my unit, there were 47 uF / 16 V parts installed. I have replaced them with Nichicon KZ MUSE parts rated 47 uF / 25 V. 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 circuit board just screams retro. Even the color scheme of the components (screws included) blends very well.

This is the hardest circuit 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.

Electrical Settings

The restoration cannot be complete without performing the electrical settings as illustrated in the service manual. Thus, there are two main settings that I need to touch before putting the case back on.

• Output of Power Amplifier Section
• Current Alignment of Power Amplifier Section

The procedure for setting the output of the power amplifier DC voltage is very simple. First, commute the Speakers selector to SYSTEM-A position. Then connect a DC voltmeter to the SYSTEM-A left terminals. Set the scale of the DC voltmeter to 20 mV. Adjust VR801 up until the voltmeter reads 0 mV ±10 mV. Repeat this step for the right channel. This time adjust VR802.

Adjusting the idling current of the power amplifier is done by removing the speaker fuse cover on the back of the unit. The next step is to remove fuse F002. Turn Speakers selector to OFF position. Connect DC ammeter on the terminals of F002 fuse. Adjust VR803 for a reading of 30 mA ±2 mA. Repeat the procedure for the right channel. This time remove fuse F003 and adjust trimmer VR804.

Needless to say that you need to power the unit off each time you remove the fuses. It can be done with the unit powered, but there could be a risk to short-circuit terminals and blowing off the nice output power transistors.

On my unit, there is a slight variation in the DC output voltage, particularly on the left channel. It is close to 0 mV, but it fluctuates repetitively around ±15 mV. This is subject to a further investigation.

Cleaning the Front Panel

Dismantling the front panel requires first removal of all knobs and switch caps. It is fairly easy, but you need to be careful in order not to damage the front aluminum mask. I cleaned everything with a window cleaner based on ammonia.

Next, I cleaned the knobs, the felt between the switches, and the frontal mask.

There was a lot of dirt and I used about twenty cotton swabs for the buttons alone. The frontal mask came up pretty clean, but there were some oily stains deep within the aluminum ridges that I cannot remove no matter how hard I tried. But I am happy the dirt is now a matter of the past.

Impressions

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

Here is the amplifier temporarily positioned on my left speaker. I have used the wood chunks because the amplifier feet are further spaced than the width of the speaker top cabinet. It looks nice, however it does have a dent in the top aluminum bevel.

Even though I cleaned the top cover as best as I could, there are some scratches that I really don't have any means to deal with. But anyway, if you want perfection, then buy new units. Old ones have their own drawbacks.

It doesn't sound exactly like an AU-9500, but it is similar in a good way. The sound is non-fatiguing, laid-back, and sweet. The bass is deep and well-contoured. Midrange adjustment is a little bit harsh for my ears. But I have found a sweet spot at around +1 gradation on the knob. The high frequencies are rendered in a clear manner.

This is my main amplifier for PC usage. This means that I listen to music through it every time I work on the computer -- or when I play games. It doesn't heat up, it is reliable, and fulfills my needs in this area. I like this amplifier better than the dual AKAI AA-5210 rig it replaced.

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

High Frequency Oscillations

Let me discuss an interesting and annoying behavior that I just spotted a few days ago. A year and almost a half ago, when I was dealing with the restoration of this unit, I spotted fluctuating DC offset, most notably on the left channel. While the variation was really minor, I decided to finally tackle it.

So dismantling the unit and connecting the oscilloscope to the speaker output revealed an HF -- mostly RF -- oscillation. Interesting enough, I said. I measured with the oscilloscope some key parts of the output amplifier section and the RF signal was there. Clear and repetitive. If you have ever heard a low-frequency audio sweeping oscillator, then imagine the same but in RF spectrum. The speakers were mute, they couldn't reproduce such high frequency. But the Sansui circuits were clearly able to generate and maintain the oscillation.

Well, I moved on to the preamplifier then. The same oscillation was present everywhere bar the entry point, before the 220 nF coupling capacitor to the input of the preamplifier. What? So the oscillation was not coming from anywhere before the preamplifier.

Then I put the scope on the F-2013-1 power supply section. Both at the output capacitor and TR003 base, there was this oscillation present. It was very unlikely, but I decided to test whether TR003 was oscillating. It was not, and as I said, it really was unlikely. That circuit cannot oscillate in that configuration. However, I saw the sweep signal with the scope. I added a low-capacity ceramic capacitor between the base and collector of TR003 and the only thing that happened was that the oscillation signal quickly changed shape and then came back steady and repetitive as it originally was. Back to step 0. The problem is not originating here.

Then, in access of curiosity, I commuted the source selector to PHONO 1 input. The oscillations disappeared completely. Hmmm, so there it is. The equalizer board is the source of these RF oscillations. But why is it propagating when the equalizer stage is disconnected from the main signal line? And why through the power supply line? For sure, there is no electrical contact -- besides the ground signal -- between the equalizer board and the input of the preamplifier, if the source selector is on AUX for instance. But the oscillations are there.

In order to be convinced that the oscillation came from the equalizer circuit, I have removed it altogether from its place and the oscillations disappeared completely.

Let's debug this circuit then. Start by looking at the schematic and using the brain first. The first thing to come to mind was the replacement of the previous transistors with modern ones. Let's start with position TR601 and TR602, formerly 2SA493. The transition frequency is 100 MHz and the collector capacity is 10 pF. I have replaced these with modern Fairchild KSA992 transistors. These have the transition frequency set at 50 MHz and the collector capacity is 3 pF. What if the difference in collector capacity triggers these oscillations? For a circuit to auto-oscillate you need an RC group. In this section, while the R is external, the C is internal. I didn't have a 7 pF capacitor in my spare parts box. The smallest one I could find was a C0G 100 pF MLCC capacitor. I gave it a try as per the altered schematic below.

