Introduction

A Pioneer amplifier that knows how to play music. From my experience, Pioneer gear manufactured during the 1970-1980 decade is refined, with a full and rich sound. Some would even say warm. I had very few Pioneer units during my life because I strongly disliked their sound. I can remember the CT-F950 or the CT-F400 cassette decks. Spectacular looks but dull and cold sound. So when I saw this SA-9100 unit on the local market, I decided to give it a chance. It was cheap because it was sold as defective.

And the seller was right. I plugged the thing in, switched on the power and nothing. No power-on light and no sound at all. From time to time, the protection would trigger and light the red indicator on the front panel. I opened it up and surprise: it was missing the original supply rail filtering capacitors. Instead, some axial capacitors of mixed brands were floating on the chassis, soldered directly to the wire terminals. What a mess. I opened the bottom cover only to find another two capacitors of the same axial type floating around. So, in total, there were four short-circuit prone parts floating in the chassis.

I have temporarily insulated the wire leads with electrical tape and decided to try to power on again. This time, the amplifier started correctly. Relay was acting properly as well. However, I was initially misled by a malfunction due to the front power indicator being burnt out. Then I decided to connect a set of speakers and a signal source. I have played a few tunes. Despite the supply buzz caused by the semi-leaking axial capacitors, the sound was powerful and warm. Now wait a minute, this one is a keeper.

This is why I decided to restore and keep this unit.

Technical Data

This amplifier has the following technical characteristics.

Parts List

I have assembled a parts list for this amplifier. While I plan to replace all old electrolytic capacitors, I will also replace some of the transistors.

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

Descriptions and Parts Listings

The power supply rail filtering capacitors are mounted on the chassis. Should they test bad, they need to be replaced. However, in my case, they were missing altogether, so I am obliged to add them.

Parts followed by the * symbol are additionally required to mount the new relay in place of the old one.

The Equalizer Amp Assembly is coded AWF-007-0 and implements the Phono preamplifier and the RIAA correction curve. You can find it constructed on dual-mono topology in the front-right part of the steel chassis. In order to operate changes, you need to extract the two printed circuit boards. The schematic doesn't correspond to the actual circuit. Nor does the layout of the printed circuit board correspond to the foil side pictured in the service manual. Some capacitors have different markings and some are even recent additions of a new design. Please note that the actual PCB assemblies installed in this unit are of type ANP-201-A and not AWF-007-0.

The parts are listed for only one channel. So double ordering is required. For this chart, I have switched the Schematic notation with Installed notation for the reasons stated above. Capacitors that don't appear on the schematic but are in the circuit are marked with the C symbol followed by the star (*), or double star (**) symbol.

The Control Amp Assembly is coded AWG-018-A and implements the preamplifier and the tone control circuits. You can find it in front of the steel chassis under the black Faraday shielding cage. In order to operate changes, you need to extract the printed circuit board. This is the long board between the equalizer amp assemblies and the power supply circuit B printed circuit board.

The parts are listed for both channels.

The Filter Amp Assembly is coded AWK-021-0 and is located under the steel chassis in between the switch boards and the power transformer. You can recognize it easily since it houses two big filter coils beneath an aluminum cage -- on some models the shielding cage is missing.

The parts are listed for both channels.

The Power Amp Assembly is coded AWH-023-B. There are two identical boards that are bolted to the big black radiators for the output power transistors. You recognize them quickly as they occupy most of the visible area that you see when you first remove the top cover. These boards are hard to get out without a proper angled screwdriver.

The parts are listed for only one channel.

The Protection Circuit Assembly is coded AWM-025-A and is located beneath the steel chassis on the right, next to the power transformer.

The parts are listed for both channels. The starred (*) part does not exist on the schematic nor does it exist on the actual circuit board. I have installed it additionally to provide protection to the relay driver transistor from inverse transient voltages from the relay coil.

The Power Supply Circuit-A Assembly is coded AWR-031-0 and is located in the center of the unit. You can spot it between the big power transformer and the metallic Faraday cage that houses the small signal audio preamplifier boards and the power supply circuit B board.

The Power Supply Circuit-B Assembly is coded AWR-032-0 and is located in front of the amplifier, under the black Faraday metallic cage. This board is located just left of the control amplifier board. You can recognize by the two regulator transistors that have undersized radiators.

The Switch-B Assembly is coded AWS-039-0 and is located under the steel chassis. It is the home of the Tape Monitor and Duplicate switches. Easy to spot on but hard to work on.

The parts are listed for both channels.

The Switch-C Assembly is coded AWS-040-0 and is located under the steel chassis. It is home of the Muting, Low Filter, Tone Defeat, High Filter, and Loudness switches. There are a lot of wires connecting this board to other electrical sub-assemblies. Since servicing this board is difficult, replacing the electrolytic capacitors with film counterparts would be good for future proofing.

The parts are listed for both channels.

The construction of this amplifier is very complicated due to a lot of wires and component layout. Thus, the restoration is not exceptionally easy. In fact, it is on the hard side. The wires cannot be removed and in some places you need to use your contortionist skills.

Restoration

As always, before I start working on a vintage unit restoration, I organize all the parts that will be changed. Furthermore, I am sorting transistors based on the hFE factor. This is a boring job, but it pays for itself. Careful with the new Fairchild transistor leg disposition. Thankfully, the Pioneer engineers designed these boards with triangle transistor leg dispositions. This means that it is extremely easy to mount every new transistor in place of the old ones. However, one needs to carefully observe the board markings and the electrical schematics first.

This Pioneer amplifier is difficult to work on due to the multitude of wires that are all wire-wrapped. These cannot be unwrapped and de-soldering the wire-wrap posts is next to impossible. The posts were mechanically fixed before being soldered. This means that you have to work with the boards in place, carefully bending all the wires at once. This operation is risky, and I wouldn't bend the wires more than a few times. So a lot of patience is required.

I did not take a lot of pictures during this restoration because the circuit boards are hard to position for good lighting.

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

On the chassis there are a few things to be done, such as the power supply rails filtering capacitors, the front panel indicator bulbs, and the speaker protection relay. It appears that there is not much to do here. But this assumption is plain wrong. Let's proceed then.

A technician installed some axial electrolytic capacitors: two Setron and two Roederstein, marked as ERO. The mounting hardware is missing as well. Oh, no! This is what the former capacitors looked like.

While I have never heard of Setron, the Roederstein (or ERO) are very familiar. I've seen a lot of them, at least in my country. These were considered very good quality capacitors back when they were new, but almost none of them aged well.

But how to explain this... mess!? I've never seen a Roederstein capacitor fail like this. It looks like it vented out all its electrolyte.

I do understand these are ground leads. But even so, I would've used some insulation if I were the technician that installed these capacitors.

Now please check this electrical connection, proudly realized by the same technician.

What hides under that tape?

But under this one?

The negative lead of that Setron capacitor might actually win the contest for worst connection ever.

That's it! Let's start to get things straight.

The junk capacitors are all out. One of the Roederstein parts that has vented reveals that, upon measuring, the actual capacity is around 970 uF out of the rated 4700 uF. ESR was a surprisingly low 8.1 Ω, but the internal loss was at 31 %.

One last look at the venting holes before tossing them in the recycle bin.

Thankfully, I was able to find Sprague Powerlytic 36DY large can series capacitors that will suit this purpose. In fact, these have identical electrical specifications to the old parts. That means 15,000 uF / 50 V.

These are serious capacitors.

Mounting hardware is available as well. And it fits perfectly the old machined holes.

When I ordered the hardware mounting kit, it wasn't clearly specified in the datasheet whether it would fit the capacitors that I chose. But after a short discussion with the Mouser technical support guys, they reassured me that they're specifically made for that capacitor diameter.

However I had to manufacture connector wires since the old ones were cut and floating around the chassis protected by electrical tape. So I took 1.5 mm stranded copper wire and soldered terminals designed for screw mount capacitor leads.

I have used thermo tube to isolate the potential short-circuit prone zone.

I have soldered the wires using a very resistant technique and then covered the joints with thermo tube.

This is what the new wiring looks from below.

And in the end this is what the capacitors look like from above.

Moving on to the front panel indicator bulbs now. The power indicator bulb is burned out. The other one works without issues. I have decided to remove the bulbs and replace them with LEDs: yellow 5 mm LED for power indication and red 5 mm LED for protector indication. LEDs require specific current and rectified voltage. I have used a simple circuit for each LED. The parts involved are: a 390 Ω / 0.5 W resistor in series with a 1N4148 diode in series with each LED. I have decided to replace the wires as well, since the old ones were very short. I have decided to solder each diode as close to the LED as possible and cover it with heat shrink tube. This makes for a very clean approach. The 390 Ω resistor is left uncovered and is mounted directly on the wire. Can you spot the tiny DALE resistors?