I performed the modification on the tracks side as there was no apparent space on the parts side. It doesn't look obtrusive and Sansui did it before. Think AU-6500, driver board. Take a look on the tracks side, and you will see a similar alteration of the circuit.

What do you know? The circuit is now not oscillating anymore. I have tested it with the oscilloscope and my ears. Dead silent. Then I tried to use the turntable to check for audible problems caused by installing this new capacitor in such a low-signal high-gain circuit stage. Everything seems good.

Now, if only I could have installed a 7-10 pF capacitor in place, it would've been close to perfect, I think. But the problem is solved, and I am pleased by this resolution.

However, it is very interesting to note the fact that the RF oscillation was propagated through the ground signal to the power supply circuit and from there it was distributed to all the other circuits in this amplifier. So, should you ever find yourself in a similar situation, then this could be a starting point for a larger investigation.

In the end, I have redone the DC offset adjustment, and now it is a steady 0.1 mV on each channel.

I have also updated the table with the parts list for the equalizer circuit at the beginning of the page. Now it includes the 100 pF capacitor as addenda.

Small Upgrades

Working on a similar AU-6500 unit revealed that some parts could have been improved from the start in order to prolong the life of the AU-7500. I am now retroactively using the knowledge I got from that previous restoration to futureproof the AU-7500 amplifier.

Let's take the circuit sections one by one.

F-2013-1 Power Supply Board

Resistor R004 was inherently replaced with a Dale precision metal film resistor. While on this unit it appeared to be in top condition, in the AU-6500 that I recently restored, this resistor was scorched. So why take the risk?

Transistors TR002 and TR003, originally 2SA678 were replaced with Fairchild KSA733 parts. Be sure to order the transistors with the C suffix. This means center collector.

Zener diode ZD was replaced with a Nexperia 14.1 V / 1.3 W modern part. The original was a 14 V / 1.5 W part. Diodes D002 through D005 were replaced with 1N5392 parts. These appear to be more of an equivalent to the original 10D-1 parts. Diodes received glass tubes as separators. Why? Rectifier diodes produce heat. While it is true that, in this case, the heat is not sufficient to damage the printed circuit board, good engineering practices say to use separators.

The power supply section should now be on the safe side. The old diodes and transistors had corroded legs, and who knows for how long they would have remained functional?

F-2041 / F-1215-A Protector Board

Transistors TR901 and TR902 were originally 2SC634A, and I have replaced them with KSC1845 modern parts.

Diodes D905 and D906 were originally 10D-1. I have changed these with 1N5392 parts.

On F-1215-A, the single 2SC634A transistor with corroded legs was replaced with KSC1845.

F-2034 Driver Board

On the driver circuit I have replaced all variable resistors with new Bourns cermet parts. R851 and R852 are 10 ohm / 1 W resistors which I have replaced with Dale precision metal film resistors rated at 3 W and 1 % tolerance.

Of course, multi-turn variable resistors would've been way better, but these Bourns 3306F series parts look very good in vintage gear. And are pretty much stable once set, albeit the fine adjustment being a bit more difficult.

Aftermath

Old parts. Bar the resistors, all of them have corroded legs. Eventually, this corrosion works its way inside the plastic package, reaching the silicon layer. At that point, the transistor (or diode) is compromised.

While mostly unnecessary at the moment, these small updates will increase the MTBF rate of this amplifier. Which is exactly what I originally wanted.

Fun Facts

After a while of using another amplifier, I took the AU-7500 back into my room. It was a nice experience to hear that familiar sound signature again.

So I launched a PC game and, while deeply immersed in the game, I noticed faint FM-like sounds in the left speaker. At first, I thought these were sound effects from the game. But I was awfully wrong. The sound was coming from the electro-acoustical chain.

Angry as I was, I resisted the impulse to knock the chassis of the amplifier to see whether this would cure a bad solder joint or malfunctioning transistor. I mean, how could this be? I paid extra care while working on this unit. Too good I stopped in time.

Because I quickly realized that I had recently changed the 6N23P vacuum tube in my DIY buffer. I tackled the tube a little bit in its socket and that was it. No more unwanted noises. In the end, this amplifier works as it should.

Once I finished the restoration work on the Sansui AU-8500, it naturally took over the role of the AU-7500 in my office. I moved the AU-7500 into our bedroom instead. We seldom use it to listen to a matching TU-7500 radio during some evenings and most mornings.

Component Value Inconsistencies

Yesterday I received an e-mail from one of the readers. The subject was related to C723 and C724 on the F-2014 Tone Control Block. In his unit, the factory installed 10 uF / 16 V capacitors. That is yet another case of classic Sansui component value mismatching.

I'll state again that normally, what is written in the service manual can also be found installed on the PCBs. But that's not always the case. There were multiple revisions of the same unit, and they silently updated part values, without issuing updated service bulletins. That is why, in general, there are inconsistencies between what is installed and what is typed in the service manual. However, in this case, there is an inconsistency between the schematic diagram and the parts list.

Now, about C723 and C724. These are emitter decoupling capacitors which provide a low-impedance path to ground for AC signals. At the same time, these capacitors cut DC completely. By going with a larger capacitance in this circuit section, you will have an increase in low-frequency response, i.e.: bass reproduction. Also, the overall frequency response range will be flatter.

So I would recommend anyone servicing an AU-7500 to replace both C723 and C724 with 47 uF / 16 V capacitors. There will be a slight improvement over, say 10 uF, but it's not really audible, nor might it even be measurable by ear. On the other hand, with a spectrum analyzer, you might be able to see the improvement instead of hearing it.

I found the conversation interesting, thus I took the time to update this page with these remarks.

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!