Yeah, I know I mismatched wire colors to the LEDs. But at least I kept dark for ground and light for positive.

The original bulbs were made by Stanley, rated at 8 V / 50 mA. Very nice! Stanley is still in business. They make automotive tail light assemblies and all sorts of incandescent ​bulbs. Should you want to take an original restoration approach, please find similar bulbs. I didn't have any similar bulbs around and 5 mm LEDs fit the rubber casing very well.

Final result.

Next up is dealing with the speakers protection relay. It is located in the lower left part of the steel chassis, next to the Power Supply B assembly. In addition to extreme heat exposure, it is also probably unusable by now due to the contacts being clogged with oxide and carbon deposits resulting from electric arcs. These arcs are generated when the mobile armature opens or closes the circuit while music is being played at a consistent volume.

Mounting a new relay requires a little bit of ingenuity. But everything is possible, as you will quickly see. Let's examine the old relay next to the new OMRON part. The first thing that screams incompatibility is the position of the mounting ears. The old one has the ears on the bottom while the new one has the ears on the top.

The quickest method to mount the new relay in place of the old one is to use some computer-grade hex standoffs. The ideal length would be 33 mm. I already had some hex standoffs in my junk parts box, so I used those instead. I have joined a 20 mm standoff with a 15 mm one to provide a 35 mm standoff. A little bit taller but nothing to be scared off.

The wires under the relay are particularly short and difficult to work on. My approach was to suck all solder from the old relay terminals with my solder pump and then push the wires as far down as I could. I have individually clipped the tips of each relay terminal by about 2 mm to expose the U-shaped cavity. Then I extracted all the wires effortlessly. Soldering them is easy. In addition, I have added small lengths of protective thermo tube over the insulation of each wire next to the soldering point.

Slowly working my way through the wires...

...finally leads to an end.

All is good now. The new relay will last a lifetime if you make sure you lower the volume while you power on or off the amplifier. Or at least, stop the music.

Equalizer Amp Assembly Restoration

The equalizer amplifier is built on two different printed circuit boards which are located on the far bottom-right side of the steel chassis. It is very difficult to work on these two boards because of the connection wires that are wrapped on two diametrically opposing sides. I decided to de-solder the top four wire wrapping posts of the right outer PCB, taking great care not to damage them.

The circuit depicted in the schematic diagram doesn't entirely correspond to what can be found on the printed circuit boards, but I am posting it here anyway.

This is the printed circuit board, before replacing the parts.

Those color-coded tantalum capacitors do look funny. Like small bees.

And after.

Those brown-reddish cfilm apacitor-like looking parts are actually precision resistors.

Control Amp Assembly Restoration

The control amplifier implements the tone controls and can be found under the black front main Faraday shielding cage.

Here is the electrical schematic diagram.

I originally thought this would've been the hardest board of them all since the number of wrapped wires is just outstanding. An entire array of wires spanning throughout the entire PCB length. But once I removed the two screws, I was able to flip the board upside-down and get access to all the parts. I have shotgun replaced every transistor and capacitor on this board.

Pictures as follows.

Soldering side. The solder pads on the lower side of the PCB have two holes for mechanically tightening of each wire-wrapping rod, before the wave soldering process. It is very hard to remove the posts without damaging the circuit board. My initial thought was to remove them all and get the circuit board out. But, hard luck!

Working my way through the component forest...

...while adding small glass beads to the stacked film capacitor leads...

...and tubular glass beads on the collector terminal of each small-signal transistor.

All good on the soldering side.

Next, metal can transistors are off the PCB. Modern Fairchild replacements are in place.

Filter Amp Assembly Restoration

The filter amplifier is accessible from underneath the unit. There are four transistors and a few capacitors to replace.

Here is the electrical schematic diagram.

Removing the circuit board from the plastic securing posts is easy if you have patience. Servicing this PCB does not raise any problems.

Before the restoration.

After the restoration.

Those wire-wrapping posts...

The FG series capacitors are a good fit in this circuit.

And this is the soldering side. On this board I have not touched any other solder joints than necessary.

Power Amp Assembly Restoration

There are two power amplifier printed circuit boards in this unit. Both are bolted to their appropriate power transistor heatsink. There are four screws per board. The two on the top are easy to remove, but in order to get out the two on the bottom you need to disassemble the heatsink and get out the power transistors themselves. The wires leading to these boards are fairly short, and it is difficult to work on the boards and also to take pictures.

I am targeting all small-signal and medium-power transistors, all small signal diodes, all variable resistors, and some of the standalone resistors. I chose Dale resistors with a higher wattage than the original specs. The variable resistors are Bourns. Transistors are either ON Semiconductor or Fairchild. Capacitors are all Nichicon FG series electrolytic types or Nichicon film types. The 1N4148 small signal replacement diodes have been mounted on glass standers. The replaced pre-driver transistors have received thermal compound between them and their heat sinks.

The Pioneer glue used to mechanically bind capacitors to the printed circuit board assembly is easily removable with 100 % acetone and cotton swabs. No mechanical removal approach is required.

Here is the electrical schematic diagram for one channel.

Let's start with the right channel, which is located on the rightmost side of the steel chassis.

Before the restoration.

There's also a fair amount of dust omnipresent.

During the restoration.

After the restoration.

Finally, I have secured the PCB on the power transistor heat sink assembly.

The power transistors received new mica insulators and a drop of quality non-conductive thermal compound.

I am continuing with the left channel, which is located on the leftmost side of the steel chassis.

Before the restoration.

During the restoration.

I like the silkscreen and the way they printed the transistor symbols. It's very easy to spot whether a specific transistor is NPN or PNP.

After the restoration.

As before, these power transistors also received new mica insulators and a drop of quality non-conductive thermal compound.

Protection Circuit Assembly Restoration

This board implements the speaker protection circuit. It is easily serviceable, and it is mounted directly underneath the big can power supply filtering capacitors. It is secured by plastic posts. On this board I am targeting all transistors and two capacitors. In my unit, the two capacitors were 220 nF / 25 V tantalum capacitors. I have replaced them with Nichicon film capacitors rated 220 nF / 50 V. In addition, I have added a flyback diode to protect the relay driver transistor from inverse transient voltages generated by the relay coil when it de-energizes.

This is the electrical schematic.

Before the restoration.

After the restoration.

Flyback diode.

Power Supply Circuit-A Restoration

I think this is the easiest board to work on from the entire amplifier. There are only two capacitors and eight diodes to replace. Normally, the diodes are not prone to failure, but since this amplifier is very hard to disassemble, I have chosen to overhaul it in terms of inaccessible semiconductors.

The electrical schematic diagram, as follows.

Before the restoration.

This is a simple circuit board with an easy layout.

ON Semiconductor power diodes are mounted on the PCB assembly.

And mounted back in its place. You can spot the two axial capacitors.

Power Supply Circuit-B Restoration

This is probably the worst power supply construction that I have ever seen in an audio amplifier. Hot transistors, small heat sinks, cooked capacitors, scorched resistors, mismatched power dissipation coefficient on certain resistors. Drifted resistor values and cracked resistors. But even with all these design flaws, the power supply was still running OK(ish).

I have shotgun replaced a bunch of resistors that appeared damaged or on their way out. I have chosen Dale resistors with higher power dissipation factors in order to ensure some derating. As for capacitors, all of them were replaced by high-temperature Nichicon BT series parts that can withstand temperatures of +125 °C while still doing their job. These capacitors also have an unusually long life if they are exposed to ambient temperatures -- think 30 to 40 °C -- while operating. The transistors were all replaced with better ones. The two power transistors used as regulators were moved somewhere else on the steel chassis so they could better dissipate heat.

The electrical schematic diagram, as follows.

Before the restoration. Notice the massive heat damage to the printed circuit board. I think these CEM (?) boards are 94V-1 flame retardant certified. It really stood the test of time and heat, but it doesn't look pretty. However, the tracks are intact, and the solder pads are in excellent condition. Heat discoloration is the only issue I can see here.

New capacitors.

New resistors with a large power dissipation reserve.

More resistors.

During the restoration. Some of the transistors have already been changed. All capacitors were replaced and some of the resistors are still pending replacement. You can see the two regulator power transistors mounted on weak heat sinks. As I previously said, I am going to relocate these power transistors somewhere else where they can better dissipate heat.

I have used some heavy-duty wires that were originally built for military high frequency transceiver constructions. These have a very thin PTFE (Teflon®) insulation. I was keeping them for tube amplifier construction, but I decided to use them in this unit as well. Thermo tube was used for insulating each transistor terminal.

I was initially going to go simple with the screw directly through the transistor heatsink mounting tab as pictured below. Then I quickly realized the tab was electrically wired to the collector. This spells disaster. So I have used a PTFE washer that dresses the screw so that there is no electrical contact between the collector and the heatsink. Note that the heatsinks will be attached directly to the steel chassis for better heat dissipation. And while the chassis is connected to ground... you figure out the rest.

I went one step further and used heat shrink tube to protect all three wires. It was fun to shrink it with my mini-torch.

I have labeled the terminal wires as E (emitter), C (collector), and B (base) with this simple color code: RED = E, BLACK = C, NOTHING = B. Easy to remember and dumb-proof.

They go in like this to the remote transistor.

Both regulator transistors are connected.

But where are the transistors? I was looking for a place on the steel chassis where to mount them. Normally, there are a lot of places, should you want to drill 3 mm holes. I didn't want to do this, so I had to improvise. I have searched for already existing screws that could accept the heatsink pre-drilled hole. I found one next to the MODE switch. This screw is part of an array of three machine screws that hold the steel chassis in place. One of them now also holds the heatsink for one of the regulator transistors.

And the other one? Check the image below. It is mounted to one of the screws that holds the tone control amplifier in place. The wires are covered in heat shrink tube and are routed through the middle of the chassis to the exit point which is directly under the Power Supply Circuit-B printed circuit board.

Which is pictured here.

After the restoration.

Switch-B Assembly Restoration

Easy to locate but damn hard to work on. I used some medical pliers to extract and place the new components in position. It is also very hard to photograph. So there you have it: I took only one picture.

The electrical schematic diagram, as follows.

After the restoration.

Solder joints were touched with eutectic solder.

Aftermath

These old parts have been removed from this amplifier. Here are the original capacitors.

And the semiconductors, resistors, and relay.

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.

• Idle Current
• Neutral Potential

In this unit, both procedures are very simple. First, commute the Speakers selector to the OFF position. Switch the input selector to AUX1. Ground each AUX1 RCA terminal through a 4.7 kΩ resistor. Let the unit warm-up for at least 20 minutes.

To set the idling current, connect a DC voltmeter between terminals 9 and 13 on the left channel power amplifier module. Set the scale of the DC voltmeter to 200 mV. Adjust VR2 up until the voltmeter reads 25 mV. Repeat this step for the right channel.

Adjusting the neutral potential of the power amplifier is done by connecting the DC voltmeter between ground (either GND signal or steel chassis) and terminal 23 on the left power amplifier module. Adjust VR1 for a reading of 0 mV ±10 mV. Repeat this step for the right channel.

Here's the device under tests. All electrical parameters seem to be OK. I have obtained a very steady 24 mV reading for idle current adjustment and 0.1 mV neutral potential on each channel.

Wires, wires, wires everywhere.

Cleaning the Front Panel

I have dismantled the panel and cleaned it with a cotton rag and some natural BIO degreasing substance. This is after the job.

Detailed view.

The knobs of this unit were particularly dirty. It appeared to be some kind of nicotine slime combined with dust and some oily residue. I haven't taken pictures before, but why should I lie? I have thoroughly cleaned them with a window cleaning solution based on ammonia and Q tips. Besides the smell, everything went smooth and a couple of hours later, the buttons were all shiny again. It is a marvel that these buttons don't have any dents or scratches on them. I am pleased to finally see these knobs after 3 years since I bought this amplifier as defective.

As I said, the knobs are in very good condition.

Then there is also this part. It is the LEVEL SET selector knob which is mounted on the same shaft as the volume control knob.

Knobs are all in.

I have also cleaned the wooden case...

...but it is suffering from time patina.

Even that metal grill is scratched. I might paint that at some point.

Those solid walnut frontal wood trims are very tought, yet they still received some ugly scratches.

When I have time, I will restore the missing chunk with epoxy rosin, then I will sand it down and reapply a small piece of veneer. But this is a whole different project that I will tackle somewhere in the future.

This concludes the restoration of this unit. I hope it will work great for a long time.

Impressions

Throughout the entire restoration process I had a strong feeling of fragility and insecurity. The entire construction itself seems fragile and prone to failures. Think about Power Supply B electrical design. It is a total no-go. I don't know who designed it and for sure I wouldn't want him or her in my design team. While this particular board proved that it could function for at least 30-40 years, the temperatures involved are nowhere near acceptable. Then again, wire wrapping is a bulletproof connection technique. But short wires? It is a nightmare to repair if a wire is broken accidentally. Failure-prone transistors installed in critical locations -- think 2SC1451 -- are a no-go as well. Let me remind you about the lack of a flyback diode for the speaker protection relay coil.

The mechanical construction is, however, very good and sturdy. Schematics are ingenious for that period of time. All-discrete resistor-capacitor tone control network is a delight. Both in terms of construction and sonic quality. The ELNA printed circuit boards are of very good quality. I would dare to say they are better than those used in contemporary Sansui units. Speaking about the protection circuit, without its drawbacks, I still find it well-designed. It was able to protect the speakers despite all the problems this unit had.

I would say serviceability is a 7 out of 10. While I don't especially do Pioneer units, this one was an interesting project. I learned a lot of things which, in the end, were a plus.

The SA-9100 even looks nice. It doesn't look like laboratory gear (think Sansui AU-x500), test gear (think early Accuphase), nor does it look like hospital instrumentation hardware (think AKAI AA series). This Pioneer looks sleek with its wooden case and walnut veneer. A successful combination of vintage and modern, in my opinion.

Now let me talk sound reproduction.

It sounds warm and dynamic at the same time. A Sansui AU-x500 unit would have a more laid-back sound. This one is at the same time laid-back, while the bass notes are punching straight in your face. It sounds very good on rock music and Eurodance as well. I think it sounds good on anything when coupled with good speakers. For late-night auditions, I would still prefer a Sansui amplifier. But I wouldn't be ashamed if I'd listened to the SA-9100.

What is special, though, is the sound with the tone controls defeated. It appears to be surreal, natural but at the same time flat and dynamic. Activating the tone controls, however, produces some sort of pleasant audible sound compression. A refined version of the old boomboxes that had a Wide Stereo switch. I bet it is the discrete tone control network that produces this kind of sound. Baxandall-style tone controls sound different.

After all, circuit topology is what gives the distinct sound footprint of a (pre-)amplifier. And this one is different.

What I like now is that I can use the unit without observing any heat build-up. The wooden case remains cool to touch. Before, it was fairly hot, especially on the lower left side where the problematic power supply circuit was originally radiating heat.

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

Impressions

These days, I have had the chance to use this amplifier in my living room. The Sansui AU-9500 is out for maintenance (again) due to me not changing some of the transistors back in 2016. But anyway, that's a different story. I brought the SA-9100 in place of the Sansui unit. And what do you know, I instantly felt in love with the sound. It has a very '80s-like sound signature. And the noise floor when idling is probably one of the lowest I've ever heard from a power amplifier.

The sound is, as I previously said, indeed laid-back with powerful bass and clear highs. One difference I've spotted though is that it lacks the insane dynamic range of the AU-9500. But hearing Eurodance through the Pioneer unit is a totally mesmerizing experience. I've listened to some old cassettes with DJ Bobo, Down Low, Blue System, and the like. Great sound indeed.

I'll be using this amplifier in the following days for casual listening and movie watching. But when the Sansui is fixed, I'll bring it back and put the Pioneer to rest again until I manage to integrate it into a new acoustic system. Last year I used it in my study for a couple of months and then came back to the AU-7500 unit. I actually enjoyed every second of listening to it. Same as I do right now. But there's something to the AU-x500 Sansui units that makes me always come back to them. In fact, I sold every other amplifier I had besides the three AU-x500 models and this Pioneer. And I don't regret it even a tiny bit. Also, now I have very little time to enjoy this vintage audio hobby. Thus, I prefer to stick to only a few units that I enjoy and know inside out, should they develop electrical or mechanical faults.

Now back to the audition. The soft but punchy bass notes reproduction is mesmerizing...

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!